compression test question

-
its more important that they are all within 10%-15% of each other.i would expect around 125psi
 
both! each cylinder should be with in the 10-15% of each other & that 125 psi is his guess at your compression number,
what you want is all cylinders close together-- not big changes between them -- for example cyl 1 = 125 psi cyl 2 = 75-- not good. Lawrence
 
its more important that they are all within 10%-15% of each other.i would expect around 125psi
For a stock street engine with the Chrysler gear reduction starter and no major carbon buildup, that would be about right. For one with a Nippon starter, that would be a little low. For a carbureted race engine, that would be pitifull......
 
I was getting high 70s in my '80 /6 but they were all pretty close together. I was pondering how to proceed. It was after doing a valve lash adjustment. What would be the first thing to explore when having low compression?
 
After making sure there is plenty of valve clearence and the engine is cranking fast enough....I'd squirt some oil in each cylinder and recheck the compression. If it stays about the same, the valves are leaking. If it goes up quite a bit, the rings are worn out. I assume this is a running engine? Does it smoke?
 
You must hold the throttle flat out when doing a compression test so the motor can draw air to compress. This will increase your numbers
 
As I said, it may bring them up 5lbs on the gauge at most. Think about it....the engine will run at idle with the throttle in the same position as when you're doing the compression test so there is enough air flow to produce compression under those circumstances plus if you have all the plugs out, you only need to draw air for the one cylinder you're testing. The others will move air in and out their spark plug hole.
 
Here are my compression numbers:
1- 100
2-90
3-100
4-105
5-85
6-70

looks like somethings up on no.6 maybe carbon buildup? Any suggestions?

Thanks Gary
 
Might try running a cylinder leak down test on 2,5 & 6. find out where your leak is.
 
Here are my compression numbers:
1- 100
2-90
3-100
4-105
5-85
6-70

looks like somethings up on no.6 maybe carbon buildup? Any suggestions?

Thanks Gary


do a "wet test" on that #6, throw a small amount of oil in the cylinder and see if the compression changes greatly, if so you should suspect your piston rings. I'd do the same thing on the #5. search "wet compression test" in your handy dandy web search bar.
 
As I said, it may bring them up 5lbs on the gauge at most. Think about it....the engine will run at idle with the throttle in the same position as when you're doing the compression test so there is enough air flow to produce compression under those circumstances plus if you have all the plugs out, you only need to draw air for the one cylinder you're testing. The others will move air in and out their spark plug hole.


:thumbrig:

unless all the vacuum from the running cylinders at idle opens the throtle valve a bit, but I dont think this happens
 
It has a miss, once in a while you hear a slight pop at the tailpipe, could it be a burnt valve, or maybe I need to redo the valve lash adjustment?
 
Pull you valve cover back off, make sure you have some valve clearence, retest the compression both dry and wet. If the compression is still low like it was, you'll need at least a valve job. While you have the valve cover off, check the springs for any that may be broken or real weak. That may cause it too.
 
This car will do 80 plus on the freeway easy, its when youre at a red light all the vibration at idle that drives me nuts.... Car dont smoke or knock...leads me to believe valve job time....
 
More than likely you're right but a tight valve adjustment will give you the same readings.
 
I recently did a comp test on my old, thought to be worn out, 225 and all the cylinders were around 150psi. Lowest was 140. This motor didn't have a carb on it, and prolly around 113k miles. I thought it was more worn out but guess not..
 
Is it just me, or do I kill a lot of threads?
 
-
Back
Top