sanstoetter

Horrible Gas Mileage

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I have a 1994 LS400, and the gas mileage has dropped 3mpg in the last month. I know starting up a car it runs in open loop which is very rich, and that is lengthened in the winter months, but 3mpg seems worse than any other vehicle I've ever owned. I've been letting the car warm up for ~5 minutes every morning so that its just running rich at idle instead of driving to rich. My cars CEL is not lit up, and I'm not familiar enough with my car yet to know if it'll throw a code w/out turning the light on. Originally I've been getting 21.7 mpg mixed driving, and in the past couple fill ups I've averaged 18.x. I've already checked my air filter, and tire pressures which are both fine. Any suggestions on what to check next, or what I can do to help with the cold weather gas mileage?

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With the recent cold weather that much of the country has experienced, including NC, it is not surprising that you have seen that kind of mpg reduction. Remember that during your repeated 5 minute warm-ups you are getting 0 mpg. You'd do better going with a 30 - 60 second warm-up - that engine doesn't need further warming before driving off.

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Well my thoughts are that the wasted gas would go up exponentially with throttle while the car was in open loop, but if not I ill be warming the car up a bit less in the mornings. It just seems odd that I can smell so much fuel out the exhaust on start up. I figured open loop was open loop, and that 30 degrees vs. 50 degrees wouldn't change how much fuel to throw in. I just assumed the time for the motor to warm up would be slightly prolonged. My other car lost <1 mpg in the past month with the cold, and I assumed the Lexus to do the same.

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Here in the UK with the price of gas the equivalent of $8.00 a gallon we always get bad gas mileage!!!!

Lexus do not recommend warming the engine before driving.

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You might need to replace the coolent Temp. Sensor. When I first got my LS a 20 mile trip would burn 1/4 tank of gas, and after I changed my coolent temperature sensor I got much more reasonable gas mileage. Like you I didn't have a CEL. Another possibility might be O2 sensors, but I have no experience with them.

BTW a 5 minute warm up at idle, even in open loop, will not affect gas milage significantly. A full size tractor trailer takes an hour to idle a single gallon of gas so assuming you are burning fuel at the same rate as a big rig in 5 minutes you burn 5/60 or .083 gallons. If you regualerly get 20mpg without idleing for 5 minutes then with idleing for 5 minutes on every gallon of gas use will only drop youir mpg to 18.34. No were near 3mpg and that is asuming you burn fuel at the same speed as a simi truck.

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I have a 1994 LS400, and the gas mileage has dropped 3mpg in the last month. I know starting up a car it runs in open loop which is very rich, and that is lengthened in the winter months, but 3mpg seems worse than any other vehicle I've ever owned. I've been letting the car warm up for ~5 minutes every morning so that its just running rich at idle instead of driving to rich. My cars CEL is not lit up, and I'm not familiar enough with my car yet to know if it'll throw a code w/out turning the light on. Originally I've been getting 21.7 mpg mixed driving, and in the past couple fill ups I've averaged 18.x. I've already checked my air filter, and tire pressures which are both fine. Any suggestions on what to check next, or what I can do to help with the cold weather gas mileage?

I feel you brother! I just got mine on the road as I have only owned it a few months. I bought my 93 LS with collision damage for dirt cheap and had a tech school put it on the laser Framer for Free! Amazing how they do that. I actually bought a parts LS, a 92 with Rear damage at the same time for dirt cheaper.. as I only needed the front body parts to fix my 93. Now that I have the 93 tagged, insured and drivable in this Cold A$$ weather (Damn How do you guy up north do it man....Geeeze!! Its Cold as PH*CK here in Greensboro NC) has breen no more than 35 degrees for almost a week! I don't know what the mileage should be on my LS but it sure does drink. I put 5 gallons in and drove 60 miles....there is about a 1 1/2 gallons left. That might be good..i don't know. I have found alot of parts from the 92 will not work on the 93. The headlight bulb connectors are different, the Alternator plug is different, the A/C condensor fan plugs are different.... Basically alot of the electrical connectors are different. Anyway, Stay clear of those Redlight Cameras over there in Raleigh. I used to work for that company. :D

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How is your engine operating temp? You could replace your thermostat and hope for some improvement. I changed mine and got

a slightly hotter engine temp and the MPG went up substantially. However, in the winter months, you can expect worse mileage than in

warmer months. The TStat is fairly cheap ($10 for stat, $5 for the O-ring gasket, another $15 for antifreeze) and easy to install that you might want to try it.

