I have a 1998 Ram 1500 with the 5.9 and I'm at a loss. The truck starts right up and runs great, drives down the road great but as soon as it gets up to operating temperature and you stop at a red light or go to park, after a few seconds the RPMS drop to really low, it starts to run really rich and sounds like it has a mild cam in it. I've changed the plugs, wires, cap, rotor, IAC valve, coolant temp sensor, cleaned the throttle body completely, pulled off all the injectors and cleaned them along with new o rings. I've done most all of the things it should be but it hasn't solved the problem. I am thinking now it might be the MAP sensor and/or the TPS but I'm not sure. I got a vacuum guage to check the plenum because I seen a guy on YouTube do it but it's good. I've already replaced most of the inexpensive things so before I start buying more stuff that might not even be the solution I thought I'd ask on here if anyone has had the same problem. I don't have a check engine light and I had my cousin out it on his good Snap-On computer but he said all the sensors were reading fine. Any help would be greatly appreciated. Thank you in advance!

 

Pre-cat oxygen sensor for the win, man. Just to confirm: Unplug the electrical connector from the thing and then drive around to confirm that it's not developing that lumpy idle. If that's what happens, stop by the dealership on the way back home to grab an O2 sensor, or at the very least make sure that it's either Denso or NTK brand -- any other than dealer, Denso, or NTK is going to make the PCM unhappy.

 

Thanks man! I'll try that tomorrow and see what happens but I've changed the front O2 sensor not to long ago. The rear one is old so could that be the problem?

 

Rear doesn't have nearly the impact the front one does. Rear is mostly to monitor catalyst efficiency.

 

I just changed my IAC and TPS last night on my 97 with 5.9 cleaned the whole throttle body up .... how long ( miles) will it take for the PCM to learn the new parts?

thinking the parts were the originals....19 yrs in there and 205,000 km

short ride yesterday after finishing the work at stop lights the idle was doing very small surges like 25-50 rpm

 

Did you reset the PCM after changing the parts? (just disconnecting the battery for 10 to 15 minutes is adequate to the task.)

 

Battery was disconnected the whole time did the job so guess that's all it needs.

 

Yep.

It will learn the new sensors almost immediately, but, the PCM will once again relearn how the engine runs. that can take a bit.

 

Any updates? Was it the front O2 sensor? My '97 has a similar issue in that at operating temperature, it idles slow and stumbles occasionally. Only does it when it's in gear, and you're stopped (like at a red light). It idles fine in park, but just too slowly when the transmission is in gear...

 

No I haven't gotten around to messing with it but when I do I will let y'all know.