96 Ls400 Bad Gas Mileage! Tried Everything

[yotoy82]

I am almost ready to honestly give up. I'm getting like 11 in the city and 17 combined city/highway. Its ridiculous considering a ford excursion which is probably one of the largest production passenger vehicles ever made is rated 14 in the city. I drive EXTREMELY light footed, grandmas regular drive by me like something is wrong haha.

I've changed both cat converters after check engine light suggested they may be bad. I've changed ALL FOUR oxygen sensors. A mechanic did a scan and told me he thought it was the MAF sensor so changed that. No luck. I changed the spark plugs, no luck. New fuel filter, and new fuel pump after it went bad, still no dice.

New air filter as well, new coolant temperature sensor. New PCV valve. I don't know what hasn't been changed except spark plug wires, but they are only 50,000 miles and 5 years old, and the car runs butter smooth!

The best mileage in a year I was able to get was 25 mpg on a 90 mile trip averaging 50 miles an hour roughly. That was a miracle for this car.

I have read all the links. Its getting quite expensive to drive this car. My only thought is maybe the car isn't getting warmed up enough for the sensors to work, but most people say the LS400 temperature gauge at that one mark before halfway is normal.

I guess the car was cheap and I don't drive much, spending money on parts and diagnosing only adds to the $$$ on top of the fuel I spend. I got the car cheap and I don't drive very much but even an average of 400 miles a month it gets a bit awkward that a 50 mile trip cost $20 bucks in gas when it should cost $10! If its normal then I guess I can live with it , but I'm worried the extra fuel would hurt other components too.

96 LS400 207,000 miles on it. I keep track of the mileage using an app on my iphone, always fill up. Screenshot enclosed.

If anyone who has an ls400 with bad mileage like me who has an idea or someone who can tell me something better than any mechanic has told me it would be appreciated. I want to spent money to change the engine mounts and shocks because everything else has been changed but so tired of spending money on the car that gets horrible mileage like this. I am sincerely thankful to anyone who can brainstorm something for me!

[curiousB]

There is something very wrong with the car based on your MPG. I routinely get around 19MPG in stop and go short trip city driving and high twenties on highway trips.

I have been having problems with emissions and one catalytic converter so it has me looking at similar things. An invaluable tool has been an OBDII interface and my laptop computer. I can set up gauges to look at fuel trim, short term (ST) and long term (LT). Look at MAF sensor output. See what ECU thinks engine temperature is. See if emissions system has reverted to closed loop or stuck in open loop.

It’s under $100 if you already have a laptop. You need something like this or you will forever be throwing expensive parts at the problem and not having much success.

The first thing I would do with such a device is check to see what long term fuel trim is on each bank. If it is very high that indicates the system is sending too much fuel to the engine for some reason. It could be as simple as vacuum leak in the intake ducting so that air is bypassing the MAF. The O2 sensors detect a lean condition and start bumping up LT fuel trim to compensate. In the end you can get a rich condition, poor gas mileage and red hot catalytic converts (which may get permanently damaged).

I’d also want to look at engine temperature the ECU sees from the code reader. If the sensor is faulty the ECU may be using the wrong fuel map and sending a very rich mix which isn’t needed. I believe the temp sensor for the dash is different than what the ECU uses.

It could be a stuck open thermostat keeping the car from warming up. That will cause ECU to send rich mix…

It could be a faulty fuel pressure regulator causing pressure to be too high so too much fuel is shot into the ports. The ECU would attempt to correct this though and you’d see LT fuel trim in negative values as the O2 sensors would be saying the mix is too rich.

So it could be many things but you need more data so you can pinpoint the problem better.

[jac430]

Maybe a stupid remark, but is the real fuel consumption in accordance with these figures?

The trip computer makes calculations, and there could very well be one bad input into the calculation offsetting everything.

If you calculate the average consumption each time after gas refills, do those figures comply with the trip computer values?

Jac

[DEATH]

I am almost ready to honestly give up. I'm getting like 11 in the city and 17 combined city/highway. Its ridiculous considering a ford excursion which is probably one of the largest production passenger vehicles ever made is rated 14 in the city. I drive EXTREMELY light footed, grandmas regular drive by me like something is wrong haha.

