2001 V6 F150 with MIL. Codes were P0171 and P0174.

I put a scan tool on it and found that at warm idle (with engine collant temperature at 197 - 199 F) the short term and long term fuel trims on both banks 1 and 2 were pegged (LTF 28.1, STF 32.8) with the idle running at about 945 rpm. After idling for 2 minutes, 45 seconds the engine shudders, the idle drops and then recovers to 823 rpm, but now the short and long term fuel trims are all exactly 0.0.

What would cause this switch in the short and long term fuel trims to occur after 2 minutes and 45 seconds idling? This is quite reproducible and can be reproduced by switching off the engine and almost immediately restarting it. The fuel trims start out pegged then drop to 0.0.

Thanks for any input on this problem.

 

Massive vacuum leak as usual.

 

This is a friend's truck and I don't have a manual for it so I don't know what's what, but I did find and repair a break in a very small diameter vacuum hose on the passenger side firewall just left of what looks like a one-way check valve with a capped-off T. It could hardly cause a "massive" vacuum leak, but I couldn't see any other leaks and vacuum seemed about normal at 20 psi.

During this operation I disturbed the positive alternator cable which arced against the alternator. Evidently the bolt had been sheared off and the cable was just held in place by the rubber boot. After I'd tapped out the bolt hole and reconnected everything (including the battery) the ECU's memory had been wiped. It restarted with LTFT 0 and STFT 32.8 (both banks). It seemed to run a bit better and no longer stumbled a few minutes after start up. The idle was also around 850 rpm. STFT backed off with increasing rpm, but LTFT remained 0, which seems a bit weird.

I didn't have time to mess with it any further.

What is the vacuum hose that I reconnected connected to? Why was there a 2 minute 45 second delay before the engine stumbled and began running better before I reconnected the vacuum hose? Surely ir doesn't take that long to switch from open to closed loop.

 

Mmm, well, fixing the small vacuum leak and resetting the trouble codes allowed the truck to pass smog. All's well that ends swell.

 

i've seen it take that long to go from open to closed loop. i've experienced a dirty MAF sensor give those codes. i've been told it could be a leak at the intake manifold gasket as well.
are you still messing with it? any new news on it?

 

My comment is the long term fuel trims normally don't move that fast. (long term).

An interesting action to say the least.
Check the vacuum line connected to the fuel pressure regulator.
If the diaphram has ruptured, raw gas may be flowing from the regulator into the intake manifold.
Also look at the fuel pressure while this is gong on.
The change in RPM is an attempt by the PCM to recover the idle from the rich or ? condition by operating the IAC. Reason is the idle dose not match the TPS and RPM and FI for the idle speed detected by the crank sensor. The system is very precise about this within +/- about 50 rpm.