I have a problem with a stumble that I can't seem to find a solution to.

Whenever I take my truck out for a drive of over 20 mins or so, I develop a stumble off idle that is caused by dropping vacuum according to the MAP sensor. The truck will surge several times with the A/F ratio going nuts and then clear up and drive normal. This seems to happen around the 1600-1900 RPM range. It doesn't happen at cruising speed, only off idle. Furthermore, if I really grandpa the pedal I can avoid the issue, but if I get on it any more than about 10% throttle it'll sputter.

I have some videos. It's hard to hear the stumble, but you can see the A/F ratio go wild when it happens:

http://www.youtube.com/watch?v=bRUNzYwmIlA

This next one was done in 1 on the shifter to rule out any potential shifting issues when it was happening for diagnostic purposes:

http://www.youtube.com/watch?v=gMHe0...eature=related

The only thing I'm finding on the Snap On scanner is that the MAP sensor's vac reading plummets when this happens which causes the fuel ratio to go sky high.

I can't figure out what can cause a vac leak, but only after driving for a minimum of 20 minutes.

I've tried a new TPS, which I thought for sure would fix it, but no.

 

I've been quietly following this over there for a while.. There ain't no way I would suggest such a thing in the presence of such 'surgeons of the magnum', because it sounds so dang silly.. but..

PCV valve? Reason I suggest this, is because everything else I could possibly conjure has already been covered over there, and I'm all about simple stuff first.. anyway:

does it not stand to reason that the valve 'off idle' and 'after some time of operation' would or could be in a state of transitioning due to crankcase pressure, and all nice and lubed due to operation? If so, would it not behave just like a vacuum leak? and, when the motor produces enough vac to pull it closed, it would do just that..

It's a long shot, and it is likely something that just couldn't interfere like that.. I'm admittedly reaching, in other words..

 

I was thinkin' maybe Purge solenoid? That seems like a reasonable time for it to be triggered...... and it WOULD create a serious vacuum leak if something were wrong there. But, I would think that should set a code....

 

once it starts doing it, does it continue, or does it clear up after a little while. if it clears up, that might suggest the evap system.

one way to rule out the evap and other related vac items is to label, then remove and cap off all vac lines from the intake manifold. this causes a lot of stuff to stop working, but its a good test.

 

i stumble less if i sit down while drinking whiskey.

 

Is definitely not throwing codes, and it does stumble a few times, then clear up, and it sure won't do it at any other point then off a stop or really slow roll.

PCV is a cheap enough fix and I'm probably due anyway.

I'll cap off the Evap system port and see if she goes away. I can actually cap those things off one by one and test drive it.

 

PCV valve did not fix the problem. Took a look thru the entire vac system back to the charcoal canisters and found no obvious signs of leakage.

If the evap return line to the TB is disconnected, shouldn't the LDP throw a code for a vac leak? Or is it only pre-solenoid that it's looking for a leak? I have run the truck with the evap return line disconnected and never tripped a code.


I'm really grasping at straws here, but do any of you think this could be a valve seat leak that only happens when the valve gets hot? It seems very unlikely that it would only act up after 20min of driving and only in an off-idle situation, but I've never had a bad seat to know what that feels or runs like.

Another theory throw out was head gasket leak, but I don't have the overheating, loss of coolant, bubbles or oil in radiator, or water in oil that is usually associated, so I'd prefer not to tear it down chasing ghosts. A compression test may tell more.

 

Do a vacuum test, and see how the gauge reacts with cold engine vs. hot.

 

Got a spare MAP sensor you can toss on there to test?

 

I do as a matter of fact. I'll try it now.