I've changed both cat converters after check engine light suggested they may be bad. I've changed ALL FOUR oxygen sensors. A mechanic did a scan and told me he thought it was the MAF sensor so changed that. No luck. I changed the spark plugs, no luck. New fuel filter, and new fuel pump after it went bad, still no dice.

New air filter as well, new coolant temperature sensor. New PCV valve. I don't know what hasn't been changed except spark plug wires, but they are only 50,000 miles and 5 years old, and the car runs butter smooth!

The best mileage in a year I was able to get was 25 mpg on a 90 mile trip averaging 50 miles an hour roughly. That was a miracle for this car.

I have read all the links. Its getting quite expensive to drive this car. My only thought is maybe the car isn't getting warmed up enough for the sensors to work, but most people say the LS400 temperature gauge at that one mark before halfway is normal.

I guess the car was cheap and I don't drive much, spending money on parts and diagnosing only adds to the $$$ on top of the fuel I spend. I got the car cheap and I don't drive very much but even an average of 400 miles a month it gets a bit awkward that a 50 mile trip cost $20 bucks in gas when it should cost $10! If its normal then I guess I can live with it , but I'm worried the extra fuel would hurt other components too.

96 LS400 207,000 miles on it. I keep track of the mileage using an app on my iphone, always fill up. Screenshot enclosed.

If anyone who has an ls400 with bad mileage like me who has an idea or someone who can tell me something better than any mechanic has told me it would be appreciated. I want to spent money to change the engine mounts and shocks because everything else has been changed but so tired of spending money on the car that gets horrible mileage like this. I am sincerely thankful to anyone who can brainstorm something for me!

Check your wheel alignment. Ask for a before and after printout. The car doesn't have to be pulling one way or the other to be out of alignment. I have fixed many fuel economy concerns by properly aligning all 4 wheels for customers with all makes of Lexus automobiles. What happens is, as the suspension components(ie; ball joints, strut bars, rear trailing arm bushings) wear, the wheels begin to point in the wrong direction as you're driving. When this occurs, it's like you're driving with the brakes applied because it requires more power to overcome the drag caused by the worn steering/suspension.

[curiousB]

Maybe check as well to see if your brakes are dragging. That would kill MPG too. Go for a 5-10 minute drive on a road where you won't need to brake. Then coast to a stop. Step outside and feel with the palm of your hand if the wheel and brake area is giving off heat (don't touch it as it can get to >>300 degrees). If the brake is dragging it should get good and hot down there.

[landar]

An inexpensive IR gun with LASER site might come in handy. You can look for proper or abnormal heating of components such as the brakes, engine, etc.

You can even tell if one cylinder might not be firing correctly by targeting the exhaust manifold at various points. Or check the cats to see if they are getting too hot. Lots of good uses.

[curiousB]

An inexpensive IR gun with LASER site might come in handy. You can look for proper or abnormal heating of components such as the brakes, engine, etc.

Thats what I use. A bargain at twice the price..... very handy tool.

[DEATH]

"Step outside and feel with the palm of your hand if the wheel and brake area is giving off heat.", "An inexpensive IR gun with LASER site might come in handy. You can look for proper or abnormal heating of components such as the brakes, engine, etc. You can even tell if one cylinder might not be firing correctly by targeting the exhaust manifold at various points. Or check the cats to see if they are getting too hot." ROFLMAO! You guys are funny! It's not that complicated. I do this !% every day! I'm sure in the world of back-yard mechanics, you guys are looked upon with high regard.

[wanderson]

You said you changed the coolant sensor, did you change the right one? There's one for the engine management and one for the dash gauge. here's a link with the procedure:

http://www.clublexus.com/forums/ls400/377171-diy-coolant-temperature-sensor-change-pics.html

If so, have you pulled the plugs to check the color? That should give you an indication if the fuel mixture is too rich.

[landar]

I'm sure in the world of back-yard mechanics, you guys are looked upon with high regard.

Yeah, D.A.Y. We know you are the automotive "expert". And BTW, this IS the world of back-yard mechanics. Lots of DIY'ers here just learning and sharing ideas. That's all. No need to be condescending.

