Hi, 

Currently I'm dealing with a misfire when hot. Mostly noticeable at idle, lower rpm and slower speeds. It will also make a crackling sound when driving at low rpm. It often hunts for the right idle rpm if I let off the throttle. If I snap the throttle, there's a delay of like 1 second and then it will blow some white smoke like the timing is off. When I go to take off, if I push the throttle too fast at low rpm, there's like a short flat spot in the apps where the truck runs weird and crackles almost like it's choking out. 

 

I have tested and replaced almost all of the fuel related components including injectors 4 times, the pump, and all the sensors except IAT. When cold the IAT reads the same as coolant temp so I don't think that's the problem. I tend to believe the problem is electrical because I did the hot wire to the pump test and the misfire seemed to go away BUT I'm not entirely ruling out some kind of mechanical issue either. I did reset the valves and a few were not quite right so that did help some but I can still hear the rough running when it's warmed up. There is no air in the fuel, per the clear hose test. The apps signal appears to hold steady on an analog meter. I have done the WT wiring mod on my truck, the alternator shows almost no noise and just to be sure I completely disconnected it. Nothing helped. 

 

Until recently, there have been no codes. It finally threw a PO122. So currently I'm doing the testing in Mopar man's article for that code. Everything checked out until step #6, which is to check for continuity between the blue/white wire on the apps harness connector and pin 31 on the ecm connector. Is there supposed to be a certain level of ohms there instead of just continuity? There is continuity there, if I'm doing it right that is. Both harnesses disconnected, key off. 

 

My question is this. In the article you state that if there is continuity, the ecm needs to be replaced. That's where it gets a little confusing. It states if there is no continuity, repair the open. If there's continuity, replace the ecm. So it sounds as if either way it's a problem. Earlier I swapped the ecm with my brother's truck, which is exactly the same as mine. It didn't change a thing. BTW, I'm also running a timbo apps which is probably 4 yrs old now. I had also swapped the apps with my brother's with the same result. 

 

I hope you can help. 

Thanks, Dave 

The only thing that could cause a misfire either low compression or fuel injector. Could you crack injector lines and see if you can stop the misfire? This might isolate it down to a single cylinder then you know what to look for and where. Did you do new crossover tubes? Might be a bad tip or seat that is bleeding into the return rail causing the misfire. 

I can try that. Although I did put in new Cummins connector tubes last time I put in injectors. So it doesn't seem to follow either injectors or tubes. 

Ok. Reaching harder. Might look at the tips of the injector lines make sure the tip don't have scratches or gouges that might weep. I've had a small scratch in one of tips and would weep. 

The top of the head is dry, no visible leaks. The injector lines are old enough that I do have to make them pretty tight to prevent leaking. 

Hi folks, 

Well I'm back again with the same problem. In the time since I lasted posted on this topic, I pulled the engine and completely rebuilt it myself. It was in very good shape for the miles but I decided to do everything right. 

 

It's bored 20 over with modified Speed of Air pistons. New head with new valves and springs. New everything. 178/208 cam. Fenley 62/67/9 turbo. 

 

It was a long expensive project but I decided to do it for 2 reasons. 1, find the misfire and 2, try the new pistons from SOA. 

 

When I put everything back together, I cleaned up all the wiring, cleaned grounds, and took my time to make sure everything is put together right. 

 

Unfortunately after all that, the misfire is still there. I even sent in the injectors to have them checked. 2 ended up being bad after just 7k miles. DAP replaced those 2 and reset the others. The truck starts instantly but as soon as it starts warming up, it runs terrible just like it did before. Rough running and hunts for idle a couple seconds when I let off the throttle suddenly. It hazes some at all rpms. 

 

I'm about at my wits end. Almost everything mechanical has been replaced at least once. The ecm I had swapped with my brother's truck. No change

 

It's hard to believe that the problem is electrical because it only shows up when the truck is warm. In my thinking, hunting for idle comes from peeing injectors, as does the hazing. The random misfire also wud seem to point to injectors. But why would it not show up in testing? And why would multiple sets from different manufacturers do the same thing? 

 

The only set that had served me well was made by Weston Shupe. They lasted for 30k before a tip cracked. Sad day for me. Every set since then as been absolute crap and doesn't last long at all.

 

Thanks for any input 

What did you have the injectors popped to what pressure? 310 bar is stock. A lot of injector builder will pop them as low as 300 or 305 bar which will give short life being by about 280 bar you can have misfire issues and/or idle issues.

 

I've also seen injectors after a short time have much lower pop pressures due to settling of parts.

 

Like my 7 x0.010 injectors (150 HP) are popped to 320 bar. 

They were popped to 310 originally when DAP built them. When I had them sent in again this last time to be checked, they didn't specify pop pressure. I'm assuming they put them back to what I had originally ordered. 

 

I've wondered, is it possible that the injectors are behaving differently at test stand temperatures than when they're several hundred degrees hotter? 

The thing is that the misfire has showed up right away, every time I changed injectors 

I'm starting to wonder if the VP44 might have issues with PSG when it get warmed up? 

 

I also got to ask which did you tightened first the unjector hold down or connector tubes? Correct is the connector tubes first then the injectors.

As far as the pump, the problem was there with old pump too. I replaced it thinking it might fix it but it didn't. 

 

And yes, I snug the tubes first when I change injectors 

You could try cooling the ECU And VP with cold water and see if anything changes . I doubt it will do any good but you might get lucky and suddenly it smooths out.  Does it go away when loaded down?

I can hear it at light throttle and low rpm when driving, but I can't tell it under a good load. Has plenty of power 

Could I hire you to take a look at it if I made the trip over there?

The only other thing I could think of that could be the issue is while hot doing a compression test. I've seen one member here @jlwelding that had a similar problem and found out later it was a cracked piston. It passed on the blow-by test but would have failed the compression test.

Have you ever tried unplugging the coolant sensor or IAT and running without them

 

Greatwork!, I don't think I have.  That might be worth a try. 

 

And moparman, the engine is freshly rebuilt with new pistons and rings. It's just now broken in well and it doesn't use oil or have any blowby. I used total seal rings. So I think I can safely rule out any compression problems