Hey guys,

This is my first post here. First of all, I have a 1999 Dakota with the 3.9l and standard tranny in it. It has 295,000km (~182,000 miles) so I've looked through all the similar threads I can find and they all seem to have issues while the engine is cold. My truck is acting up after it reaches operating temp.

Symptoms:
-Starts great when cold and runs perfect for about 5-10 min until it gets close to operating temp (according to gauge on dash)
-Once it gets close to operating temp, it starts to run really rough at lower RPM and idle, but still runs OK above 2.5k RPM
-When Engine is warm, problem gets progressively worse and runs rough through entire RPM range and will die if I try and let it idle (Sputtering/missfiring/backfiring) and get random spurts of decent running then goes right back to running like garbage. This will continue for 5-20min depending on outside temp. (The colder it is the worse it is) If its really warm outside, I have no issues at all. (Above 15-20C or ~60-70F)
-After this 5-20 minutes of hell (If i try and let it idle it will stall and not start for about 5 min, then start no problem) it starts to smooth out in the reverse order it started. It'll start running smooth at higher RPM, and slowly smooth out through the RPM range until it's running properly throughout, then I will have 0 issues with the truck until I let it sit for a few hours without running (Overnight usually) then start this all over again.

I'm at my wits end with this as I drive 65km (~40 miles) each way to work and if it acts up on the highway, I can barely maintain 80km/h (50mph)

Parts I've replaced since owning the truck: I've had it for about 100,000km (60,000 miles)
- TPS (1 week ago)
- Distributor Cap and Rotor (Yesterday)
- Spark plugs and wires (Yesterday)
- Cleaned up the throttle body (1 week ago)
- Recent oil change (yesterday)

She was definitely overdue for a tune up as the plugs had essentially no electrode left, cap contacts were all corroded, and wires were starting to degrade. I have noticed a big improvement in power when it's running properly, but still have this sputtering issue.

The only other things I can think of to try are the Coolant temp. sensor, and maybe the IAC Valve, But I don't want to keep throwing parts at it unless it has a good chance...

If you guys have any thoughts or ideas that would be greatly appreciated!

Thanks in advance,

Gabryel

 

Oh I forgot to mention, the CEL does not come on the majority of the time. It does come on the odd time while it's acting up, but goes out by the time I go to have it scanned and no codes show up... I may have to buy a scanner and leave it in my truck.

also could it be the crankshaft position sensor?

Thanks

 

I keep a cheap $15 scanner handy:
Sounds like a closed loop issue. That would steer me to think it may be an O2 sensor gone awry. 1999 should be a 1 cat 2 sensor system unless it has california emissions (3 cat / 4 sensor). Usually a bad O2 will throw a code, but not always. at 182,000 miles there are alot of service items that should have been done... O2's, crank sensor, cam sensor (though mine is about same mileage and fine), MAP and TPS sensors (honorable mention to IAC). I probably missed a few.

 

X2 on the O2 sensor.
I had a bad one with no code.
Chrysler 02 sensors in these years have been known to cause wierd problems, and should probably changed every 50,000 miles, assuming the vehicle isn't burning oil excessively.
Make sure you get the upstream O2! The one in the cat has nothing to do with how the engine runs, it's just their to throw a code if it doesn't like what's in the exhaust.

 

I thought the front O2 was actually in the cat on the 3.9?

Front O2 is the one BEFORE the cat in the exhaust stream. I know dodge used a couple different configurations on the piping......

 

Hey Guys, thanks for the input I appreciate it! Sorry I haven't replied earlier, work got really busy for a bit.
Update: I replaced the upstream O2 sensor with a new NGK one. (My truck is the 1 cat, 2 O2 sensor setup). It did look to run a bit smoother after replacing the O2 sensor, but still keeps sputtering and backfiring. When it was running smooth, it was idling higher as well, around 1100rpm. I found the cap for that vacuum port off to the side of the left head (just below the coolant line beside the coolant temp sensor missing. Replaced that and that fixed my idle, but still have the same problem.
I just replaced the coolant temp sensor last night and still no change. It has not thrown any CEL codes since my last post.
@Magnethead, I am planning on replacing all the sensors anyways this spring anyways, but I hope I can figure out whats going on soon. Do you guys have any recommendation on what to try next? I'm thinking possibly the crank sensor?

Again thanks!

 

Deleted

 

Update: The check engine light came on again finally and it is throwing a P1391 code. From what I can find that's the code for the crank/cam sensors correct? How likely is it that this code would be caused by actually a bad sensor? My truck has had a slight rattle/knocking sound since I got it and never really gave it much thought. It sounds like it could be coming from the timing chain, but I will try and pinpoint the location of the sound this weekend. Could an excessively loose timing chain offset the timing enough to cause my symptoms in this truck? The only thing I find weird is it only does it when it gets warm.
Thanks

 

I knowthis thread is old,but just curios what the out come was. I'm having very similar issues with my van now.

 

My 98 3.9 has the front O2 sensor before where the cat should be.