400 compression?
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Freshman User
Joined: Mar 2007
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400 compression?
hi all , I am a beginner at all of this engine stuff ... i read and research to understand... the how and why the best i can . So my 79 350 has been running rough as of late , decide to do a compression test . The readings were as follows: #1-120 #2-135 #3-100 #4-140 #5 -140 #6 - 140 #7 -140 #8 -140 . I'm assuming #1 and #3 are my problems . Tried a cap full of oil on each
#1 went up to 130 and #3 stayed the same . Where do i go from here ? Is this something I can figure out on my own , I enjoy learning and working on my truck... or this out of my league . Valves , rings etc. thanks for any input or guidance!!
#1 went up to 130 and #3 stayed the same . Where do i go from here ? Is this something I can figure out on my own , I enjoy learning and working on my truck... or this out of my league . Valves , rings etc. thanks for any input or guidance!!
Posting Guru
Joined: Nov 2008
Posts: 1,937
Likes: 2
From: wyoming
compression stuff!
yes your #3 is a valve pretty sure. Oil test is a goog way to confirm. Sounds like its time for some upgrades to the 400. need to confirm your cam is good. but if your preparing for overhaul this will all be changed out. Tim meyer good place to start.
Laughing Gas
Joined: Aug 2010
Posts: 896
Likes: 55
From: Bothell
100# is low but is that actually low enough to cause it to run rough? I agree that a rebuild is in your near future but I'd also do a tune up and look for another cause. It's not like that cylinder went from 140-100 in a day or too, odds are it's been there for a while.
If your cam is warn down that could be contributing to the low compression too (if your are not efficiently taking air in then you can't efficiently make compression). Do you have the ability to check the cam lift for each exhaust and intake lobe?
Did you try oil in the just bad cylinders or all of them? going from 120 to 130 isn't that big of a jump and I would expect the other ones would go up about that much too.
At this point it could be your cam or valves (my best guess). Are you familiar with a leak down test? That would be a good way to check for leaking rings or valves.
For reference I just went through this with my boat motor, had compression numbers that were all over the place (100-145). Cam was a bit warn on some of the lobes. After changing the cam the compression went to 140-145 on all cylinders.
As far a learning goes, if this isn't your daily driver, it's a great place to start. These engines are simple to work on. If you have confidence in your mechanical skils then you will be able to handle this issue even if a full rebuild is needed.
If your cam is warn down that could be contributing to the low compression too (if your are not efficiently taking air in then you can't efficiently make compression). Do you have the ability to check the cam lift for each exhaust and intake lobe?
Did you try oil in the just bad cylinders or all of them? going from 120 to 130 isn't that big of a jump and I would expect the other ones would go up about that much too.
At this point it could be your cam or valves (my best guess). Are you familiar with a leak down test? That would be a good way to check for leaking rings or valves.
For reference I just went through this with my boat motor, had compression numbers that were all over the place (100-145). Cam was a bit warn on some of the lobes. After changing the cam the compression went to 140-145 on all cylinders.
As far a learning goes, if this isn't your daily driver, it's a great place to start. These engines are simple to work on. If you have confidence in your mechanical skils then you will be able to handle this issue even if a full rebuild is needed.
Laughing Gas
Joined: Dec 2012
Posts: 801
Likes: 7
From: Toledo, Ohio
. Did you have the throttle wide open while testing each cylinder for compression?
. If an intake valve is leaking combustion fire back into the intake manifold, that can cause rough running... although it's usually the exhaust valves that start burning/leaking first... although a broken valve spring could harm the intake valve sealing... as could a bent intake valve from overrevving causing lifter pump up and valve hitting piston...
. If engine is in good tune via carb. and ignition system, but still running rough, removal of valvecovers or even heads for further testing may be necessary... how many miles on that rascal?
. If an intake valve is leaking combustion fire back into the intake manifold, that can cause rough running... although it's usually the exhaust valves that start burning/leaking first... although a broken valve spring could harm the intake valve sealing... as could a bent intake valve from overrevving causing lifter pump up and valve hitting piston...
. If engine is in good tune via carb. and ignition system, but still running rough, removal of valvecovers or even heads for further testing may be necessary... how many miles on that rascal?
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