This problem is really testing my patience. Truck runs a little rough when started cold, then smooths out when it goes into closed loop fuel status. Drives great - tons of power, very responsive.

No matter how long I drive, when I stop for a few minutes (to run in the store, whatever)... when I come back, the car starts horribly. Basically it can't figure out how to idle. Revs to about 1100rpm, then drops down to 200, chugs and barfs for a sec, (sometimes dies), then surges to 1100 again, does this for about a minute, then smooths out and runs fine again.

I've checked everything I know to check, and am going crazy. Someone please throw me a bone here.

Details:

• New MAF - voltages and air flow read normal
• Crank PS and Cam PS both test acceptable resistance
• throttle body is *pristine* inside
• Vacuum is strong, no leaks I can find.
• Have checked the entire intake for leaks
• fuel pressure seems fine, new fuel filter - haven't checked actual PSI (don't have a tool)
• OBD scantool shows O2 sensors alternating in acceptable range
• OBD has no stored codes, no freezeframe

The only thing that has me a suspicious is the short term fuel trims. Both read average -23% when in closed loop. However, long term trims consistently average only about -2%. Someone told me not to worry too much about the ST trims, but to make sure the LT are near zero? (That sounds like bad advice.) Regardless, could this be the problem and what's the solution? (New MAF, remember?) Negative SFFT means it's too rich and the ECM is trying to lean the mixture, right? If it were an injector, wouldn't the trim be really low on one bank only? Both banks show the same negative trim.

Man, I miss carburetors. Gimme a screwdriver and a can of spray and I'll have her purring like a kitten.

Help?

 

It would be helpful to know what year and engine this is, but some general things pop in mind.
You're correct about the fuel trims. EEC is trying to pull just about all the fuel it can out of this engine when it's running -23% (really, really lean). As someone whose done a ton of driveability on these, whenever I see either LFT or SHTFT go really far (more then 10% either way), I start looking for problems.
I would say that it's time to invest in a fuel pressure gauge. If EEC is trying to pull out this much fuel, the long fuel trims will move to this lean position in time (and driving). I've seen these fuel pressure regulators stick intermittanly, and they can be really hard to pinpoint.
This could also very easily explain the starting and stalling; However, if it was running really rich for a long time, I'd think the lft would move to a more lean position.
It could also have 2 (or more problems), like a worn throttle body and sticking IAC.
So, a little more info would be appreciated.