3rd Gen Fuel Trim Data

2000 SR5 4Runner, I scanned the check engine light data (P0172) and found a Short Term Fuel Trim of -16.4% and a Long Term Fuel Trim of -32.8%. I know this is far from ideal and I’m wondering whether the numbers narrow down the cause for any of you 3rd gen owners?

 

ECU thinks you are running very RICH. Could be a few things.

Fuel regulator may be faulty. Check for an overly clogged air filter. Upstream O2 sensor could be bad. A bad injector.

 

Any experience as to what might be the most likely cause?

 

Not really. Just start with the easy stuff. New air filter. Clean the MAF, throttle body and IAC. Make sure you do it correctly. MAF is very sensitive.

Next I would look at the O2 sensor. Make sure you get TOYOTA ones and check with your dealer if you need an O2 or A/F sensor. Cali cars were different than federal and there was a switchover in 2000 from O2 to A/F for the 2001 model year. Give the dealer your vin and they can tell you the right part.

You can get a fuel pressure test. Not sure on the numbers you need but I am sure they are found online.

Injectors probably last as they are a bit of a pain since you have to take the intake off. I'd probably run some cleaner through the system. Not sure if 1 injector would cause such a rich condition without a misfire.

One other thing...you may want to do a compression and leak down test. If the internals are simply worn, and the valves are not working right, you can get a rich condition.

 

It is a California car so it’s A/F, I already replaced the downstream one. Guess I’ll just have to go by process of elimination.