Hey guys. A friend of mine has a 98 ram 1500 with a 5.9 in it. Lately, he has been having a problem that he says happens at random. It will run smoothly, then all the sudden the idle will drop and the truck will run extremely rough, the exhaust will STINK to high hell, and the trucks sounds like its sucking air heavily, but it has no vacuum leak which makes me think its the IAC opening up in the throttle body, right? He replaced the IAC because he thought that was bad, and I noticed that the rough idle situation happened shortly after he started the truck after replacing the IAC. After some time in my chiltons manual i see the upsteam O2 controls fuel mixture, and I am leaning on the upstream o2 sensor being bad. The truck has no codes. Does it seem like im on the right track?

 

O2 sensor doesn't really start doing anything for two to three minutes after cold start.

I would have a look and see what the temp gauge is doing when it starts running bad. If the temp gauge drops to nothing, that would explain a lot. You can check the connector for the temp sensor, is just outboard of the fitting on the intake the heater hose goes to.

If the truck were a couple years older, I would suggest EGR... but, the 98 doesn't have EGR, so, one would think that isn't the problem.

 

I understand that the engine doesn't take a reading from the O2 sensor until it is in closed loop idle mode. However, he changed the IAC when the truck was warmed up. He started the truck, coolant temp was in the middle, and then within a minute of starting the rough idle stopped. So could I be looking at an upstream O2 sensor issue?

I originally thought it could be coolant temp sensor related, but checking it with a scan tool, it is reading correctly. O2 sensor or something else?

 

Did you reset the PCM after changing the IAC?

 

No, what is the procedure for resetting the PCM? I just talked to him, and he said it commonly happens when the truck is warmed up and he stops at a light, comes to a parking lot, etc. He said it is usually when the truck is warmed up.

 

Just disconnect the battery, turn on the headlights for a bit, turn off headlights, reconnect battery.

When you go to start the truck, turn the key to "On" (but, not start) count to ten, THEN start the truck. PCM will relearn the sensors. Don't know if it will fix your issue though.....

If it still misbehaves, try driving with the pre-cat O2 sensor disconnected, and see if the problem goes away. (will set a code though.)

 

Dude just seafoam, I think it'll fix it

 

I have been dealing with the same issue, i have replaced the ICA twice and still the same issue. tried to see if it was the o2 sensors and that didn't fix the issue. going to try and put a new throttle body on and see if that fixes the issue ill let you know if that fixes my issue and if it does this might be his issue.

 

The magnum engines are a Speed Density system not a MAF system so a vacuum leak would cause a high idle not a low rough idle. Is there any smoke coming out the tailpipe? How would you describe the smell? Sounds like an a/f mixture problem. You can try unplugging the ECT (temp sensor) and drive it around a little bit. If the problem goes away then the 02 sensor could be the culprit.

 

Any changes to air requirements (vacuum leaks or for that matter fixing vacuum leaks) changing sensors or just doing the plenum fix require you to reset the PCM (by doing what HeyYou said, simply disconnect the battery and turn the headlights on for a few minutes.

I fixed a vacuum leak under my dash, the plastic line broke and it was JUST enough to cause the truck to surge when the a/c cycled on and off...reset PCM, fixed. Very simple.