[curiousB]

.....I'm sure in the world of back-yard mechanics, you guys are looked upon with high regard....

I just do this car thing for fun. Landar and I are professional engineers. We actually design things that guys like you fix.... So spare us your condescending rhetoric.

[SRK]

I do this !% every day!

Like I said before, what you do every day is sell lemonade at a stand out front of your mother's trailer. She's calling - off you go.

[yotoy82]

There is something very wrong with the car based on your MPG. I routinely get around 19MPG in stop and go short trip city driving and high twenties on highway trips.

I have been having problems with emissions and one catalytic converter so it has me looking at similar things. An invaluable tool has been an OBDII interface and my laptop computer. I can set up gauges to look at fuel trim, short term (ST) and long term (LT). Look at MAF sensor output. See what ECU thinks engine temperature is. See if emissions system has reverted to closed loop or stuck in open loop.

It’s under $100 if you already have a laptop. You need something like this or you will forever be throwing expensive parts at the problem and not having much success.

The first thing I would do with such a device is check to see what long term fuel trim is on each bank. If it is very high that indicates the system is sending too much fuel to the engine for some reason. It could be as simple as vacuum leak in the intake ducting so that air is bypassing the MAF. The O2 sensors detect a lean condition and start bumping up LT fuel trim to compensate. In the end you can get a rich condition, poor gas mileage and red hot catalytic converts (which may get permanently damaged).

I’d also want to look at engine temperature the ECU sees from the code reader. If the sensor is faulty the ECU may be using the wrong fuel map and sending a very rich mix which isn’t needed. I believe the temp sensor for the dash is different than what the ECU uses.

It could be a stuck open thermostat keeping the car from warming up. That will cause ECU to send rich mix…

It could be a faulty fuel pressure regulator causing pressure to be too high so too much fuel is shot into the ports. The ECU would attempt to correct this though and you’d see LT fuel trim in negative values as the O2 sensors would be saying the mix is too rich.

So it could be many things but you need more data so you can pinpoint the problem better.

All good ideas, unfortunately nothing I can do myself. I've changed the correct temp sensor though, not for the dash but the regular one for the ECU using a genuine lexus one. I had a friend put in a thermostat for me last winter, a lexus original one, and unless he put it in improperly, It might have to be double checked. But the car seems to heat up just fine, I mean the gauge in the dashboard shows it almost to the halfway point, but not really quite a bit less than half. I had a local mechanic check the trim levels for me, and they were off, then put another one off from another LS400 and said the trim levels looked to be fine now, so don't know, maybe I should let my toyota dealer to all the scanning for me, they said it would cost $70.

Maybe a stupid remark, but is the real fuel consumption in accordance with these figures?

The trip computer makes calculations, and there could very well be one bad input into the calculation offsetting everything.

If you calculate the average consumption each time after gas refills, do those figures comply with the trip computer values?

Jac

This is not from a computer. This is from my iPhone application. I just fill the car up all the way, and look at how many miles I had driven since the last fillup and calculate my mileage. I do it by hand too, the app on my phone just helps me keep track and everything. The 96 LS400 doesn't keep track of mpg for you like the newer ones.

Check your wheel alignment. Ask for a before and after printout. The car doesn't have to be pulling one way or the other to be out of alignment. I have fixed many fuel economy concerns by properly aligning all 4 wheels for customers with all makes of Lexus automobiles. What happens is, as the suspension components(ie; ball joints, strut bars, rear trailing arm bushings) wear, the wheels begin to point in the wrong direction as you're driving. When this occurs, it's like you're driving with the brakes applied because it requires more power to overcome the drag caused by the worn steering/suspension.

Just had a wheel alignment done at my toyota dealership. They said the rear was fine and the front was slightly off.

Maybe check as well to see if your brakes are dragging. That would kill MPG too. Go for a 5-10 minute drive on a road where you won't need to brake. Then coast to a stop. Step outside and feel with the palm of your hand if the wheel and brake area is giving off heat (don't touch it as it can get to >>300 degrees). If the brake is dragging it should get good and hot down there.

