Replacing the original fuel pump that had 188,000 miles on it resulted in the LTFT dropping from 19.5+ at idle to 14.1+. I still have an exhaust leak and a P0420 code that showed up. I'm guessing the crazy high LTFT I had caused the engine to dump more fuel than necessary into the engine and ruined the catalytic converter.
I am pulling the entire exhaust system and replacing the gaskets and hardware and will inspect the exhaust manifold for leaks or cracks. I will then put it back together and drive it for a few days. If the P0420 comes back I will replace the the downstream O2 sensor and/or the catalytic converter.
any gaskets down stream of the O2 sensors won't matter for LTFT.
Maybe you've got a bonked ECM? I had that once in a Chevy truck, it would NOT control fuel trims at all. Trims were always zero and it was running like crap, new ECM and it was fine. Something had gone wrong with the ROM inside it I guess.
I don't think so, but I'm not confident it's a bad ECM, it seems to be responding when you add propane to push the trims around. Maybe go for a drive with some propane added to the air intake via a tube and see if that does in fact push trims toward zero or even toward negative. That would confirm the PCM is working properly.
Ok lads, so I'm pretty sure my catalytic converter was clogged because I had a noise that sounded like one of those silly cold air intakes the tuners put on their car, hesitation, and the fact that I rain it with 35+ LTFT for 200 miles and the car has 190,000 miles on it.
So, I gutted the cat. I reinstalled the exhaust system and reset the computer. I allowed the car to come up to operating temperature and go into closed loop. The change in LTFT was almost instant. I took it for a 20 mile drive and let it idle, drove in normal stop and go traffic, and got on an interstate to go fast and to accelerate quickly.
The change in throttle response and MAF flow was noticeable. I have attached a screen shot of my trims and MAF readings after I gutted the cat.
I plan on replacing the cat after researching prices and finding a good brand. I don't mind paying for quality to get this thing fixed once and for all.
Can anyone explain how a plugged exhaust system would have such an effect on fuel trims?
That's weird but I guess if it's not passing exhaust it wouldn't be able to empty and refill the cylinders with the proper mix of air and fuel, exhaust could have been partially displacing the fresh charge and making it a lean mix, like if the EGR valve was stuck open.
I had that problem. It turned out to be a vacum hose running off the MAF, the end cracks causing an air leak. If you find cracks cut them off and see if that helps. If it does buy a new one from the dealer. good hunting!
Updated: still have persistent 19.5 LTFT. No vacuum leaks detected with smoke machine. New-remanufactured injectors didn’t help. Car runs fine. Sometimes shudders when stopped and in gear (auto). No codes.