Low rpm stumbling?

so my DD is a bitchin 00 explorer that has been a reliable workhorse for the 12 years and 250k miles it has now but on my way home it decided to after driving perfectly fine for an hour, to stumble from between idle and 2000 rpm, it hops and jumps cutting in and out until i can get it above 2500rpm, then its fine. So basically im a lurching traffic hazard from every light, at least i can check that off my bucket list. Im tired and kinda going around in a loop so im hoping for some outside opinions, heres all the symptoms and things ive noticed:

No check engine light

Seems to do it at higher temperatures( it did this once last year but randomly stopped for no apparent reason)

Does it in park as well as in drive and it doesnt matter what gear its in, as long as its between 900-2000rpm

Seems to only do it when the engine is hot.

It has had a random impending lean fault randomly that doesnt trip the check engine light

If this was a 70s ford truck i would assume vapor lock. I did the old wrap the lines in aluminum foil to no effect last year. It randomly stopped doing it on a 102 degree day and didnt have a problem til now. It was hotter than normal but it was 82 today.......its not that hot. The problem manifested and went away when it was hot as berkeley in town driving last year.

Fuel filter maybe? Its got near 100k on this filter but i would assume no fuel pressure would trip the check engine light. Also high rpm is perfectly fine

Its a 4.0 OHV with the plastic intake and i know they are prone to leaking but the idle is fine.

I replaced the IAC, cleaned the MAF, and wrapped the lines with no effect. Then a few days after i did this it was fine for many months.

Thoughts? 

Check grounds. Sounds electrical. 

TPS come to mind.

Dusterbd13 said:

Check grounds. Sounds electrical. 

Hmm, i had to play with the negative battery terminal to get it to start just before this, but it drove fine for an hour afterwards at different rpm and my voltage meter didnt change so i didnt think of that. Ill check all the other grounds to as well , thanks

KyAllroad (Jeremy) said:

TPS come to mind.

Good point and i should have thought of that although i thought that would show as rough idle too?

In reply to Antihero :

Does this still have a distributor?  ( sorry I don’t know when Ford went distributor less) if so try simple first.   Replace the rotor/ cap then check the vacuum hose. 

frenchyd said:

In reply to Antihero :

Does this still have a distributor?  ( sorry I don’t know when Ford went distributor less) if so try simple first.   Replace the rotor/ cap then check the vacuum hose. 

No distributor but vaccum hoses are a possibility. Last time it did this i was driving with the torque app on and vaccum wasnt doing anything strange but its worth a look again

Next time you run with the torque app- check the fuel trims.  See if it's a lean stumble or a rich stumble.  

If neither, i'd start by finding a way to clean the injectors, since that's the cheapest.  Then change the ignition system.

The other possibility is EGR, but I'm not sure what the system looks like on that truck- never worked on one,

My Protege5 recently experienced this issue...

horrific stumble at idle, smoothed out as RPM’s climbed. 

My EGR valve was stuck open, acting like a giant vacuum leak. Pulled it apart, cleaned the carbon out, lubed it up, and now the car runs like a champ.

Update: started the car in the morning, no hesitation or stumbling but it finally popped a lean code on both banks. 

I'm thinking intake gasket keak just from the problems I've heard about the plastic intake having but I grabbed a few screen shots to see if anyone else sees anything.

Vaccum was holding steady at around 19 or so without rapid fluctuation, i apparently grabbed that at high idle rather than low. The red arrow is me revving the engine to about 5k to see if it would stumble

isnt a minus on fuel trim a bit odd? Torque doesnt seem to think so, i dont remember seeing that before.

So you have a leak.  And it's a partial, leak, as it seems to want to slightly go away.  Check all of the rubber.

Vacuum leaks are powered by vacuum. Vacuum goes away as you open the throttle. If you need more throttle to run higher speeds, the effects of a vacuum leak will decrease under those conditions. IF it were a manual, you could floor it at any rpm (thus 'turning off' the vacuum leak) and verify that the engine ran fine with no vacuum/no leak. But, with an auto you are limited in how much load you can put on the engine below 2000 rpm without triggering a downshift. 

If your poor running is being caused by a lean condition being caused by a vacuum leak somewhere after the MAF (i.e. air the computer doesn't know about and hasn't supplied fuel for) then you should be able to make the engine run better by increasing throttle. Your maxed out fuel trims would also shrink under high load/throttle. Go test it!

If you have an idle air control valve, take it out and clean it. I know you’ll be thinking “but that only controls the idle when cold”. Yes, it does. But if it’s malfunctioning, it’ll allow MORE air in and run lean and screw with the idle. 

I just dealt with this. 

In reply to Vigo :

The engine runs fine at idle, nothing weird at all and at above 2500. I would have thought it would idle poorly with a vaccum leak but I must be wrong.

Trackmouse said:

If you have an idle air control valve, take it out and clean it. I know you’ll be thinking “but that only controls the idle when cold”. Yes, it does. But if it’s malfunctioning, it’ll allow MORE air in and run lean and screw with the idle. 

I just dealt with this. 

I actually just replaced my IAC, mine wouldn't let the car run for more than a few seconds after it started. I tapped out with a hammer to get it to work short term to great effect actually lol

Throttle shaft leaking? May be the source of the vacuum leak.

Ill finally have a chance to look at it tomorrow, ill look around,spray with some wd40 and find out whats up.

Thanks for all the ideas so far

 I would have thought it would idle poorly with a vaccum leak but I must be wrong.

The main problem may not be a vacuum leak, but unless you also have some other problem (like low fuel pressure) then your max-positive fuel trims suggest a vacuum leak. Low fuel pressure is not as likely because the symptoms it creates would get worse with rpm, not better.  

Yeah, I'm thinking IAC valve too.  When they flake out they cause all sorts of stumbles.

So i started looking around and spraying some wd40 to see if the intake is leaking...nothing. Im not seeing a vaccum line broken so i went ver it with a shop light....

A berkeleying spark plug is loose on one side, apparently instead of over thinking it i should have just looked at it. Since both banks are lean im betting theres one on the otherside thats a little loose. Hows that for a stupid problem ?