How do you diagnose a miss in a Cummins 855 engine?

Folks, as always, thanks for all the help with these Cummins engines. We have an 855 Cummins, 400 hp, and the engine developed a miss. We pulled the injectors and found a lot of metal to metal wear so we replaced them. Made no difference. We also purchased a compression test adapter for the 855 engine and took a compression test. We got a low number of 320 psi (cyl. #3) up to a high number of 460 psi (cyl. #4). So, the first question is, what should I be getting as far as compression goes? Next question, what strategy do you use on these Cummins engines when you have a miss and the valves are within specs, the injectors have been replaced and their travel set at .175, and the fuel filters have been replaced? At least with the other engines you can crack open the fuel lines and figure out which cylinder you are dropping. How do you go about diagnosing a problem with these 855 engines?????:Banghead As always, thanks to any one that can educate me.

Chris

I use an electronic temp gun.... shoot the exhaust manifold and find the cold port.

I have little experience with Cummins engines, but I'm like gramps, check the temp of manifold ports. If that's still inconclusive, seems I recall the 855 uses a rocker for the injector. Just wondering if you could do the same thing we used to do on Detroits and use a screwdriver to hold the injector down while idling and find which cylinders are firing and which is not.

Willie nailed it. Use a large screwdriver or pry bar and hold the injector down to find you miss firing cylinder. I have done it that way for years.

I dont recall the compression specs off top of my head. Do you have a lot of blowby? Have white smoke from the exhaust?

Might check the injector push rods to see if one is bent.I worked on a Cummins L10 that had a bad cam lobe funny it smoked a lot but no miss.

Hey Folks, thank you for the great feedback and ideas. We have a temp gun and found the 5th and 2nd cylinders running much colder. I like that idea about the pry bar on the injector but won't the pushrod fly out when you hold down the injector? Just wonderinbg how to prevent the pushrod from popping out. Also, let's say we figure out the bad cylinder(s) and I have already replaced all the injectors, am I wrong to assume it must be compression (e.g., burnt valve)? The bad news is that we already had the back head off and I looked at the valves myself and the valves for #5 and #6 were in good shape. So, with 189,000 miles on this engine, what else could cause this #5 cylinder to drop? You know, these Cummins engines are a rather simplistic system and are not bad to work on until something fails. At least a gas engine with a nice complicated electrical system gives you a year's worth of stuff to keep you busy diagnosing. So, any more info? Again, thanks to all for taking the time to educate me.

Chris

If there is a lot of blowby you might consider broken rings

tbone1471, good point. Checked and we don't feel anything coming out of the dipstick. Thanks.

CJLennon said:

I like that idea about the pry bar on the injector but won't the pushrod fly out when you hold down the injector? Just wonderinbg how to prevent the pushrod from popping out.

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It's not necessary to completely depress injector with pry bar or screwdriver with engine running. If a cylinder is firing good, all you have to do is slightly interrupt the up/dwn movement of the injector with a pry bar and it will make that cylinder drop. Rocker will click a little from the wide clearance, but it won't pop out pushrod at idle.

I bought a truck a few months ago and it had a miss when I bought it (Big Cam Cummins) I was told the engine only had 50,000 miles on it and all heads were replaced. I pulled the back head and found the valves were not seating, it looked like the valves were ground at a different angle than the seats were. I took it to the machine shop and had the head reworked, installed and everything OK.
I found the cold cylinder, with a temp gun
Good Luck

Yep, valve leakage can cause poor ignition of cylinders. Quick test if heads are off is flip head on it's side and pour water in the ports. A good tight sealed valve won't let water leak by.

does this engine has top stop injectors or mvt timing on it or stc injectors

Folks, great advice thanks. This engine has does not have top stop injectors. It has the old style. We will give the pry bar on the injector a try. Thanks for the idea. Also, while I have you folks here. I get that there are two ways of setting the travel on these injectors. The one way, torque method, I never felt comfortable with because I never reached the torque until the injector bottomed out so I have been using the dial indicator method. For the big cam engine which I have here, I have been using .175" for the injector travel setting. Is this correct? Any opinion or feedback on the torque method? I think the torque setting is 48 in/lbs. (cold), correct? Could really use some education here. As always, thanks for taking the time here.

Chris

If you folks are OK with it, I'd like to close this thread down and open up a new one that addresses only injector travel setting. I'm starting to better understand this Cummins 855 injector setting and I have a couple of questions. Thanks.

Chris

One more thing. Don't think you can set top stop injectors using the travel method. I'm living proof that it does not work! It turns out that if you do it, then you have set the injectors about two turns too tight. I redid the injectors using the torque method and each one had to be backed off about two turns. Hope some one else can benefit from my stupidity. Thanks guys.

Chris