Rough idle on 88 3.9 - Dakota Durango Forum
2.5L/3.7L/3.9L Engines Questions regarding the I4 and V6 Engines

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post #1 of 1 Old 07-25-2010, 07:23 PM Thread Starter
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Join Date: Jul 2010
Location: Milton Freewater, OR
Vehicle: 65W200, 93 D250, 88 Dak
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Rough idle on 88 3.9

I bought this thing as "scrap" from a dealer, and so far I've gotten it mostly good running, but one last thing eludes me. It idles rough. Previous to me, I have been able to see that a set of wires, plugs, dist cap, rotor, fuel filter were all put on, and yet the dealership could not get it to run.

I had to move the timing many degrees (probably around 20-30) to get it to start. Just lining up the 0 mark and locating my finger at the #1 plug spot on the dist cap showed the rotor pointing nowhere near it.

The truck is now responsive, has good power, and seems to not suck much gas, but it idles rough. As in, it has some random misfires at idle. In gear, the truck has an uneven sort of shiver, and when not in gear, you can feel the "putt" out the exhaust when it misfires. It's not steady, and not always the same cylinder.

I don't have any reason to think it has low compression, the plugs are good (new), I've run Marvel Mystery Oil through the gas, which improved things a lot. I replaced the rotor, as the previous "mechanic wannabe" damaged it. Still, no change, really. When you start it cold, it fires up and is smooth until it slows down to normal idle speed, and then it slowly gets "rough". I took it out and did a hard run up the mountains (to over a mile high pass) and back and it's somewhat better, but still has this shivery idle.

Is that by chance normal for a 3.9 TBI? I know old 2.2 and 2.5 Mopars would NEVER idle fully smooth.

Also, is there anything you have to do to set the timing? Wires to remove, plugs to plug, etc?

BTW, I changed plugs to NGK V-Power, changed the O2 sensor as it wasn't working right, and other minor fixes. Still has the uneven hot idle.
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