My old man has an '02 1500 Ram 4x4 with a 5.9. I drive it quite a bit because I usualy take him to his VA appointments and I've noticed this "stumbling" it does at random times mostly while cruising at highway speeds (55-60mph) but sometimes even idling or very low speeds. It can go months between times it does it, other times it may do it twice in a day. This has never set any codes over the last year and a half that it's been doing this. I've used a scanner on it multiple times and now I'm just following a trail of breadcrumbs.

If you can imagine a lamp that's turned on in your livingroom, if you were to reach down and pull the plug from the wall outlet and quickly plug it back in so that the light goes off and on, that's what the engine is doing. It happens so fast that there's no way to know when it's coming and it's back to normal before what's just happened can register in your brain.

I tested the TPS thinking there may be some issues with it. It showed a voltage that was within range with the throttle sitting closed. However when monitoring the voltage changes during full travel from idle position to full throttle, it showed just over a volt below what it should have been. So I figured it was getting weak, going out, something's going on with it and just replaced it.

The truck is still having the same issue. He had another appoinment so we headed up there and while sitting at a light, the engine "stumbled" again. But I noticed it was right as the clutch on the A/C compressor engaged. Right away I remembered every time that the truck has done this, the A/C had been on which would also explain the randomness of it, it does it when the compressor clutch engages and sometimes it doesn't. I drove it the rest of the trip with the A/C off and never had an issue. My old man had been driving it since then with the A/C off and it's never done it to him either.

So I learned that the IAC will be used to slightly idle up the engine to compensate for the extra drag on the engine and it may need cleaning. I pulled the IAC and the plunger was caked with carbon. I cleaned it off until I could see clean (but stained from the carbon) metal and also cleaned out the IAC port in the TB which was packed with carbon too. I checked to see how far the punger was extended on the IAC and it was less than an inch, I had been told if it was over an inch the IAC needed to be replaced.

I put it all back together and drove the truck for a bit with the A/C on, never had a single "stumble". When I got back to the house, with the engine and everything warmed up, I made the compressor clutch kick in and out by using the dash controls, never had a single "stumble".

Today, my old man drove it with the A/C on and it "stumbled" on him. Should I just go ahead and put a new IAC in it, maybe pull the TB and clean it thoroughly while I am at it or should I be looking at something else? I can foot the bill for the first couple of parts and am really trying to keep him from having to start dumping money into it chasing down an issue.

 

Would I be wrong in guessing the TB has never been cleaned since new? I'd definitely do that. How much less than an inch are we talking about on the plunger? Is it an expensive part?

 

I think the TB was cleaned about 4 years ago when we put a heavier intake plenum plate on the engine. I remember that we did some cleaning on most of what we took off. I think I'm just gonna pull it and clean it anyways. The plunger measured a hair over 3/4".

 

Just posting an update to this and hoping someone might have some ideas.

I took the TB off and cleaned it til it was sparkling again. I also replaced the IAC while I was at it.

The truck still has the stumbling issue, but now it will also randomly keep a high idle after driving and putting it in park. It will drop back down to normal idle once you tap the gas pedal. I thought alright, the IAC I bought wasn't doing its job so I took it back and switched it out for another one. I'm still getting the same issues except they are happening more often now.