I thought about this too. I regularly take my foot off the pedal and my LS400 drives forever on its own, almost made me worry about the transmission. I can literally coast forever even if I take my foot off the pedal on a flat road going 40 miles per hour, it just keeps on going and going and going! lol. But will check it as you say just to be sure.

You said you changed the coolant sensor, did you change the right one? There's one for the engine management and one for the dash gauge. here's a link with the procedure:

http://www.clublexus.com/forums/ls400/377171-diy-coolant-temperature-sensor-change-pics.html

If so, have you pulled the plugs to check the color? That should give you an indication if the fuel mixture is too rich.

Had the plugs changed last month at the toyota dealer as part of regular maintenance after changing everything else. Didn't ask the dealer how they looked though. I did notice that the car sounded at least 25% more QUIET after changing the plugs though.

At this point I'm personally thinking there is some problem in the thermostat letting coolant come through, or a major vacume leak somewhere. I appreciate all the replies.

[curiousB]

I thought about this too. I regularly take my foot off the pedal and my LS400 drives forever on its own, almost made me worry about the transmission. I can literally coast forever even if I take my foot off the pedal on a flat road going 40 miles per hour, it just keeps on going and going and going! lol. But will check it as you say just to be sure.

You will coast some with the car in drive as the engine will transfer some torque to the wheels even with engine idling. That could extend your glide distance. You could pop it into neutral and see how far you glide.

I like the heat idea better though because if brakes are dragging they're going to get hot. Its simple physics. Energy dissipated as heat.

[yotoy82]

I thought about this too. I regularly take my foot off the pedal and my LS400 drives forever on its own, almost made me worry about the transmission. I can literally coast forever even if I take my foot off the pedal on a flat road going 40 miles per hour, it just keeps on going and going and going! lol. But will check it as you say just to be sure.

You will coast some with the car in drive as the engine will transfer some torque to the wheels even with engine idling. That could extend your glide distance. You could pop it into neutral and see how far you glide.

I like the heat idea better though because if brakes are dragging they're going to get hot. Its simple physics. Energy dissipated as heat.

If it was dragging though wouldn't the RPM's be higher for a given speed? But ya going to take some advice here and give it a check. Is there any other way to diagnose if the brakes are dragging besides looking for heat? I mean its summer anyway so I don't know how much heat extra I'll be able to tell a difference. Can I have the wheels taken off and check anything in particular?

[1990LS400]

IMO, 17 mpg in combined city/highway driving is pretty good for a 97 LS in summer driving. I drive with a light foot and get barely 18 mpg in combined city/highway driving in summer. The 98-00 LS400 like mine with the 5-speed tramsmission generally gets a bit better mileage than the 4-speed 95-97 LS. I'd be more interested in what your gas mileage is over five to ten tank fulls. Don't fill your tank until the fuel gauge is below 1/4 and then manually calculate. Try to fill you gas tank the same way each time ... i.e. fill slowly and stop with the pump automatically turns off to be consistent. I've tracked every gallon of fuel that has gone into my cars for 46 years and have found a high degree of variability with mileage being better in Spring and Fall and worse in Winter and Summer.

[landar]

IMO, 17 mpg in combined city/highway driving is pretty good for a 97 LS in summer driving. I drive with a light foot and get barely 18 mpg in combined city/highway driving in summer. The 98-00 LS400 like mine with the 5-speed tramsmission generally gets a bit better mileage than the 4-speed 95-97 LS. I'd be more interested in what your gas mileage is over five to ten tank fulls. Don't fill your tank until the fuel gauge is below 1/4 and then manually calculate. Try to fill you gas tank the same way each time ... i.e. fill slowly and stop with the pump automatically turns off to be consistent. I've tracked every gallon of fuel that has gone into my cars for 46 years and have found a high degree of variability with mileage being better in Spring and Fall and worse in Winter and Summer.

Jim...better check your alignment. :P

To the OP. I was wondering if your LS is reporting the correct mileage since you are using the Iphone app. Is a mile really a mile? You don't have "20's" on this thing, do you?

[DEATH]

"Just had a wheel alignment done at my toyota dealership. They said the rear was fine and the front was slightly off."

