Besides the absolute compression value, all should be within about 15% or so of each other. Maybe even as much as 20%, but that's getting borderline.
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- Thread starter Dan2002
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Alex_M
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I'd check spark plugs first - I've changed many a factory set at 250-280k miles and it makes a world of difference. My next guess is fuel injectors. I've had 2 or 3 with misses like that I had trouble tracking down and it was clogged fuel injectors. I'd swap coils around cylinder to cylinder before springing for injectors though, assuming spark plugs and wires are not the culprit.
So, ("T") an LM7 non-Flex engine. I was going to suggest running some E85 (or E30-50) but that's out.
If you get to the point of thinking it's an injector issue, I would recommend an injector balance test. If you pull the injectors to clean them, be sure to use new o-rings when reinstalling them.
If you get to the point of thinking it's an injector issue, I would recommend an injector balance test. If you pull the injectors to clean them, be sure to use new o-rings when reinstalling them.
Alex_M
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And pay the premium for injectors from the parts store. There are fake "AC Delco" injectors floating around. I had one stick open and kill a motor.
Did compression check on one side of the engine,
Cylinder 8 == 90 psi
Cylinder 6 == 100 psi
Cylinder 4 == 110 psi
Cylinder 2 = 90 psi
TollKeeper
Full Access Member
And thats the harder side of the engine to do. I would say you have some blow by going on by those numbers. Its within spec cylinder to cylinder (barely), but those are some pretty low numbers.
got all cylinders tested.
with engine running
Cylinder 8 == 90 psi Cylinder 7 == 90 psi
Cylinder 6 == 100 psi Cylinder 5 == 100 psi
Cylinder 4 == 110 psi Cylinder 3 == 95 psi
Cylinder 2 = 90 psi Cylinder 1 == 105 psi
thats with the engine running. i watched a couple of videos and they did a compression test but with the engine just craking. not running, so i removed the fuel pump relay
and re checked all cylinders again, they avarage from 135 PSI to 150 PSI,
so its safe to say compression is not an issue
rockola1971
Full Access Member
Compression checks are done with engine cold, not running and throttle wide open. (Pull fuel pump relay to disable engine start).
Update..!!
Got a set of injectors on Amazon (prime) so I could get them quick,and cheap and installed them today.
Most of the misfiring cylinders are not misfiring anymore, I have my scanner which shows each cylinder, how many misfires it generates. I have only 2 cylinders on bank2 that are misfiring a little bit. And one cylinder on bank1 that is also misfiring a little. Bit
I believe those little misfires are due to the bad spark plugs because they were dark when I checked compression.
Now im about to order, new coils ,new cables,new spark plugs and new injectors, maybe a good name brand
Got a set of injectors on Amazon (prime) so I could get them quick,and cheap and installed them today.
Most of the misfiring cylinders are not misfiring anymore, I have my scanner which shows each cylinder, how many misfires it generates. I have only 2 cylinders on bank2 that are misfiring a little bit. And one cylinder on bank1 that is also misfiring a little. Bit
I believe those little misfires are due to the bad spark plugs because they were dark when I checked compression.
Now im about to order, new coils ,new cables,new spark plugs and new injectors, maybe a good name brand
Tonyrodz
Resident Resident
Hopefully you got genuine AC Delco injectors. Amazon is notorious for selling knock off/bogus car parts, especially spark plugs. You really should make sure--as stated above--you buy genuine parts.
