Hi, I’m new to the site and a huge Dakota fan. I’m hoping to get some help with some trouble I’ve been having with my 1999 SLT 4x4with the 3.9L V6—154,000 miles.
I’ve been having a rough idle problem since about 145k miles. After not driving for a day or so, the truck becomes difficult to start (takes 2-3 tries compared to the 1 time every time it used to be) and once it finally starts, idles really rough. It has also been throwing a P0306 trouble code (misfire on cylinder #6). Once pulling out of the driveway and heading down the road, the truck runs like a champ until I stop and the rough idle appears again. I’ve tried to fix the problem by doing the following:
-New plug wires
-New plugs
-New dist. cap and rotor
-New coil
-New idle air control valve
-New throttle position sensor
-New injectors
-New cat converter and O2 sensor
-Synthetic oil
-New battery
-Checked for vacuum leaks
-New relay for the auto shutdown relay
The truck will idle perfectly after I reset the PCM (by disconnecting the neg terminal on the battery, turning key to start position for 20 seconds) and drive around town. It only starts idling rough after I drive around at 45+ MPH (I believe the speed at which the idle speed is calculated from). Seems like the PCM keeps “forgetting” the idle speed. I was wondering if the PCM could be going bad? Is it worth trying a new one? Also, this problem started on my way from Illinois to my new home in Lubbock, TX, a +2500 ft change in elevation. Could that have something to do with it? Thanks for the input.
I’ve been having a rough idle problem since about 145k miles. After not driving for a day or so, the truck becomes difficult to start (takes 2-3 tries compared to the 1 time every time it used to be) and once it finally starts, idles really rough. It has also been throwing a P0306 trouble code (misfire on cylinder #6). Once pulling out of the driveway and heading down the road, the truck runs like a champ until I stop and the rough idle appears again. I’ve tried to fix the problem by doing the following:
-New plug wires
-New plugs
-New dist. cap and rotor
-New coil
-New idle air control valve
-New throttle position sensor
-New injectors
-New cat converter and O2 sensor
-Synthetic oil
-New battery
-Checked for vacuum leaks
-New relay for the auto shutdown relay
The truck will idle perfectly after I reset the PCM (by disconnecting the neg terminal on the battery, turning key to start position for 20 seconds) and drive around town. It only starts idling rough after I drive around at 45+ MPH (I believe the speed at which the idle speed is calculated from). Seems like the PCM keeps “forgetting” the idle speed. I was wondering if the PCM could be going bad? Is it worth trying a new one? Also, this problem started on my way from Illinois to my new home in Lubbock, TX, a +2500 ft change in elevation. Could that have something to do with it? Thanks for the input.
Record Breaker
Welcome to the forum. There are some good guys here who can help you.
Personally I doubt the PCM is bad, you would probably have other problems if it were.
It could be the Manifold Absolute Pressure (MAP) sensor is bad. It monitors barometric pressure in the manifold and is mounted to the front of the throttle body. If it is bad or sluggish, there can be a decrease in both power and gas mileage. It can also cause a rough idle and it can definitely be related to the change in altitude from Illinois to Texas.
I don't know thw testing procedure by heart and am at work now, all my manuals are at home. I know you can test the MAP sensor by checking the voltage on the center wire at the 3-pin plug on the MAP sensor. Be sure you have 5 volts across the 2 outside wires on the plug with the ignition on. I think the center wire should have about 4.5 to 5 volts with the ignition on and engine not running and slowly drop to no less than 1.5 volts when you start the truck and let it idle. I probably have some of that procedure wrong but the Haynes book for your truck will tell you the exact proper voltage. When I replaced my own MAP sensor, I did it just for maintenance but it helped out on MPG, power and the truck did seem to run a little better. Autozone had the best price on the MAP sensor, I think it was about $66.00.
Not sure on the misfire code. It may be that since you put in new plugs and wires the misfire code may clear once you solve the rough idle problem. If you do get a new MAP sensor clear the codes from the PCM like you did before and see how the truck runs and whether you get a code again.
There's a thread on here somewhere called "MPG and power loss solved" or something like that. It had a lot of info on the MAP sensor and is where I learned about the MAP sensor and its functions. It should come up if you do a search on it here. Or search for "MAP sensor".
Good luck and let us know how it turns out.
