I have a 1999 3500 Ram van with the 5.9L. The vehicle runs pretty much fine, a little low on power, but fine. I know these are not known to be the fastest, but as a v8 I feel it should have a little more “go”. At idle you can feel a slight misfire, like you wouldn’t notice unless you are really feeling for it. No check engine light illuminated. At WOT right before the trans kicks in to overdrive (only in this gear) the check engine will flash.

I held the pedal and got it to stay illuminated, had cylinder 6 misfire. Changed plugs (OE Denso), wires, cap, button and air filter, as this vehicle is new to me. Cleared code, test drove and again check engine started flashing at WOT right before it kicks to over drive. If I drive normal, all seems fine. This time the check engine light didn’t stay illuminated, but had a pending code of cylinder 6 misfire again.

A few notes: none of the plugs had oil on them, as I know these engines have plenum gasket issues that cause this. I recorded temps on the cat at high 500’s on the inlet side and low 600’s on the outlet side. Is this normal temps for these cats?


Next I was going to go with injectors, specifically for that cylinder. I feel if it was a fuel pump/ filter issue, it would have thrown a multiple misfire code instead of a single cylinder.

Any help/insight would be greatly appreciated. Just seeing if anyone had ran into a similar issue on this engine/vehicle before. I’m a heavy equipment and truck mechanic. So you can hit me with the technicals, I’ll understand lol!

 

Swap the injectors on 6 and 8, see if the misfire follows the injector.

I would check the plenum anyway. Blown gasket causes a vacuum leak, and may lead to a lean-burn miss.

There was also a technical service bulletin on plug wire routing. (it's available in the 2nd Gen Ram FAQ/DIY section.)

How many miles on the van? How old are the O2 sensors? Does anything change if you disconnect the front O2?

 

The van has 168,000 and has been very well taken care of (It was previously owned by a church). I don't know the age of sensors as its a new to me van. The wire routing has been fixed before hand. I'm going to jump back on it this weekend, do a drive cycle to monitor the o2 sensors and fuel trims. This way should tell me if I have a vacuum leak or if the o2 sensors are acting up. I did see in another thread where someone was having a cyl. 4 misfire and it was the plenum gasket. So I'm not saying I'm out of the water yet with that, just didnt know if anyone had dealt with this issue before. I pray its nothing too serious and will keep you posted on what I find.

 

Checked it out this past weekend. Swapped 6 and 8 injector and still cyl. 6 misfire. I went ahead a did new o-rings while I was there. I did monitor the o2 sensor via scan tool and it was within parameters. I did figure out that on these vans when WOT the fueling goes to open loop; which it would then not rely on the o2 sensor for fuel but another sensor. Next I will be changing the plenum gasket and see if the issue persists. I also found bad engine mounts so I will have a fun day ahead of me.