Hey guys. Idle problem on a 99 dodge ram 1500 4x4, 5.2L.

No codes, no lights.
I have been having some intermittent issues of the truck not wanting to idle in the morning. It cranks and fires right up every time, but it seems that the truck doesn't always want to enter warm up mode where it should be idling high, and slowly dropping rpms as the engine warms up. When this happens I can keep my foot on the gas to keep the rpms high for a few minutes, then let off it, and the engine will drop immediately to a normal rpm range of 600-700. The engine doesn't like it that low when it is cold, but won't die either. Once the engine reaches operating temp, everything works fine, drives fine, idles fine.

Now, the issues has gotten worse. Not only will it not idle high on start up, but it actually slowly idles faster as the engine warms up, and won't go down. So now, at operating temp the truck is running at 1150 to 1200 rpms all the time. It's almost like the truck has the whole idle process backwards.

I've swapped out IAC motors, no change. I've pulled TB and cleaned everything, it's spotless. I have a new battery in the truck, have had the battery, starter and alternator all tested, and they are good.

The coolant temp and air intake sensors all check out fine.

Just because I could, I pulled the TPS when the engine was idling at 1100 rpms at operating temp last night, and the engine bogged down for a bit, but recovered and kept on idling like it was without the tps connected. Not sure if that tells me anything.

I also pulled the connector from the IAC last night while it was running, and there was no change. When I checked the voltage at the IAC connector, I had two powers and two grounds, that were constantly changing polarity, which if I understand correctly is how the PCM keep the IAC from moving the pintle around. That makes me think the PCM had the pintle where it wanted it.

Where should I go from here? What am I missing? Thanks.

 

What brand IAC did you get? Think I would look for a better brand, replace it again, reset the PCM, and see what happens. You have all the symptoms of a failed IAC.

 

I know right! This should be simple. Driving me nuts trying to figure it out.

I replaced my IAC about 1 year ago with one from the dealership...kinda pricey, but I know how these trucks don't always play well with aftermarket parts.

Last night, I found the old IAC I had replaced a year earlier. Cleaned it up and threw it in, exact same conditions existed. Is it possible to burn through an IAC in a year, and I'm swapping out bad parts?

Can a vacuum leak cause something like this? Engine idle is high, but it is smooth.

 

Vacuum leak will give you a high idle. Finding it is the fun part. Have you fixed the plenum??

Dealer IAC should last more than a year.... but, given the voltages you are seeing at the connector, seems the PCM is doing the right tricks...... Might try just resetting the PCM, then, turn the key to "on", count to ten, THEN start the truck, and let it re-learn the idle...... if it will...... won't hurt to try it.

 

I think I may start spraying some carb cleaner around tonight, seeing if I can find a leak. I'll try resetting the pcm again, won't hurt.

I fixed the plenum 2 years ago roughly. And when I had the TB off to clean the other night, I checked her out then, and no oil down in the manifold. So I feel confident the plenum hasn't gone to leaking again.

 

Went searching for a vacuum leak, and didn't find anything. Tried resetting the PCM for the 4th time, no luck there. So I took it to a mechanic.

Just got a call, and he's telling me my pcm has an internal short, and I need to replace it. I don't remember the code he gave me, but evidently my scanner wasn't able to read it, because I checked for codes a few times.

He was still looking to find a used unit, was going to call me back with a price.

Any suggestions where to get a computer for cheap? Preferably a fully programmed computer? 15 year old truck...kinda hard to dumb a whole lot of cash into her.

 

Go to another mechanic

 

Local parts store.

IAC would be a P0505.

 

I've switched out the IAC with no luck.

The only time the truck set a code for the p0505 was when I unplugged the connector from the iac. I erased the code at that time, and it never came back.

I changed out the heater core a few months back. I wonder if something didn't get damaged when I unbolted it from the firewall. Though if that was the case, I suspect I would have had problems right away.

 

What brand IAC, and did you reset the PCM after you changed it?