Did you get the 'Before and After' printout?! Did they check the suspension components?! I can guarantee that if you have never replaced the ball joints, strut bars(front camber adjusting bars), and rear trailing arm bushings, they are most certainly worn out by now! No tech likes having to make all the adjustments/repairs necessary to correct the alignment on an old LS400 since they don't get paid any extra for adjusting front and rear camber(bolts are usually stuck by now), but when it is done properly the results are amazing. For most of these guys(dealerships included), it's "Set the toe and let it go!". An i-phone app for fuel economy? Seriously? Nothing works better than pen and paper.

[yotoy82]

IMO, 17 mpg in combined city/highway driving is pretty good for a 97 LS in summer driving. I drive with a light foot and get barely 18 mpg in combined city/highway driving in summer. The 98-00 LS400 like mine with the 5-speed tramsmission generally gets a bit better mileage than the 4-speed 95-97 LS. I'd be more interested in what your gas mileage is over five to ten tank fulls. Don't fill your tank until the fuel gauge is below 1/4 and then manually calculate. Try to fill you gas tank the same way each time ... i.e. fill slowly and stop with the pump automatically turns off to be consistent. I've tracked every gallon of fuel that has gone into my cars for 46 years and have found a high degree of variability with mileage being better in Spring and Fall and worse in Winter and Summer.

Jim...better check your alignment. :P

To the OP. I was wondering if your LS is reporting the correct mileage since you are using the Iphone app. Is a mile really a mile? You don't have "20's" on this thing, do you?

Stock 16 inch chrome wheels. I manually calculate as well, the app just helps me keep a record. I get from 9-12 mpg in city and from 17-23 mpg highway on a good day. I'm pretty sure thats how much big SUV's get lol.

"Just had a wheel alignment done at my toyota dealership. They said the rear was fine and the front was slightly off."

Did you get the 'Before and After' printout?! Did they check the suspension components?! I can guarantee that if you have never replaced the ball joints, strut bars(front camber adjusting bars), and rear trailing arm bushings, they are most certainly worn out by now! No tech likes having to make all the adjustments/repairs necessary to correct the alignment on an old LS400 since they don't get paid any extra for adjusting front and rear camber(bolts are usually stuck by now), but when it is done properly the results are amazing. For most of these guys(dealerships included), it's "Set the toe and let it go!". An i-phone app for fuel economy? Seriously? Nothing works better than pen and paper.

I had the strut bars replaced, lower ball joints replaced, on the front end, both sides right before the alignment. Had the other components checked out, surprisingly everything else was in pretty good condition.

I'm NOT sure about rear trailing arm bushings, what are those? The car has 207,000 miles on it and I would say that 90% of that is highway so a lot of components on it have held together pretty good, but not sure about rear trailing arm bushings.

And the iphone app, it just helps keep a record for me rather than pen and paper and calculator lol.

And I took the car to 50 mph today, put it in neutral and let it coast, seemed to stay up at a high speed for quite a long time.

Video enclosed here of it.

[1990LS400]

Jim...better check your alignment. :P

Per Randy's lame comment, the condition of suspension components has virtually nothing to do with fuel economy - unless your car is bouncing all over the road.

I'm a scientist and I'm not seeing much good data here. Fuel economy can be accurately calulated only over a substantial number of miles and tank fulls. And 23 mpg on the highway is right at the EPA revised estimate.

Those with 98-up LS cars have the advantage of having an instant mileage readout which can show how horrible fuel mileage can be for these cars. I watch my fuel mileage range between about 5 mpg and 15 mpg when in stop and go traffic but that's normal.

I'd be more interested in what your gas mileage is over 1,000 miles. If it is in the 18-19 mpg range, that is normal.

[landar]

Per Randy's lame comment

Never, ever, kid a scientist. <_<

[yotoy82]

Jim...better check your alignment. :P

Per Randy's lame comment, the condition of suspension components has virtually nothing to do with fuel economy - unless your car is bouncing all over the road.

I'm a scientist and I'm not seeing much good data here. Fuel economy can be accurately calulated only over a substantial number of miles and tank fulls. And 23 mpg on the highway is right at the EPA revised estimate.

