Taking another stab at stalling issue

[Mike1261]

I've posted before about this issue and have improved it over the years by fixing things that contributed to the issue (leaky injectors for one).

But I haven't completely solved the problem. It's very specific. Wondering if anyone else has experienced this and knows what it is?

Car starts up just fine (even though it was parked for 11 months! Started right up!)

Drives just fine around town.

Get on the highway and go on a road trip... Couple hours in, pull over into a rest area, leave car idling, have a sandwich, let dog pee, get back in car, slowly leave the parking area, engine bucks and stalls.

Restart the engine and it's good to go no issues.

Rinse and repeat.

Something about idling after a prolonged highway drive causes it to stall under low rpm-low throttle.

But restarting the car solves the issue.

My running theory is MAF sensor misreading, and restarting the engine is reverting to using the ECU's tables instead of MAF reading. But I thought that only happened on cold engines, not just a restart.

I've also got a persistent O2 sensor "out of range, too high" engine code on the pre-cat O2 sensor. It's been replaced twice, so it's not the sensor. Wondering if that is confirming the MAF being bad.

But that's just a guess. Does anyone have any other theories to explore?

[nick325xit 5spd]

If you have an O2 code, it's usually something else. This is where it's really useful to have INPA/ISTA to look at live data and adaptations. You'll be able to look at MAF readings, fuel trims, and a lot of other values that will give you a much better sense of where to look.

It could be the MAF, it could be a vacuum leak (very likely), it could be something else. I'd highly recommend getting INPA up and running to dig in to this. Otherwise, smoke test the intake and / or start throwing parts at it.

Edit: I will note that since I DIY, I tend to preemptively do things like pull the intake manifold and replace all the hoses and gaskets. It's generally pretty cheap to do and will often eliminate a bunch of headaches on an older car.

[ahlem]

I'd do two tests first.
I'd check your fuel pressure at the fuel rail anticipating the possibility that it's the early stages of a fuel siphon o ring issue and review Andrew Wynn's write up on that.
I'd make a low cost smoke tester and do a smoke test. I found vacuum leaks in three locations that caused similar or identical symptoms to what you are experiencing. One was the Disa O ring, the second was a cracked little plastic fitting in the hose going to the dipstick tube and the third was a vacuum leak after reinstalling the rubber elbow on the Idle Control Valve improperly in 10 degree winter weather..
Additionally I'd check the CCV system for old age and potential clogs.

[Mike1261]

I have INPA so I'll check MAF readings.

Probably there's vacuum leaks. I'd be shocked if there weren't. I'll pick up a smoke machine off Amazon.

CCV is new.
Disa O Ring is new.
Rubber elbows are new.
Dipstick is new.
Fuel pressure is perfect, already dealt with all the fuel system issues lol.

If Vacuum can make such a big impact (and oddly cease to be an issue when restarting the engine) then it'll certainly be that. I'm almost scared to put smoke on the engine for all the rabbit holes it'll send me down LOL.

[ahlem]

Check your left and right fuel tank levels by opening the test in the instrument cluster.
Did you clean the Idle Control Valve recently?
Some electrical components, wires and sensors don't like getting hot but they are a challenge to locate. If you run various live data selections while you drive safely and not distracted, something may pop up when warm.
Materials expand and contract as well.
With the age of these vehicles, oxidation can affect relays, connections and wires.

[X5chemist]

Is the throttle body clean? Check the fuel pump amps at idle and higher RPMs. How old is your fuel filter?