1998 Ram 2500 5.9L. Currently, it starts and runs beautifully when cold, but 5-10 minutes later it barely idles and sounds like its about to come apart. I just did the plenum job but it was doing the same before so that didn't help.

Just completed plenum job.
Clean throttle body.
New O2 sensor.
New TPS
New IAC
Original MAP
New thermostat
New coolant temp sensor
New Intake air sensor
New Cam sensor
New rotor and cap
Original Crank sensor

I'm at a loss and would appreciate any thoughts. The fact that it runs so nicely for 5-10 minutes and then goes to hell when warm suggests some problem related to the switch to closed loop. Ive tried running it with the new O2 unplugged. No help.

Some of the OBD readings while idling nicely are:

IAT 31
TPS 12.5
MAP 40kPa
RPM 670
Spark Adv 4 degrees
SHRTFTI -9.4

The spark advance seems low for idle? The only codes I get are a CAM/CRANK sensor code but those only come up when it starts sputtering and so I think they are an effect, not a cause. I don't think a bad crank sensor would only come into play 5 or 10 minutes after start but its about all I haven't replaced besides the MAP sensor.

Appreciate any ideas

 

Heat can do odd things to sensors. If it starts acting up as the sensor warms, that's a pretty significant indicator.....

IAT is 31?? Is that degrees C?

Plug a vacuum gauge into it, if you have one.... See what it looks like at idle. Should be around 20" of vacuum.

 

Yes, IAT is in degrees C. If that is intake air temperature, that is a pretty odd value given that it's a cold start with outside air temp around 50F. Not sure if that comes into play after the loop closes, or how much a hot air reading would effect fuel management. But that is a strange reading.

 

I think IAT only really matters in open loop... and even then, more at startup than anything else, from there, PCM generally just uses engine temp. (coolant sensor) I am kinda used to seeing them read higher than I would expect, but, if you are getting 31c at startup on a cold engine, that's just wrong. I would think at 50F, it should be high teens C, shouldn't it??

 

It would be helpful to see some data from when the truck acts up: O2 sensors and long and short term fuel trims. In open loop the PCM doesn't care about these and that can mask issues like bad O2 readings or vacuum leaks from evap problems.

31C is about 88F which is plausible depending on weather and how long the engine has been off. You can test the IAT sensor with an ohmmeter, there's a table buried somewhere in the service manual.

 

Thank you for the feedback. I took a video of the OBD data while it was running rough. The rear O2 sensor was unplugged at the time, so that explains is reading in the data below.

While running rough:
RPM is jumping between 400-1000. It almost conks out a 400, then it just catches itself and jumps up to 1000, then back to 400 and so on.
While the RPM is jumping around, spark advance is switching between 0 to 20. While it is idling nicely when cold, spark advance is steady at 4.
MAP is pretty consistent at 40 in all scenarios.
Front O2 Jumps between 0.000 and 0.750. Jumps seem to coincide with RPM troughs and peaks
Rear O2 1.000 (unplugged)
SHRTFT is -5.5 to -7.5
LONGTR is 0.0 to -5.5

I will reiterate that this condition is present with O2 sensors plugged or unplugged. Front O2 replacement did not change condition.
Also, while it is in this rough idle condition, I can increase RPM to 1,500-2,000 and it runs beautifully.

I haven't replaced the crank sensor yet, but It runs fine during warmup and at higher RPM ranges so I don't see that as being a suspect.

Thangs again for your thoughts everyone. This is a great resource.

 

MAP is pretty consistent at 40 in all scenarios

That is pretty suspicious.