1979 302 compression test results
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1979 302 compression test results
Finally did my first ever compression test, on a rough running 302.
Engine was rebuilt about 10,000 miles ago, ran great until suddenly power dropped on highway and started to hiccup.
Here are the readings I got:
1- 130
2- 120
3- 35 !!
4- 130
5- 155 ??
6- 135
7- 120
8- 140
So clearly cylinder 3 has a big problem, but 5 also looks too high no?
What is the general rule on acceptable highest/lowest reading?
If 3 is SOO low what am I looking at?
Engine was rebuilt about 10,000 miles ago, ran great until suddenly power dropped on highway and started to hiccup.
Here are the readings I got:
1- 130
2- 120
3- 35 !!
4- 130
5- 155 ??
6- 135
7- 120
8- 140
So clearly cylinder 3 has a big problem, but 5 also looks too high no?
What is the general rule on acceptable highest/lowest reading?
If 3 is SOO low what am I looking at?
Lead Driver
Joined: Aug 2003
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From: Boulder Creek, Ca
Not that knowledgable but just happened to have done a compression test recently; beleive the ratio is around 20% difference between high/low. 
If the max reading is running around 155 then the min. should not be lower than between 115-117. Except for #3 appears the other's are within limits. Definately somethings going on with #3, suggest try adding a drop of oil in the cyl. and if there's an increase then possibly rings; although, with that much of a difference possibly head gasket but suspect valve, or valve seat issue. A ' leak down' test would probly pin point the problem.
If the max reading is running around 155 then the min. should not be lower than between 115-117. Except for #3 appears the other's are within limits. Definately somethings going on with #3, suggest try adding a drop of oil in the cyl. and if there's an increase then possibly rings; although, with that much of a difference possibly head gasket but suspect valve, or valve seat issue. A ' leak down' test would probly pin point the problem.
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If I add a few drops of oil to the cylinder, there you should a small increase in pressure either way right? If the rings are still good?
Is it a big increase in pressure that would confirm a ring problem?
Is it a big increase in pressure that would confirm a ring problem?
Lead Driver
Joined: Aug 2003
Posts: 6,305
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From: Boulder Creek, Ca
3-5 drops of oil with an increase in compression is an indication rings are worn, no significant increase in compression is an indication the problem is with valve or head gasket. The results of the compression test on my 302 is that the cyl. were all within limits; however, there are a couple of cylinders where it would take 5-6 cranks to reach max.( increase in reading with each stroke) which according to neighbor (machanic) is an indication the rings are slightly worn.
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Hi willow billy
A buddy of mine was saying a valve too
Im going to take the valve cover off on the side which was number 3 and turn the engine over by hand, see of I can see anything abnormal on number 3
What are specific things I should be looking for? Broken spring? Bent or broken valve?
A buddy of mine was saying a valve too
Im going to take the valve cover off on the side which was number 3 and turn the engine over by hand, see of I can see anything abnormal on number 3
What are specific things I should be looking for? Broken spring? Bent or broken valve?
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So I did a wet compression test on #3 and there is no increase in compression whatsoever.
Valve problem then I assume?
I took the valve covers off, don't see anything abnormal at quick glance, will have a helper tomorrow to turn engine over by hand while I check valve movement on 3
What should I be looking for exactly??
Valve problem then I assume?
I took the valve covers off, don't see anything abnormal at quick glance, will have a helper tomorrow to turn engine over by hand while I check valve movement on 3
What should I be looking for exactly??
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Whether or not it's sticking would be about the only thing you can see with the head on the vehicle.
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We use the vehicle as the band van on tour, leaving next Sunday!
If I do happen to see a sticking valve what are my options?
I was hoping to see a broken spring or something else I know how to fix.
Someone local is selling a set of barely used 302 heads for a bargain, so I guess I could swap em out by next sunday and deal with this problem later.
If I do happen to see a sticking valve what are my options?
I was hoping to see a broken spring or something else I know how to fix.
Someone local is selling a set of barely used 302 heads for a bargain, so I guess I could swap em out by next sunday and deal with this problem later.
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I don't have a leak down tester, only compression gauge.
Can I just hook up my compressor to the #3 cylinder while at Tdc to listen for leakage at carb, tailpipe, or pcv?
If so, how much psi on compressor?
Can I just hook up my compressor to the #3 cylinder while at Tdc to listen for leakage at carb, tailpipe, or pcv?
If so, how much psi on compressor?
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So i hooked up my compressor with the #3 cylinder at TDC and I can hear/feel air rushing out of the top of the carburetor!
burnt/bent intake valve?
I have someone locally here who is selling some new aftermarket 302 heads, thinking of swapping old ones out, get em fixed later.
Considering this is an econoline van, not a mustang/race car, what should I be looking for as replacement heads? going to see them later today, don't know if they are performance heads or .....
burnt/bent intake valve?
I have someone locally here who is selling some new aftermarket 302 heads, thinking of swapping old ones out, get em fixed later.
Considering this is an econoline van, not a mustang/race car, what should I be looking for as replacement heads? going to see them later today, don't know if they are performance heads or .....