Random miss cm2350

[Lonestar10]

I have a random miss. Have ran fuel injector leakdown test, cylinder cutout, and performance test. The only thing I was able to diagnose was inj 3 failed test 1 during performance test. Cutout didn't really show anything, and leakdown was around 700-800 psi over several minutes.

Any other ideas?

[Rawze]

As my engine got some age on it, it would have a random miss/stumble at idle. It got worse over time ... especially when it was cold ... but was not quite bad enough for me to chase it down.

BTW: Engine stumble itself is not going to be in any of the the bottom end components, unless it is the crank position sensor (or associated parts) is causing it.

When I did my inframe at 980k miles, I literally re-used everything in the fuel system, including injectors, etc., and also re-used all sensors including same crank and cam sensors, same cams, same top end components, .. literally everything, except the head itself.

The miss completely disappeared after the inframe and that surprised me actually. The only thing that it seemed to have been at that point that contributed towards the miss/stumble, and it suddenly going away, was the head itself wearing out.

400k miles later on that head, that same random miss/stumble has begun to start to show signs of wanting to show up again. It is about the same mileage on the head that I noticed it the first time, so it is right on par, I suppose.

What did cummins used to say?... after 435,000 miles, the ISX heads are throw-away, because 4 out of 5 of them at that point will fail a thorough magnaflux?. - Could this be related I guess?.

Since then, unrelated to the stumble, I have also changed all 6 injectors and all of the top end components .. the cams, rockers, rocker-shafts, everything in top of head including the cam sensor. This was due to more than 1.3 million miles on all of that stuffs, and them just finally wearing out mechanically. It has not however changed this stumble that has started to set back in. On the other hand, literally everything in my IFSM (the entire fuel feed system) is still 1.4million miles original at this point too though. This including the fuel pump, solenoids, diaphragms on the back of it, etc. - so who knows what would happen if I started to replace all of that. - I am somewhat afraid to touch it, as the truck still breaks 8+ and 9+ mpg with 80,000 lb loads of coils from Kentucky to Georgia, including over mont-eagle pass,.. and who would want to disturb something like that unnecessarily, with such kind of fuel mileage.

That is just my own experience any ways... and as long as it still gets its 8+ mpg, and continues to pull just fine without any excessive vibrations, I could personally care less.

[schISM]

(10-16-2023 )Lonestar10 Wrote:  I have a random miss. Have ran fuel injector leakdown test, cylinder cutout, and performance test. The only thing I was able to diagnose was inj 3 failed test 1 during performance test. Cutout didn't really show anything, and leakdown was around 700-800 psi over several minutes.

Any other ideas?

Try monitoring the fuel pump control actuator commanded vs measured along with fuel rail pressure. Look for spikes in the measured fca that correspond with dips in the rail pressure.

[Lonestar10]

Well if it was just the stumble it wouldn't bother me much, but there is what I would call a fuel knock at 1300 rpm and above that gets louder with rpm.

About 50k on overhaul where head, cam, rockers, and everything except for injectors is new.

Last weekend did overhead Incase valves got to tight but that didn't change anything.

Was fooling around last night in the bunk doing a cut out of 1 cylinder at a time because apu 120v conked out and I needed heat "carrier". At 900 rpm idle the truck was shaking due to miss fire.

While doing the test cylinders 1,2,5,6 would make the shake worse when cut off. But 3 seemed to make no change and 4 would make the engine run smooth as silk

So I'm thinking 3 or 4 is my issue. I've got a new injector on order with a few new quill tubes Incase I guess wrong and need to change the injector to a different cylinder.