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Compression question.

3K views 34 replies 9 participants last post by  Dirk 
#1 ·
For those of you who HAVE tested the compression, I have a question...

Have you seen valve\valve seat and still had decent compression (above 390psi)???

Engine was tested at ambient outside temp.
Very little to no oil leaked into the cylinder when glow plug was removed.
Pressures held for 2-3 min before gauge was removed to move the the next cylinder.
 
#4 ·
Have never done a compression test on a deisel. But I have never seen bad valves give good compression on anything I have ever tested.

Unless the damage was so slight you could clean it up with rubbing compound.

Good luck.
 
#5 ·
Mine had no noticeable bleed down when I left the gauge at pressure for about 4 minutes. I have run comp tests four different times on mine. However, I would think that given the comp numbers on a Diesel, you should notice a faulty valve which doesn't seat properly. Which could be a blessing in that you could then blame a bad valve for crappy comp numbers.

I do know that I have not exceeded 1300* F for more than a couple seconds for the last 140K miles so I would expect my exhaust valves to be in ok condition and not burnt.
 
#6 ·
1600+ on many runs here.

I'm trying to chase down a cylinder contrib failure on 3 cylinders (noticeable stumble/skip at idle). I replaced 2 of the injectors that are failing, still failing. Compression seems to be good. Passes buzz test.
 
#8 ·
Dirk, just a wild shot. But, could the grooves that the CPS reads off of be a little dirty, not even sure if this could cause the problem or not. If not, another bump.
 
#9 ·
what are the numbers on the three failed cylinders? With high heat comes burnt valve seals, not to mention aluminum melts at 1220*F, haha But I know lots of guys have held at high egt's and have been fine.

BTW, Which cylinders are failing? And do the same Cylinders fail on successive contrib tests?
 
#10 ·
Cylinders that always fail 1 an 5. Sometimes 8. Happens on any tune. CPS has been changed out (recall). Stumble/skip is most noticeable at operating temp, not so much cold. I thought I had some damaged rockers, but after inspection, they seem fine. I haven't replaced the push rods yet. I will be doing that when I replace the valve springs.

All cylinders are 390-400psi.

I agree with the high heat comment, I just haven't seen damage that didn't come with low compression/leak down issues.
 
#18 ·
I'll give you a shout when I get everything back together. A friend just brought over a ped and some other random pieces I need for this "version" of the truck. I still need some boots and piece the big oil together.
 
#21 ·
When the truck is idling does the engine shake? Visually looking at the motor, does it run smooth or is it rocking back and forth? Is the radio antenna wiggling more than it *should*? Who's hybrids are they and have they been flow balanced?

I ask all this because my engine rocks back and forth at idle since I installed my DIY 2's and added the tuning. I had some serious tuning issues that took a while to get sorted out when I first installed the injectors about a year ago.

With stock injectors my truck idled like a brand new rig, smooth and no rocking (the rocking almost feels like a miss, but I'm pretty sure I'm running A-OK on all 8 although the last time I did a CCT I failed on 2 cyl's but I forget which ones. Maybe 1 & 5). I think live tuning would get rid of my idle issue, but if it doesn't then I'm thinking that it may be the injectors that haven't been flowed, which is why I asked you about your injectors.

Dirk, if your injectors haven't been flowed and balanced, COULD that be your issue?

Sorry for the rant, just trying to help.
 
#24 ·
ding ding ding, we have the same freakin problem. Must be an early 99 thing.
 
#30 ·
Mine is doing it too (the shaking thing). Also sounds like a miss, but AE says otherwise. Mine didn't do this until a couple of months ago.

So what are you guys thinking?
 
#26 ·
*looks through cell phone to make sure he still has the number for Swamps' injector hotline*:evil
 
#28 ·
heres a question for you guys...

i read a post from a factory tech on some other website and he said if the compression is below 325, the injector won't fire, therefore creating a knock. what's measuring the cylinder pressure? and what's telling the injector not to fire? kind sounds like bs to me...
 
#29 ·
It wouldn't cause the injector to not fire, the injector will fire on a test bench if you have the electronics and oil/fuel setup. You don't need compression to make them work, you need compression to make an engine run right.
 
#31 ·
All 3 of us have early 99's, which shouldn't have a damn thing to do with it, but its a commonality we all have. Both Dirk and my injectors are unbalanced, so we're leaning toward that as a possibility. I'm keeping all my options open, I don't know why mine does the shake, rattle, roll.
 
#33 ·
Spill the beans already! :poke: :D
 
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