Stall After Fill Up

[Amarilu84]

2008 Dodge RAM 1500 Quad Cab V8 5.7L TRX4

I have been chasing this famous phantom for a while now. Upon filling gas tank (not topping off or over filling) I'll start truck, and within 5 minutes or up to 10 miles, when I idle at a stop sign or traffic light, engine shutters and sometimes stalls out unless I gently rest on the accelerator. It only happens once after a fill and will not hesitate or do anything wrong at any other time.

This has been my observation and diagnostic process:

First checked the purge valve to see if it was stuck open. It was replaced last year and appears to be opening and closing properly.

Checked charcoal canister in EVAP system. After a fill, and sitting overnight, I took off hose from canister and smelled no evident gas fumes from a saturated canister. I inspected for cracks or breaks, all looks good.

I looked for a PCV hose that goes to the intake behind the throttle body but could not find one, other than a hard plastic tube that goes from purge valve to (intake?) I wanted to inspect hoses for cracks or breaks that might be causing a vacuum leak. Only hose I found is from air filter box to oil cap. Looks good.

I replaced the MAP sensor also.

What's happening is the ECU is constantly correcting fuel ratio for an extremely lean condition, and it's dumping fuel into the intake especially at idle. It seems to me a vacuum Ieak would be overly effective at idle in producing these symptoms.

O2 sensors are working fine.

Short Term Fuel Trims are somewhat normal, they swing between -5 and 5 while regular driving but Long Term Fuel Trim is up at 18 or 20 at idle. When driving or holding RPM at 2000 it will go down to 5 - 8.

As a note, after the fill and first drive, that purge valve opens right up and stays pretty solid at 50% to 75% and that makes me feel like between that valve opening so hard and for so long coupled with the ECU thinking it needs more gas immediately is literally causing the engine to stall at idle, especially if there's a vacuum leak and unmetered air is entering the system triggering the "lean" condition the ECU sees.

I am stumped now. I see the PCV contraption behind/beside the throttle body, it's just a twist lock cap that I assume pops out and can be replaced if needed.

What else can I do to pinpoint the issue?

The 2 pics are random while driving. One is at idle. Note the Long Term Fuel Trim at idle, showing how the ECU is still over correcting a false "lean" condition and killing my MPG.

EDIT: It might be unrelated, but when I looked at the air filter there was a small spot of motor oil on the air filter itself, as though it was pulling oil (suctioning) out of the oil fill neck. Don't know why but that doesn't seem normal. It's not much, just a splotch, but worth mentioning I suppose.

 

[Sherman Bird]

Disconnect and block off the purge valve. Repeat the testing. If fuel trims return to normal, you have isolated that the engine corrections are not the problem, that the EVAP system needs attention.

 

[MrBonez]

Exact same thing here.
In my case it is my fault since it only does that if and when I'm trying to stuff it/get it to the next dollar from where it pops off out of sheer habit.

If you are in the habit of trying to ram it full or like I've been doing out of habit, thats your answer.
I caught on quickly to why and since I've corrected myself about it, the truck has been doing OK.

If I forget and try to round it up to the next dollar/half dollar on the pump thats when I may, or may not have a problem.

When it pops - That's it regardless of what the pump says since the tanks on these will fill right up to the tippy-top before the pump pops off anyway, at least mine does.

 

[DILLIGAF]

Evap issue, just delete that junk

 

[FaceDeAce]

I occasionally experience the same. Meaning a stall after fillup. Exactly as MrBonez said, it happens only when I have overflowed at fillup and backfilled into the evap tube on the filler neck. Don’t overfill and problem goes away.

Now onto your fuel trims issue. I can think of the following things that will cause exactly what you describe (lean at idle good enroute)
1. Intake manifold gasket leak. Sucking air in - Between the bottom of the manifold and the head. (Replace intake to heads manifold gaskets)
2. Cracked intake manifold (replace the intake manifold)
3. Vacuum hose leak (replace or repair hose/fittings)
… all of the above to check for and easily diagnose using a smoke machine.

4. EGR sluggish and/or sticking partially or fully open. Replace the EGR or block it off. The EGR can and often does go bad without throwing a CEL. No code does not mean the valve is not failing.

I sincerely doubt the evap system has anything to do with your symptoms - other than slugging and stalling after a fillup when you’ve overfilled as noted.

PS: the bit of oil at the air filter from the crossover hose coming from the oil filler neck is absolutely normal. See that on mine too. To me it occurs only on wot high rpm when the PCV is being overwhelmed or is a bit dirty.

Hope that helps!