Long Term fuel trim question

[Rob megee]

I have a 2001 chevy suburban 5.3l with higher than expected fuel trim on both banks. This started out as po171 and po174 errors. Ltft both banks over 25. I suspected a vacuum leak so I made a smoke tester and found a leaking intake gasket. I replaced the gasket and ltft’s were much lower. I smoked the intake again and found a leak on the throttle body shaft, replaced the throttle body. I think the intake was leaking for a very long time. I replaced the cats because they would severely rattle when lightly struck on the pipe. After replacing the cats I started getting slow switching o2 cell errors. With 160000 miles I figured they were also bad. I have checked fuel pressure and believe it to be ok. I cleaned the maf then got the sensor replaced under warranty. I don’t think it was bad. The issue is I am not getting any codes and the car runs fine. I just don’t like the ltft numbers. Bank 1 is 7.8 at idle and increases with rpm sometime as high as 14. Bank2 is better at 6.5 and also increases with rpm slightly. I just don’t want to ruin the new cats with too rich of mixture. Oh yeah, the stft crosses zero to about+- 3 or 4. I would do an injector balance test but I don’t have a pulser tool. I have heard of unplugging each injector one at a time and recording the rpm change. I’m not sure how we’ll that works, I’m thinking dirty injectors.

Topic

[John Hugon]

I would monitor the pre convertor o2 sensors voltage to see if the engine is running efficient. If the O2 Sensor stays 0.500 mV (1/2 Volt) and above very long, the PCM will try to reduce the amount of Fuel short term trim to change the O2 Sensors values. On GM vehicles the Oxygen Sensors values (closed loop) will switch between Lean and Rich (0.100-.0.005 maybe 0.007 mV) condition several times every few seconds the whole time the engine is at operating temp idling.

Long term trim reacts slowly on a GM engine. Like I stated above… I always looked at the pre convertor o2 sensors voltage to see what’s going on. If the short fuel trim keeps adding fuel and the o2 sensor(s) are averaging 0.005 mV and switching quickly…something is wrong with the ECM logic. If your long term trim stays high, reset it with a scan tool or disconnect the battery for 20 seconds and start from scratch.

Most people only find the intake vacuum leak and not the throttle body shaft….good job on diagnosing. Most of those throttle body shaft’s leaked when new.

I never tested fuel injectors by disconnecting them like that; I’m not saying it can’t be done. I always used the injector pulse tester and recorded the pressure on the pressure gauge.

I don’t know if Eric has a way illustrated to clean injectors or not …maybe check that out.

[EricTheCarGuy]

LFT numbers are not as critical as SFT numbers. It also takes a while for them to come back into line after a repair such as what you mentioned. As stated above, if your O2 readings are in a happy place, I don’t think you’ll have to worry about your cats being ruined. If there was an issue that could damage the cats, you would likely see a code for it. That is the way the system is designed after all.

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