Those with 98-up LS cars have the advantage of having an instant mileage readout which can show how horrible fuel mileage can be for these cars. I watch my fuel mileage range between about 5 mpg and 15 mpg when in stop and go traffic but that's normal.

I'd be more interested in what your gas mileage is over 1,000 miles. If it is in the 18-19 mpg range, that is normal.

I'd say over a 1000 miles I'm averaging about 14 mpg in the city at best. All winter was getting about 9 mpg in the city, and 11 mpg in the city during summer right now. If I go on the highway I might get 20 mpg max. Only on one lucky day I was able to squeeze 25 out and that was a miracle, but other than that nope.

But my father has a 95 LS400 which is almost identical to my 96 LS400 and they get 19 in the city pretty constantly and about 26 on the highway no problem. They have never gotten 11 mpg like I have gotten, and I am consistently getting 11 mpg unless I get on the highway to make a trip, than my mixed mpg is about 16-17.

I would LOVE to get 16-17 mpg in the city! If I fill my gas tank up fully and just start from scratch on a 2 hour trip averaging 55 mph only THEN I'll get 25 mpg. Otherwise regular interstate driving I'm getting 20 mpg =( Something is wrong for sure because makes no sense another LS400 we have that is actually much less maintained gets twice the miles per gallon I get.

[yotoy82]

Here is my current printout, they didn't give me a before printout, but this is the current settings they gave me. I don't know how to decipher it so if you can tell me if it looks ok?

[landar]

Something is wrong for sure because makes no sense another LS400 we have that is actually much less maintained gets twice the miles per gallon I get.

You have another LS that gets 40 MPG highway!? Sweet.

Couple of considerations. If the A/C is on, that will chew up a couple of MPG right there. Another prime suspect, as CuriousB pointed out earlier, is the thermostat. I know it was recently replaced, but IMO, it still is a prime suspect. A cool(er) running engine is a rich running engine. I had a 98 LS400 and changed the thermostat, not because it was bad, just because I was doing some routine maintenance. To my surprise, the MPG jumped up about 2 in combined city/highway. Go figure.

[1990LS400]

I'd say over a 1000 miles I'm averaging about 14 mpg in the city at best. All winter was getting about 9 mpg in the city, and 11 mpg in the city during summer right now. If I go on the highway I might get 20 mpg max. Only on one lucky day I was able to squeeze 25 out and that was a miracle, but other than that nope.

But my father has a 95 LS400 which is almost identical to my 96 LS400 and they get 19 in the city pretty constantly and about 26 on the highway no problem. They have never gotten 11 mpg like I have gotten, and I am consistently getting 11 mpg unless I get on the highway to make a trip, than my mixed mpg is about 16-17.

I would LOVE to get 16-17 mpg in the city! If I fill my gas tank up fully and just start from scratch on a 2 hour trip averaging 55 mph only THEN I'll get 25 mpg. Otherwise regular interstate driving I'm getting 20 mpg =( Something is wrong for sure because makes no sense another LS400 we have that is actually much less maintained gets twice the miles per gallon I get.

You seem to still be talking in generalities rather that specifics. Keep detailed track of your fuel mileage over a number of tank fulls and then report back. Trying to determine what gas mileage you are getting in city driving vs. what you are getting in highway driving is very difficult.

If your father consistently gets 26 mpg in highway driving in his 95 4-speed transmission LS400, I'd call it a miracle. The very rare times I've exceeded 26 mph in my 2000 5-speed LS400 have been on long (e.g. 700+ mile football weekend) trips mostly on cruise control where I did viturally no low speed driving other than to pull off the interstate, fill up the tank, and continue on down the interstate at speeds within the speed limit, in low/no wind conditions, in a lightly loaded car carrying only two people and a couple of small suitcases. Even a little low speed stop and go driving or a head wind makes my overall average fuel mileage plummet.

Disregard your iPhone app, buy a small booklet and record every fuel purchase, date and and odometer reading for the next 1,000 miles. I've done this for about 700,000 miles of driving and can attest that doing this eliminates guessing.