I am NOT even a shade-tree mechanic, and am often proud of myself for changing even a simple sensor. However, I have neighbors that do most of my work and I do research. I have been surprised to find some things they were not even aware of. However, I have an ongoing issue that multiple neighbor's and friends, and three different garages, have not managed to figure out. I hope this isn't too long, but I think the history has some bearing on the current issue.

Truck is a 1998, 1500, 4x4, 5.2L

About two years ago it started stalling if letting up off the gas quickly, usually at a stop. Is slowly got worse until revving it while in park would also kill it, or letting off quickly even at higher speeds as well. The IAC was removed and cleaned, and the throttle body "sort of" cleaned. The port was cleaned the without being able to see in, and other than spraying cleaner into the TB only the top was cleaned. (Cap, rotor, plugs, and pcv changed that day as well) This fixed the stalling issue..

However, it then started to stumble and backfire through the throttle body when under load and moderate acceleration. Slow or heavy acceleration seemed unaffected, and it was also random and sporadic. It slowly just went away and seemed fine.

THEN the stalling returned, along with RPM hunting both when at idle and driving under load. This time a neighbor swapped the coolant temp sensor (since it seemed worse when warmed up and a cheap part). He also pulled the TB so while he was doing that I scrubbed the TB entirely clean and replaced both the IAC and the Throttle position sensor. The MAP was removed while cleaning, but was not replaced. The K&N filter a shop supposedly cleaned only a few weeks ago, as well as a few months ago, was also filthy and cleaned (that shop is off my list to ever return to). The truck started and ran great!! No RPM hunting, and when letting off the gas it dropped RPM's in increments. Never overshot or stalled.

But, the chugging, lack of power, and backfire's through the TB returned And worse than the previous time. It is almost consistent now and seems to be at all temp ranges. There have never been any codes found for either issue throughout all of this.

Both neighbors and a coworker (who once owned his own shop but lives 1,500 miles away so only verbal help) all agree it seems to be a lean condition.

My question is, if running lean, should the O2 sensor not be reporting a lean condition? Also, and the reason I gave the history, it does this when the TB and Air filter cleaned, so could it be too much air? Or, it is always running lean, but when air was also starved it was at least close to being balanced, other than performance and mileage stunk?

It is going into a new shop tomorrow, but I am still wanting to learn and understand. If nothing else to protect me from being given incorrect info or just another guess list from the shop that my neighbors could just keep doing.

Any ideas would be great!!! I'm tired of beating my head and even others who look at it seem to find nothing!! Even with no code, maybe now that it is so easily reproduced I'll have better luck.

Thanks!!!!

 

Have you checked the fuel pressure? Might have a fuel pump that's on it's way out and once it's warmed up it starts to lose pressure.

 

No, I have not. For some reason not even my more auto repair friendly neighbors seem to have a gauge I am also not sure if the shops it has been brought to checked, as fuel pressure wasn't mentioned until talking to that distant co-worker of mine a few days ago. It is for sure one thing I want verified on this coming shop visit, once I limp the truck the 10 miles to get there.

However, I am still confused. If it is a failing fuel pump why would it start so easy, idle so easy, accelerate slow so well, as well as take off with plenty of power when I floor it? It is only at moderate ie: normal acceleration where it really seems to act up.

Not that I want to have to replace a fuel pump on it, but it is good to see even the first input matching my co-worker who is somehow usually right even though he can't even touch the vehicle. He couldn't explain the fuel pump question though, other than saying he has seen it only be an issue in moderate acceleration before.

Oh, and the hesitation and backfire is at any temp range. It was the stalling that is now fixed that seemed temp dependant, at least at times.

 

Hopefully one of the more knowledgeable members shows up. The fuel pressure was just one thing that struck me when I first read your post. I hope they find something!

Good Luck!

 

Sounds like maybe a bad front O2 sensor. It is giving false readings causing your truck to stumble and backfire when in closed loop (O2 is used be computer). Closed loop is only activated when the engine warms up (can't remember trigger temp). The truck is in open loop at start up, when cold, and at full throttle.

The backfiring makes it sound like a timing issue. Ignition timing is entirely controlled by the computer, so possible bad computer, crank sensor, or cam sensor. The cam and crank sensors should either work or not work but I have seen otherwise on this site.

 

+1 on the fuel pump. My ram had those very same symptoms when my pump was dying. Backfiring stalling ect. Also i did not have a Check engine light when my pump died. So it was hard to diagnose. I hope this helps...

 

Thanks again to all so far!!!!!!

Again, I don't do most of my own work. I can't seem to turn a wrench without harming myself, lol. But, I have an Avionics background and love to learn and do enjoying doing what I can. I just lose it when I can't get a bolt to move.

It is starting to look like the fuel pump is the most likely.

Secondary possibilities are the forward O2 sensor, MAP sensor, or possibly cam or crank sensor so far?

Otherwise, just to vent, I wish it would just break and throw a code!!!!!!

 

I would vote down the MAP sensor. If that was acting up, you'd probably notice it right away, not after it warms up. I unplugged mine once just to see what would happen (years ago), and the truck wouldn't even idle or had a really hard time idling.

My wife's Durango had similar problems, and it turned out to be the front O2 sensor.

 

As a corollary to the front O2 sensor.... have you checked your plenum? Are you using much oil?

 

id say, goto autozone get a fuel pressure gauge, and hook it up to the fuel rail and see what ur gettin when the truck is wearm and let us know. if its good then id go with the front o2.