Ok, my nephew's truck is killing me. Never ending mis-fires! All cylinders, intermitting.
This is a new one for me, I simply can't figure it out.
97 3.9 V6
Misfires started about a year ago on cyl 1 and now we get random mis-fires across all cylinders.
We replaced the following:
Plugs, Wires, Cap/Rotor, Coil, Distributor, all 6 fuel injectors, also replaced several vacuum lines as some looked beat.

The entire ignition system is new at this point and still getting mis fires, throwing the same codes and you can actually feel it mis under acceleration.
WTF?
No smoke in the exhaust, no moisture, no foul'd plugs, not burnin oil.
I am at a loss!

Edit: We also replaced the Crankshaft Pos Sensor as part of another code that has been resolved. Still miss fire.

 

If you have an OBDII scanner, hook it up and see what your MAP reading shows. It should be somewhere around 18inches-mercury while running(basically vacumm). This will tell you if you have an leaks and can also tell you if you have an exhaust issue. The cat could be clogged.

 

No vacuum leaks, no other codes either.


The funny thing is, when it started more than a year ago, it was one cylinder, threw a code every week or two, then it went to 2 then 3 cylinders on one side, then 4 5 and 6.

I am starting to lean towards possibly a problem with the ECU while firing the injectors. That is the only thing that has a global control over engine management.

the exhaust system is solid, not getting any bad smell like the cat is going bad.

I can physically feel the misfires now, that tells me it is fuel or spark. the misfire is intermitting, maybe under acceleration, it isn't so bad when crusin but still happens.

I have never seen a misfire problem like this one, most of the time it is a coil/wire/plug etc etc etc.

 

Mine has done similar for a couple of years and I replaced cap/rotor/wires/plugs to no avail. Very intermittent - like once a week. Only does it at partial throttle. I think it's a bad connection to one of the O2 sensors because it occurs mostly when it's dewey out and I'm off first thing in the AM (I park it outside). I never have codes on it. If I go full throttle it runs perfect.

I'm probably taking mine in to the dealer in a couple of weeks for another issue and I'll inquire. Please post your conclusion if you ever find the root cause.

Good luck,

Karl

 

http://www.dakota-durango.com/forum/...19&postcount=8


I just found this, and it does appear to be the problem.