1996 Ls400 Gas Mileage Issues, Slow To Warm Up

[R White]

I managed to boost the mileage on my 96 LS400 (103,000 miles) from between 18-19 mpg to 24.5 mpg (mostly highway miles) by changing oil to Amsoil 0-30w and running Amsoil's PI injector cleaner through the engine (you put the cleaner into a full tank every 4000 miles). I'm NOT an Amsoil distributor and I am a leadfoot driver so I never get the best mileage, but this improvement totally sold me on Amsoil products.

[Ondro1977]

Is it:

1) bad temperature sensor fooling ECU?----->>>>>>Maybe, pretty inexpensive to fix (there are two temp sensors replace the ecu one...)

2)rusted or stuck up cooling system( needed flush)? --------->>>> doubtful this would lead to overheating not slow heating

3)stuck up open thermostat? --------->>>>> maybe and easy and inexpensive to replace. I'd start with this

4)cooling fan working constantly (faulty on/off switch??) might be a problem but not the reason you are slow to warm up...

I managed to rule out thermostat... gas mileage did not improve and after start I smell gasoline from exhaust that mostly disappear after vehicle warm up... still feels like it is running on waaay too rich fuel. Fuel economy in the city is still not better than 14-15MPG and i am very lightfooted driver.

[yotoy82]

Ok, I'm just curious.. My 1996 LS400 takes literally 14 minutes from a completely cold morning (30F) from starting the car to the completely full normal operating temperature. It takes 6 minutes for the needle to even move. I even had the thermostat changed and its still taking that long. But my fathers 1995 LS400 gets to full operating temperature in 6-8 minutes.

How long does everyone elses LS400 take to warm up from completely cold engine to normal operating temperature?

[jaswood]

On my '95 LS there is a servomotor that control the blend door and via a cable through the firewall a coolant flow valve to the heater core. This valve will stick due to coolant residue and then not only will the valve not always open the position of the blend door might affected.

You can also get better heating response if you TURN OFF the A/C.

Low MPG is more likely than otherwise the result of a blocked exhaust flow in the catalytic converter. Over time and use the honeycomb structure of the catalyst crumbles and will oftentimes, even interittently, block the exhaust ourflow from the catalyst.

Generally no CEL to indicate this fault.

[jaswood]

Just a quick update for everyone, I took my LS400 to a local mechanic, has good reviews but a little expensive. He checked the fuel trim levels and said it had a biased lean. After looking around, he said the mass air flow sensor didn't seem right or something like that. There were no codes or anything like that, but he said its possible it was out of calibration etc. We had another LS400 of the exact model that was getting great mileage. Changed the MAF sensor w/the other car and checked the fuel trim levels, and viola, no more biased lean. I didn't want to buy a new one or a remanufactured one, I mean how often do these go bad on a 96 LS400, bought one on eBay for $100, and so far it seems to have helped, fuel trim levels are great. Now I've driven a little bit in the city and the interstate, and the gas gauge is at 3/4 still after driving 80 miles, where usually 80 miles was close to half empty. Let's hope my gas mileage problem is solved!

When you switched in the "other" sensor was the battery disconnected long enough that the ECU had to "relearn" all of the sensor operating parameters, ALL sensors.

And it will take up to 200 miles of "mixed" driving to relearn.

At idle or simple cruise it is the upstream oxygen sensors that are used to control, SOLELY used, the A/F mixture. Only under "power" production mode is the A/F mixture control switched over to the MAF/IAT module.

[yotoy82]

Just a quick update for everyone, I took my LS400 to a local mechanic, has good reviews but a little expensive. He checked the fuel trim levels and said it had a biased lean. After looking around, he said the mass air flow sensor didn't seem right or something like that. There were no codes or anything like that, but he said its possible it was out of calibration etc. We had another LS400 of the exact model that was getting great mileage. Changed the MAF sensor w/the other car and checked the fuel trim levels, and viola, no more biased lean. I didn't want to buy a new one or a remanufactured one, I mean how often do these go bad on a 96 LS400, bought one on eBay for $100, and so far it seems to have helped, fuel trim levels are great. Now I've driven a little bit in the city and the interstate, and the gas gauge is at 3/4 still after driving 80 miles, where usually 80 miles was close to half empty. Let's hope my gas mileage problem is solved!

When you switched in the "other" sensor was the battery disconnected long enough that the ECU had to "relearn" all of the sensor operating parameters, ALL sensors.

And it will take up to 200 miles of "mixed" driving to relearn.

At idle or simple cruise it is the upstream oxygen sensors that are used to control, SOLELY used, the A/F mixture. Only under "power" production mode is the A/F mixture control switched over to the MAF/IAT module.

AC is OFF always in the winter, I don't have a problem with heating. About 10-12 minutes the heat is coming out nicely and the car warms up just fine on the inside, plenty of hot air blows. I've disconnected the battery for hours over night.

I'm not as concerned so much but it doesn't seem right. The car doesn't drive or sound as good until its COMPLETELY warm, and that little 1/4 of an inch higher that it eventually gets to close to little less than halfway is when the car seems to settle down, get smooth, quiet, fast throttle response, and that "lexus feel and power".

But soon as I turn the car off again, even if its 40 degrees, in like 20 minutes the car is back to COMPLETELY cold and I have to start the whole slow process of warming the car up.

This car just does NOT want to get warm! :-( People have overheating problems, I have the opposite lol.