I have a ram conversion 1996 3.9 2500, when on the last few miles of a 1500 mile drive engine dropped to 5 cylinders. have all new plugs and leads 3000 miles ago. Isolated which cylinder was not firing so replace lead and plug again, no luck, so changed cap and rotor no luck, power is arriving at plug. have just done comp test and cylinder is the same as others just under 150. Not sure what comp should be but as it is the same as working cylinders i assum its not that. whats next any ideas.

 

I will guess debris in the fuel rail or a bad injector. Try swapping injectors and see if the misfire moves with it.

 

Thanks landyacht, not being a great home mechanic i have a question . this model has throttle body injection , will this make a difference to your reply

 

Are you positive?? I'm pretty sure that TBI went out and MPFI became standard in 92 or 93.

 

i will check to make sure then get back to you

 

Just noticed in my origanal post i neglected to mention it is a conversion van

 

AFAIK, my TBI model will not even display a misfire code, much less even specify a particular cylinder.

 

it isTBI

 

You checked the compression, so the valves and rings are good.
It is a TBI, so that eliminates a single injector as the source of misfire on just 1 cylinder
You have replaced cap, rotor, and wires, and it still misfires on a particular cylinder.

The only thing left is the plug which I don't see that you have replaced.

The cheaper caps have aluminum contacts and wear out much quicker. Pay a little more for the copper/ brass contacts.

Your 1996 did not come with a TBI, your engine is from an earlier Vehicle.

 

sorry forgot to mention i first changed plug with another cylinder and it worked but missfire cylnder still no joy so put new plug just in case ,still nothing. Also interesting fact you mention is it has an earlier engine, when i bought it it had 50,000 miles and have had 110,000 miles almost trouble free motoring , till now