Low compression causes?

GrandmaLS

GrandmaLS

Club Member
What can cause low compression? Bad rings or valve issue is a given but what else could be wrong with it? The guy got the car with a motor with "low compression" so he replaced it from day 1. He's willing to trade me the SBC for a set of Eibachs I have.

I'm wondering what it could be. He didn't remember what cylinders it was but said the motor turns over just fine. A bad head gasket would cause two side by side cylinders to be low, correct?
 
possible causes


bad rings
bad(bent, burn, etc.) valve(s)
blown head gasket
cracked head
cracked block
cracked/melted piston

thats about it, the cracked stuff is rare, at least in a non race engine
 
It's a tiny little V6. I highly doubt it's a crack or bent rod. My guess is a ring, head gasket, or problem with the valves. The motor has 89k on it.

He isn't mech. inclined at all so he just paid to have a reman'd motor put in instead of having it fixed. He bought the car with the motor and the guy said "It's got low compression, but it still runs" so he just said f**k it and replaced it.
 
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SBC or 3.4 v6? little difference there, scooter. If it's the 3.4, you're looking for a motor to rebuild anyway. This gives you a couple months to go through things and see what needs to be fixed. without knowing more about the specific motor, i'd be inclinded to say head gasket as it was not un-common problem, right after the notorious intake gaskets.

Think a leak-down test would be a good idea to see how bad the compression really is? take a look at the motor.... Did it use coolant at all?
 
CLC85TA said:
SBC or 3.4 v6? little difference there, scooter. If it's the 3.4, you're looking for a motor to rebuild anyway. This gives you a couple months to go through things and see what needs to be fixed. without knowing more about the specific motor, i'd be inclinded to say head gasket as it was not un-common problem, right after the notorious intake gaskets.

Think a leak-down test would be a good idea to see how bad the compression really is? take a look at the motor.... Did it use coolant at all?
Click to expand...
I know there is a difference, dumbass. SBC 355....
 
GrandmaLS said:
I know there is a difference, dumbass. SBC 355....
Click to expand...


Im still really confused as to what engine your talking about a v6 or 355...


btw

low compression...


Hole in the cylinder(s)
hole in piston(s)
Rings
Cracked ring glands
Bent valve(s) <not as uncomomon as you'd think
Carbon build up on valve seat and face(causing valve to hang open)
Bad valve guide (hanging open valve)
To long of push rod
Improperly adjusted rocker arm
Broken springs
Head gaskets
If two cylinders are low next to each other then prob blew between the two.
If one is low and both right to lef tof it are "normal" prob blew into collant or oil passage. (check for excessive blow by or collant carbon contamination)
Cracked block
Cracked head
 
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