No guarantees, its a crapshoot.

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The tractor trailer comparison is flawed since they burn diesel fuel. Diesel engines at idle use very little fuel compared to gas engines. I agree that the coolant temp. sensor and/or O2 sensors may be the cause - have them checked out.

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The tractor trailer comparison is flawed since they burn diesel fuel. Diesel engines at idle use very little fuel compared to gas engines. I agree that the coolant temp. sensor and/or O2 sensors may be the cause - have them checked out.

Thats true, so I did a little more research and found that the 6.0L GM LS2 V8 burns around .75-.8 GPH at idle, and the average economy

4-cylinder burns between .2 and .35 GPH. Both figures are from cold starts. So I would guesstimate the trusty old 1uz to be some where around .5-.6 GPH.

My first guess would still be the coolent temp. sensor, and seeing as it doesn't have to completlely fail, just become slightly inaccurate to send the engine into open loop mode, if you dont have a CEL it would be a good place to start. The T-stat is also inexpensive and simple to replace, so those two would be first on my list. I believe the O2 sensors will through a code if they get bad enough, but the temp. sensor seems to be more common so I would start there. Best of luck

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How is your engine operating temp? You could replace your thermostat and hope for some improvement. I changed mine and got

a slightly hotter engine temp and the MPG went up substantially. However, in the winter months, you can expect worse mileage than in

warmer months. The TStat is fairly cheap ($10 for stat, $5 for the O-ring gasket, another $15 for antifreeze) and easy to install that you might want to try it.

No guarantees, its a crapshoot.

You know what?...Thats a good question because even though I have very good heat inside the car, the temp gauge is not quite 1/2 way up...in fact it points to where a four would be if it were a clock hand...or between 4 and 5. I figured it is because its so fricken cold here. We don't usually get this cold until late February. So I'll change the sensor and the thermostat and see what that does.

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Keep in mind that the cluster temp gauge and ECM information come from two different sensors. If your car is idling very high and getting bad gas mileage while the gauge stays in the cool range suspect the thermostat. If your gas mileage still sucks and it idles high with your gauge in the normal range suspect the ECT sensor. FWIW my '96 temp gauge doesn't go above the halfway mark. If you replace your thermostat just goto the Toyota parts department and stay away from chain autoparts stores. They're cheap at the dealer don't worry. They may be cheaper at an autoparts store but you need the OEM part. It has a jiggle valve which must be installed at the 12 o'clock position to get as much air out of the system as possible. Most all aftermarket thermostat's do not have the jiggle valve.

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Well my idle drops down like normal after driving a bit, and the temp. gauge points just a little up from the middle. I do notice that the air coming from the heater blows cold while I'm stopped, but gets warmer if I'm cruising around... its immediately cold though when I stop. Is the heater control valve operated by vacuum somehow? I'll go ahead and check the O2 sensor, but other than codes how do you check? I've never worried to much about testing since I can buy new ones at $15 for my other car.

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Well my idle drops down like normal after driving a bit, and the temp. gauge points just a little up from the middle. I do notice that the air coming from the heater blows cold while I'm stopped, but gets warmer if I'm cruising around... its immediately cold though when I stop. Is the heater control valve operated by vacuum somehow? I'll go ahead and check the O2 sensor, but other than codes how do you check? I've never worried to much about testing since I can buy new ones at $15 for my other car.

Cold heater air when idling (but hot otherwise) usually indicates an air pocket in the system. And perhaps why the pocket exists in the first place is that

the pressure in your cooling system is low. It might be as simple as replacing the reservoir cap which maintains the correct pressure. You may also be a

bit low on coolant.

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Hello sanstoetter,

9 years past this topic and here's someone "me!" with the same problem on the same model year. I haven't seen any comments on what eventually worked, appreciate any update here.

Thank you, 

Madi

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On 11/5/2019 at 10:14 PM, Madidoe said:

Hello sanstoetter,

9 years past this topic and here's someone "me!" with the same problem on the same model year. I haven't seen any comments on what eventually worked, appreciate any update here.

Thank you, 

Madi

There is no one solution for this problem. It could be a lot of things. For example, if the brake is dragging, MPG is also bad. 

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