My elderly dad has a '98 3500 with a V10. It just turned 100k. He bought it many years ago with ~85k on it. Before the covid deal he would drive it to town once a week (20 mile round-trip). We started cutting hay this week. He has driven it several trips between his house and the hayfield and it has run exactly as it always has. Yesterday, it made two trips between his house and the field. It sat for approximately two hours in my front yard (I live next to the field). He tried to start it and it cranked much longer than usual. When it did start, it ran very rough and would hardly pull itself. Initially, i could hear an odd sound on the drivers side. It was like a muffled tap-a-screwdriver-handle-on-a-heat-shield kind of sound, but it went away before I could investigate. I attached my code reader and found there were no trouble codes. The CEL wasn't on either. Not knowing when the spark plugs were changed, the first thing I did was pull the plugs. They deserved to be replaced but it was pretty obvious they weren't the issue. They were all dry. I would've expected some to be damp since some cylinders were obviously misfiring. I picked up a set last night and installed them this morning and wasn't surprised to find it had no effect. I should note that plugs 2, 4 and 6 were much hotter than the rest. I fired it up again and disabled each cylinder, one-by-one. At an idle, 2,4 & 6 were the only ones that showed any effect when I disabled them. Using an infrared thermometer showed that all of the drivers side cylinders' exhaust ports and the two rear passenger side ones were much cooler than 2,4 & 6. I can't believe it is only running on three cylinders? It does sound like some of the other cylinders hit intermittently when you rev it up some. I've googled and searched here and found nothing similar. I also suspected that mice had possibly chewed through some fuel injector leads. (That has already happened on his '01 1500), but aside from some ancient acorns, there's no chewed wires that I can find, and the 1500 immediately had a CEL on when that happened. Can a cam position sensor fail so selectively? I have to get back into the hayfield shortly, but I plan to check fuel pressure and compression this evening, but I don't expect to find anything with either of those tests.

 

Test for spark on all cylinders. Sounds like maybe one of the coil packs has failed.

 

After stewing on it all day in the hayfield, I was convinced I was going to find the truck had very low fuel pressure (the three consistently firing cylinders are at the end of the passenger side fuel rail), but no. When I checked it this morning I found 45-50 psi at the fuel rail. I checked for fire on every terminal and each cylinder jumps a 0.300" spark tester nicely and consistently. The three cylinders that aren't misfiring share a coil with three that are and the other four misfiring cylinders are on their own coil. While it might be statistically possible for two coil packs to fail simultaneously on a 75° day at 50% humidity, I would say the chances are close enough to infinitesimal that we can consider that an impossibility. Besides, every cylinder has good spark and none of the plugs were fuel fouled like you would expect if the misfire were spark related. Also, I've never had a coil pack failure on a Dodge, but I would expect one to cause an immediate CEL/code and I have neither. I am investing in some noid lights and going from there.

 

On my 8.0, I had a cylinder misfiring because the spark plug wire was burnt in half. That cylinder simply was not firing, at all. It never set a code.......

Does the engine run alright when it is stone cold? Or is the problem there immediately at startup?

 

Noid light is a good idea at this point. Given that the plugs were dry it might show the injectors not firing which would point towards the PCM or wiring to it. Also check if you have full battery voltage on the dark green/orange supply to the injectors.

 

I have been working on this thing in fits and starts in between other things. Long story short, I had fire to every cylinder, used the noid light to confirm that I was getting a consistent signal to the (driver's side bank) of injectors. I confirmed multiple times that (at idle) only cylinders 2,4 &6 had any effect when I grounded the high tension lead (I was beginning to suspect that at least some of the other cylinders were hitting occasionally above idle, but couldn't easily confirm that working by myself). Thinking that a trouble code might not be set until the engine reached operating temp multiple times, I warmed it up and cooled it down several times. It took forever to warm up with only three cylinders firing consistently. As a part of due diligence, I checked compression just knowing I wouldn't find anything but was pleasantly surprised to find that #5 cylinder had zero. Then it dawned on me what I would find when I pulled the driver's side valve cover. The intake valve spring on #5 is broken. That explains the weird noise I heard briefly when it happened initially. With the intake valve hanging part way open, it creates a massive internal vacuum leak of a sort. The 2,4 & 6 intake strokes must coincide with the #5 piston approaching TDC so the effect isn't so dramatic for those three. Killing #4 didn't have as big of an impact on how it ran, so something else must be going on for it to hit since #5 piston is only at the top of the cylinder twice in a revolution. Anyways, I have the spring and gasket coming. I'll chime in again if this doesn't cure everything, but I am fairly certain that it will. I wanted to get the solution out there in case someone else goes searching for this problem in the future.

 

Are all those cylinders on the same coil pack?