low compression on cyls 4 and 5

[neo]

I have had suspicions of a compression issue for some time, but confirmed it this weekend. I was not able to test cylinders six and eight because of the difficulty getting into them. I found cylinder 4 at 300 and cylinder 5 and 350. The engine was cold when I did the test, so I figured I would try it again when warm.
What I don't understand it why those two would be low. All the rest I could test came in at 400 solid. Engine has very good oil pressure, and it pains me to think I have to pull it and rebuild. I did score a Navistar AMG military take out over the summer so I guess it is time to check it and get it ready to put in.
Any thing I should check to validate the low cylinders or anything I can do to insure they are indeed not low because of something simple? I just really don't have the funds to do this right now.
THe main symptom that made me suspect low compression is that it puffs heavy white grey smoke at start up if I let the normal glow cycle start the engine. If I hold the second solenoid down to use a longer glow cycle, it won't smoke near as much. It chugs and runs very rough when puffing out the grey white smoke. I can push the glow button down once running and chugging and it will clear up very quickly. I attributed this symptom to low compression not generating enough pressure and heat to get it to run/start correctly unless I add glow to get those low cylinders firing.
Any thoughts or guidance? Could the injectors in those cylinders cause the issue? I have recently (within the last year or so) replaced them. I am running a DB2 pump just recently converted.
Thanks!
Neo

[DmaxMaverick]

The compression test is very specific. If the ECT isn't at or near normal operating temp, the test won't be reliable. Not saying you don't have low compression, only that cold engine tests are generally not accurate.

Charge battery to FULL. Get the engine up to temp. Remove all the glow plugs (yes, even those). Record compression values AFTER 6 "puffs". If you have everything ready, it takes about 10-15 minutes to do all 8 cylinders (Yes, even those), and the cooling off during that time is not significant.

[neo]

Will do sir. thank you for the quick reply. I'll probably have to knock this out over the weekend. Any suggestion for getting tothe back two (6 and 8?) SHould I drop the downpipe? It looks like that would be the easiest way. Thanks again!

[Robyn]

Getting to 6 and 8 can be accomplished relatively easy by removing the right front wheel and going in through the inner fender.

Removing the rubber flap in the inner fender area will give great access to the RH glow plugs..

8 is the only tough one though as its behind the down pipe.


Good luck

[neo]

THanks Robyn, I plan on doing this one evening if I can get motivated. On another note, I thought some time back I read about a practice the military used when rebuilding placing white "X" on parts they messed with. I honestly can't find the post/thread. Was there some truth to this? My 6.5 Navistar block has white "X" in grease or pain pen all over it. Thanks again, I will let you know what I find on this compression issue.
Neo