Just Did All This Work & Now MPG in the Toilet!
 

I was averaging a pretty good 22mpg combo city/hwy, and completed the following:

replaced spark plugs (used correct Denso Iridiums)
remove & clean throttle body, MAF (12 light sprays with CRC MAF cleaner, no physical contact),
replaced temp sensor
replaced fuel filter

Now averaging 18.5mpg and that's with the warmer weather which usually gives me an avg of 24.5mpg.

My initial reaction was that I damaged the MAF but I did the procedure correctly,
also thought a small vacuum leak from throttle body but everything looks good.
Not sure how to test for this or other problem.

No CEL's.

I too would like to know what the most common repairs are on these cars to help fuel mileage.

The 1992 LS400 we just picked up has 228K miles on it and doesn't feel anywhere near as powerful as the 1990 we still have that has 407K miles on it. The 1992 isn't slow, but it certainly doesn't feel as powerful as the old one.

give the ECU some time to readjust.

Replace the O2 sensors, pull the negative battery terminal to reset the ecu and drive it normally for a week and report back.

If you replaced the plugs and temp sensor, chances are the O2 was also pretty old and needed to be replaced due to age as well as wear!

Re-check that the ignition wires are in proper sequence and seated on the plug. Disconnect the neg battery cable or pull the FI ECU fuse to reset it.

All i know is I killed a MAF with that CRC stuff and it ran like crap

I agree with the O2 sensors.

They tell the motor how much fuel to give it.

I would normally agree about the 02 sensors, but I was averaging at least 24mpg combined before the changes,
so I don't see how they would suddenly cause such a large drop.
I was worried about the MAF but the car starts, idles and runs beautifully. I did have the problem
of a high idle just after the throttle body removal & cleaning but it self adjusted within a few miles.
Did the ECU self-adjust the idle? - If so, should I still try the battery disconnect, or just drive the car for a week?
If the ignition wires are out of order, wouldn't that cause a more noticeable symptom? I will check that.
Thanks

Yeah you would think it would run rough if it was missing at all. More air = more fuel. Maybe she is just breathing better now.

I'm also not sure how you were averaging 24 mpg. I mean the car is rated at 24 highway. I consistently see 28 driving at 65 mph but I have never averaged 24 mpg.

only use the MAF cleaner on 95 & newer LS400, for earlier models compressed air & perhaps a chamois only!

when you replaced the engine coolant temp sensor (ECTS), did you use the real Toyota dealership part?

a common cause of bad fuel mileage is running too cold, does the temp gauge inside the car read what did before (should be around half way.)

a real Toyota thermostat would probably be high on my shopping list if I were you. (in addition to the ECTS)

I did not use an OEM coolant temp sensor...
But great news. Disconnecting the battery seemed to do the trick!
I just drove about 40 miles, 75% highway, 25% suburban roads with few stops,
70 degrees outside, and averaged a whopping 28.8mpg!!! Granted, I drove like
a grandma, trying to make every mile count, coasting way before a light to avoid braking,
and so on...but looks like at this rate my tank may average 25 to 25.5 mpg. I will report back,
but it looks as if I've gained 1.5 to 2 mpg from the recent maintenance.
My 24mpg avg. is usually from spring to fall, not winter, the cold seems to have an effect,
and again my driving habits are extremely conservative.

Also wanted to add to LSCowboy that the temp gauge is in the correct range. I did do a coolant flush
but saw no reason to replace the thermostat at this time, but will when I do my timing belt next.
Also my car is a '98LS so I guess that makes it safer to use the CRC MAF Cleaner.

glad that the ECU reset helped things - PS - I don't trust the little gauge calculator thing that much, I trust how many gallons i burned vs. how many miles, calculated the old fashioned way, successive topping of the tank!

and as far as parts, I always use the real OEM parts on critical fuel injection system components such as the temperature sensor, they are just too critical to save $20 on something like that, at least to me. - A lot of aftermarket LS stuff is just crap,