Truck has random misfire when hot

[DaveS]

Injectors could definitely be a possibility. It's hard to believe that is tho because it didn't change when I put them in brand new from DAP. 

Anyway here's the latest update. 

I changed my timbo apps with a new one and the flat spot and stumbling went away. Since I adjusted the valve lash and changed the apps the truck definitely improved overall. So those 2 things were actually  separate problems. But guess what, that persistent misfire still shows up when hot. 😒 

So Idk if it's an injector or a cracked piston. I do think the injectors need to be reset because they haze too much. But I don't think they're causing the misfire because it was there when they were new. And through several sets before that too. 

I'm beginning to think I'm just gonna have to pull the engine to see what's going on. 

[Mopar1973Man]

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. 

[DaveS]

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. 

[Mopar1973Man]

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. 

[DaveS]

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. 

[DaveS]

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 

[Mopar1973Man]

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. 

[DaveS]

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 

[Mopar1973Man]

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.

[DaveS]

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 

[Great work!]

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?

[DaveS]

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?

[Mopar1973Man]

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.

[Great work!]

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

 

[DaveS]

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