This truck always ran fine until auto trans gave out (230K). Got it back after trans rebuild but it has this weird problem. Starts fine cold and runs well; you can drive truck for several minutes. Soon, the engine runs really rough. Cannot hold an idle and will most likely stall several times. Once truck reaches normal temp (about 15-20 min of idle) and temp gauge is half scale (important); engine runs great. Subsequent start/operate/stops are normal and everything is fine until truck totally cools down again. Trans rebuilder put new crank sensor thinking it was cause. PCM code indicated O2 sensor that I replaced. No more check engine light but still no good. Measured resistance of engine coolent temp sensor and it seemed off so I replaced that as well; no good. Pulled a junkyard PCM and it reacts the same as original. Seems like engine wants to stall until fully up to temperature although initial start is good and runs great for about 3 to 5 minutes before problem begins. When in this period of rough operation, you can gun the throttle and engine speeds up but there is still indication of engine missing, also there is a strong smell of gas vapors. Please help, I want my old 95 Dakota (3.9L) back!

 

My guess is a bad coolant temp sensor. You said you replaced it, but did you replace the correct one? There is 2, one for the gauge one for the PCM.

Next guess, is pinched wires between the engine and transmission, it is common.
Or a bad IAC Idle Air Control valve since it dies at idle.
Or a vacuum leak since you smell gas, maybe the gas vapor line.

 

Sounds to me more like a bad coil, as the engine heats up a break in the windings is opening causing the engine to run poorly and die, once it completely warms up the open is possibly closing back up. That is where I would start, wait until your truck is running poorly again and check your coil.

 

Thanks for all of the suggestions. The engine coolant sensor that I replaced is small and was located low, between the alternator and the air conditioning compressor. The only issue is that the manual indicates there should be two wires but mine has only one wire for only one contact. The replacement part matched the original by having only one contact. Where is the second coolant sensor located, I cannot locate another within the manual? There is a intake manifold air temperature sensor; is this the second sensor you are referring to? I have not changed the intake manifold air temperature sensor.

Can a sensor for the transmission have an effect on idle as this whole routine takes place without having ever shifted out of "Park"?

The engine really seems to be "cutting out"; that seems to fit with the gasoline smell. I will certainly test the coil just as soon as I find a description of how to do so in the manual. I always wondered about the coil...

 

You replaced the gauge sender unit. You need to replace the PCM coolant temperature sensor.

 

Here is the current status. One suggestion was investigate the coil. Purchased one from junkyard and it made no difference in the original symptoms. Did see the actual engine coolant sensor; never would have found that thing if I had not seen one on a junkyard motor that had some air condition apparatus removed. So yes, I replaced the temperature meter sensor which had been working just fine. Checked my PCM for codes and now it reports EGR solenoid or EGR problems. Still exhibits the same symptoms of perfect start and initial run followed by rough idle until fully warm (15 to 20 minutes). What next?

 

Any luck solving this. I have exactly the same problem. 1989 Dodge B250 318 van
Neil

 

Not yet, still have to replace the EGR valve and solenoid; on my list for this week. I will post my results after doing this job. Have already done these: 1. new plugs, wires, and distributor cap; 2. crank sensor; 3. O2 sensor; 4. wrong coolant temp sensor; 5. coil; and 6. different PCM for same model.

 

Happy New Year to all. Yep, still trying to fix this one. Friend of mine wants me to add fuel injector cleaner to the fuel. I put little hope in this solving the problem but since he gave me a bottle as a present, what the heck...

 

Just curious. After finding out you had replaced the wrong coolant temp sensor, did you check out the right one to see if it was working right? If you are getting egr codes its a good place to look too though.