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low compression on one cyl, but steady vacuum read

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R

recordsj

Jedi Warrior
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I did a compression test and saw the following on both dry and wet compression tests:
cyl 1) 120 psi
2) 140
3) 90
4) 140

But when I did a vacuum test, it held steady @ 15 inHg, but jumped +/- 1 inHg.

From the two tests, the compress tests points to a valve problem. But with the vacuum there isn't enough jumping of the vacuum gauge to point to the issue.

I have also have issues of oil coming out of the dipstick tube which points to a valve or piston issue, but the tests above is inconclusive....

Any ideas?
 
Blowby is cylinder/piston/rings.
If you have enough pressure at the intake stems to get blowby, the valve and guide are shot along with the face and seat, and only with throttle closed and all other valves closed.

Did you oil the cylinders and do it again?
Compare wet/dry.
If it comes back to correct (or near), rings.

THAT will give you blowby (oil coming out the dipstick)
 
I did do the wet and dry compression test, results were the same. From that it pointed to be a valve issue. But I am a little confused since the vacuum test showed 15 inHg +/- 1 inHg, which I am thinking is realatively steady. So from the vacuum test I would expect a jumping readout if it was a valve problem
 
Do a blowdown test or just put pressure on #3 with #3 at top dead center on the compression stroke. You may be able to hear flow at the exhaust or intake or dipstick hole.


Marv J
 
don't have the equipment to do a blowdown test (which I assume you mean a leakdown test?).

If there is an issue with a valve, would you expect to see excessive blowby?
 
If you have oil coming out the dipstick hole, I would suspect some blowby. I don't have a leakdown tester either but I busted the guts out of an old sparkplug and brazed in a connector so I can hook up compressed air. It's been handy. I wouldn't rule out anything from what you've seen so far.

Marv J
 
This is another vote for a leakdown test. Even without a tester, just feeding compressed air in as Marv suggested will be enough to tell you where #3 is loosing it's compression.

15 +/-1 is still low. A healthy engine properly tuned will be closer to 20". What does the vacuum gauge do if you blip the throttle?
 
don't have a compressor either...

if I blip (I assume you are talking about just a quick rev of the engine and then letting the throttle go to idle) the throttle (I am kind of going on memory of what happened earlier today), the vacuum gauge drops and then goes up from the 15 inHg
 
You can get an adequate leakdown tester at Harbor Freight. But then you would still need a compressor. It may be time to find a friend with the tools or maybe (heaven forbid) even a professional. Actually, it shouldn't cost much to have it checked by a pro. Then you'll at least know how to approach the fix.
 
Google Leak Down Tester and you can see how to convert the compression tester to a leak down tester. < $25.00 as I recall.
 
If I am getting excessive blowby, wouldn't that lead to mean that it is a piston ring issue and eliminate the issue of an exhaust valve?
 
--that would be my guess. It won't eliminate the valve question but it would put the focus on the rings.
 
Bought equipment to do the leak down test.
The leak down test showed a 40-50% leak rate on all cylinders(I have a two gauge system from Harbor Freight).In doing the test I did make sure the valves were closed (opened the valve cover and checked to see if I can move the rockers).

I did a compression test and saw the following on both dry and wet compression tests:
cyl #1) 120 psi
#2) 140
#3) 90
#4) 140

I have seen oil come out of the dipstick tube when the engine was running (none coming out during the tests).

But when I did a vacuum test, it held steady @ 15 inHg, but jumped +/- 1 inHg.


I feel air coming out of the dipstick tube, no noticable air from the carb, hard to tell if any air is coming if at all from the exhaust (the exhaust manifold is still connected with the engine in the car).

Any thoughts on the root cause? I would of expected a much higher leakage rate out of cyl #3.
 
I reran the test after realizing I had a broken gauge. Found a high leakage rate on #3 as one would expect and it looks like bad piston rings or cylinder bore.
 
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