Jimmy
Personally I doubt the PCM is bad, you would probably have other problems if it were.
It could be the Manifold Absolute Pressure (MAP) sensor is bad. It monitors barometric pressure in the manifold and is mounted to the front of the throttle body. If it is bad or sluggish, there can be a decrease in both power and gas mileage. It can also cause a rough idle and it can definitely be related to the change in altitude from Illinois to Texas.
I don't know thw testing procedure by heart and am at work now, all my manuals are at home. I know you can test the MAP sensor by checking the voltage on the center wire at the 3-pin plug on the MAP sensor. Be sure you have 5 volts across the 2 outside wires on the plug with the ignition on. I think the center wire should have about 4.5 to 5 volts with the ignition on and engine not running and slowly drop to no less than 1.5 volts when you start the truck and let it idle. I probably have some of that procedure wrong but the Haynes book for your truck will tell you the exact proper voltage. When I replaced my own MAP sensor, I did it just for maintenance but it helped out on MPG, power and the truck did seem to run a little better. Autozone had the best price on the MAP sensor, I think it was about $66.00.
Not sure on the misfire code. It may be that since you put in new plugs and wires the misfire code may clear once you solve the rough idle problem. If you do get a new MAP sensor clear the codes from the PCM like you did before and see how the truck runs and whether you get a code again.
There's a thread on here somewhere called "MPG and power loss solved" or something like that. It had a lot of info on the MAP sensor and is where I learned about the MAP sensor and its functions. It should come up if you do a search on it here. Or search for "MAP sensor".
Good luck and let us know how it turns out.
Jimmy
Sounds like it could be the problem. I'll check it out tonight. Thanks for the response!
Replaced the MAP Sensor last night. Still same symptoms: rough idle, difficult to start, CEL (code #P0306) coming on. Truck seemed to run slightly better overall, but this rough idle thing is really getting annoying. Seems like it is an electrical gremlin somewhere. I'm thinking of maybe going to the dealer to see if they can find anything. Any other thoughts?
Record Breaker
Sorry the MAP sensor didn't fix it. Two things come to my mind here. One is to have the dealer check the PCM and reset the timing. It could very well be that the PCM is bad and the timing does not stay in its memory. Usually though with a bad PCM you have other problems too and you get a "No Bus" trouble code. The other thing is maybe you have a bad intake manifold gasket which is causing a coolant leak in cylinder #6. A compression test might tell you for sure. With new plugs, wires and distributor cap and still having a misfire code there has to either be a problem in that cylinder itself or the PCM is not firing that cylinder. I would guess the PCM is bad because the truck runs OK for a while after you reset the PCM, then seems to forget the firing order and cylinder #6 goes nuts. It seems to me that if the intake gasket were bad, cylinder #6 would not fire at all, whether you reset the PCM or not.
The dealer can use the DRB scan tool and communicate with the PCM and probably find out for sure what is wrong. It would be worth the diagnostic time to find out what is really going on. If you need a new PCM, I have heard and read online there is a place here in Florida somewhere that sells re-programmed PCM's for a good price and I think they have a good warranty. I don't know their name but if you Google search Florida, PCM, etc it may come up.
Jimmy
The dealer can use the DRB scan tool and communicate with the PCM and probably find out for sure what is wrong. It would be worth the diagnostic time to find out what is really going on. If you need a new PCM, I have heard and read online there is a place here in Florida somewhere that sells re-programmed PCM's for a good price and I think they have a good warranty. I don't know their name but if you Google search Florida, PCM, etc it may come up.
Jimmy
Professional
Does your truck have California emissions? (Three cats). I have had the same problem with random misfires on the #1 bank. Replaced every sensor in the truck, and finally figured out the cat is clogged. Runs great (will rev up without missing) with the exhaust disconnected. I'm putting in a new cat next week when it comes in, I'll let you know what happens.
Replacing the cat and O2 sensor didn't make a difference for me (needed to be replaced anyway). I'm pretty sure it is either the PCM or some sensor somewhere. Possibly the battery temperature sensor? Had a battery go dead last October, replaced it, then had a bad cell in the replacement last month. I've read that the idle speed is sensitive to small changes in the battery voltage for some reason and that a new battery did the trick getting rid of the rough idle for some folks. Bad sensor might be my problem--going to test the batt. temp sensor later tonight--I'll post if I figure anything else out.