If not plenum gasket, then what?

1999 1500 4x4 318/5.2 manual

When I brought her home she had a bad downstream o2 sensor and a gas gauge that jumps to full on startup buy slowly comes back down to show E. It would start perfectly on 2 or 3 turnovers if it wasn't cold out, maybe 5 on cold mornings. Replaced o2 sensor and MAP sensor (because I'm an idiot, long story but it didn't need to be replaced), my CEL went away.

Now what I'm noticing is when I let her idle for more than about 7-8 minutes, she will drop down in rpms (I don't know an exact number because all I have is an idiot light, no tach), sputter, and then I hear what SOUNDS LIKE a vacuum leak, and then it revs itself back up to a high idle for about 30 seconds and the suctioning/vacuum leak like sound goes away, and then repeats this whole process. I haven't gotten her to die yet. When I get driving, if I stay in my power band (again, no tach, so I don't know where that is) or cruising in 3rd 4th or 5th, she purrs and runs fine. But if I let her get down in revs in 1st or sometimes 2nd, it is a little rough at times, it almost feels like the trans/clutch trying to engage (but I'm pretty sure it isn't). Where this is most noticeable is on dirt roads where I'm in 2nd gear trying to stay around 5-10 mph.

This morning, after researching the plenum gasket problem even more, I pulled off the airbox/cleaner and took a peak down into the manifold. Yep, oil, but definitely not Lake Exxon. I then reattached my airbox/cleaner and fired her up to listen to the engine while with the hood open as I hadn't done that yet, and nothing. Runs fine. A little quick in the normal fluctuation of idle speed, but it wasn't drastic and it certainly didn't sound like it was about to take a ...

The only thing that changed was I didn't clap down the box/cleaner assembly as tight as it was before. Nothing else. I'm letting her cool off for a while then I am going to start her up again and see what happens. Any thoughts as to what, if not the plenum gasket, could be causing this? I really hope it isn't, because I do want to upgrade my intake in the spring and don't want to drop the money and my time into replacing the gasket/plate right now.

Any help and insight is greatly appreciated! And yes I read DDs sticky... great info but I'm still wondering...

Sounds almost like a failing IAC valve.

Have you pulled the throttle body to inspect it yet?

I have not, what should I be looking for?

Pull the throttle body of and start inspecting everything for excess carbon build up is a start. You will need a new TB base gasket tho.

Two things I would check are battery and TPS. Your belly pan gasket is a separate issue that needs attention.

WarrenRam the battery is pretty new, what would indicate a problem wit the TPS?

DD Just pulled the airbox off intending to pull the TB, and holy dude man the bottom half (under the throttle doors, if thats the right term?) is pretty gunky. Guessing that isnt a good thing? Ha.

Surging, odd idle. Surging while using cruise control is also an indicator. I mentioned battery because weak batteries have always caused strange problems. Experienced both myself, actually, at different times. Had a weird idle issue, and the truck (98 Durango 5.2) would die when idling to a stop or turning sometimes, turned out to be the battery. Also had the weird idling issue and surging while driving and also using cruise control, turned out to be the TPS. I also had the belly pan issue and that didn't cause any of your described symptoms.

WarrenRam said:

Surging, odd idle. Surging while using cruise control is also an indicator. I mentioned battery because weak batteries have always caused strange problems. Experienced both myself, actually, at different times. Had a weird idle issue, and the truck (98 Durango 5.2) would die when idling to a stop or turning sometimes, turned out to be the battery. Also had the weird idling issue and surging while driving and also using cruise control, turned out to be the TPS. I also had the belly pan issue and that didn't cause any of your described symptoms.

Click to expand...

Ahh, I should have clarified; not using cruise control, but at cruising speed, like 55 in 5th, but CC off. Sorry. But now I'm intrigued by the TPS.

I also didn't mean only in cruise control. Just curious though, does it surge while using cruise, also? Still worth it to look into. Could try a good cleaning, but a replacement is quick and cheap.

I have yet to use CC, but I might have to go try it now since its all put back together.
I should have clarified in my original post; it doesn't surge when just cruising at low Rs, it sometimes stutters when accelerating out of that range. I drove it with the back window open for a bit yesterday and I can hear it in the exhaust more than I can feel it.

Thanks both you dudes for being so helpful, I really appreciate it!

IAC will whistle at ya when it's ready to be changed- don't clean it, just swap it. That will affect your surging in low r's and idle issue.

Make sure you got spark to all cylinders, check out your intake manifold when you pull the TB off, and buy a new TB gasket!

The IAC will whistle?

Because Last Thursday I had surging during idle, but only experienced it that run. Last weekend I cleaned the IAC and whole TB, no issues till this morning when it was surging at idle again. Wasn't surging during the ride home. I haven't had any strange whistling besides the whistling from what I've always thought was the CAI. I guess I'll just replace it. I was told that if I was experiencing stalling or if it felt like it was about to stall then that was the TPS failing as well. I've had none of that. Only happens at idle, otherwise drives completely normal. Yeah ADudeOnSkis, my TB was nasty too.

I would literally stall out at random times as well, but would hear a distinct whistle at various times, coming directly from the IAC- had the TB bare and saw it.

Well, I opened her up again, and then remembered that the plug wires are finicky, so I poked around and followed the wires, and discovered some *********** had zip tied the passenger side spark plug wires along with something else all in a bunch. So I decided to start it up and listen again. Sure enough, I can hear a consistent ticking. And then the rough idle started again. I figured I'd record it ruining for a while, here's a clip. My phones mic is fouled up, so the audio is pretty horrible. Tomorrow I'll use a real mic and see if I can get it better... Dodge ram 5.2l rough idle: Dodge ram 5.2l rough idle - YouTube

It's idling down then you can hear the IAC trying to compensate. Interesting....

Yeah, weird. To me and my non mechanic brain that says it's not getting enough of something, either fuel or air? I need to deal with this wire routing issue and see if that helps anything. Zip ties, pffsht, please.
I'm starting to wonder if it's a fuel delivery issue. My gauge being inoperable is what makes me think that. But then it probably would show symptoms at all rpms, wouldn't it?
I'm planning on doing the plenum gasket within the next few days, as well as cleaning every inch of the intake, hopefully that will fix this or at least help.

Fuel delivery usually shows up while driving or at high rpm, but the way it idles down it almost sounds like it's choking.

I've had my fuel pump run 7lbs of pressure and still run my truck... and that its not showing your fuel level makes me point at the pump. Can you rent a manifold and check pressure at the rail?

Plan on it in the am. I'll report back once that's done.

This truck still have a cat catalytic converter in play?