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sprinkles08, Chrysler/Dodge/Jeep master tech
Category: Dodge
Satisfied Customers: 22813
Experience:  ASE Master and Advanced Certified, Chrysler/Dodge/Jeep Master Certified, Trans and Hybrid Specialist
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1999 Dakota 3.9 V6 engine. I have a rough Idle #4 and #6

Customer Question

1999 Dakota 3.9 V6 engine. I have a rough Idle #4 and #6 cylinders are setting a "misfire" code. Engine runs smooth as glass above idle. No skip, no misfire. Only at idle. Background: The original engine spun a bearing. I purchased a "used" engine from a junk (recycling) yard. I used it as a "long block" I put the intake manifold (complete with injectors and throttle body), exhaust manifolds, all sending units, and distributor on the replacement engine. The original engine ran perfect until the bearing problem. Now the misfire. I took it to a garage where they performed several tests. They said that it is most likely the exhaust valves in 4 & 6 that have rust or some other problem and I needed to replace the head. My question ? why replace the head? And , Why only at an idle ?
Submitted: 1 year ago.
Category: Dodge
Expert:  sprinkles08 replied 1 year ago.

Hello and welcome to Justanswer!

Misfiring, unless severe, which be much more noticeable at idle. The 3.9's always ran a little ratty at idle anyway, but misfires from cylinder sealing issues, will always be more noticeable at idle.

They wouldn't know if the head needed to be replaced until it's removed and the valves are removed from it, but the shop doing the work may not have a machine shop readily available. If there was anything deeper than just needing to replace and lap a valve then a machine shop would be needed. 3.9's were bad about valve seat wear and the Magnum engines all had issues with cracks developing between valves, if they're experienced with these engines that may have been a factor in recommending head replacement.