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Old 12-08-2019, 03:02 PM   #1
old31
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Default Compression test and Vacuum test readings need help

Can you help decipher what the numbers are saying, and what would you do?

I have a 6.1 head, bcarb, intake bored.

I forgot to put oil in the cylinders after each cylinder test, throttle is wide open, engine is warmed up.

Compression test. 1:100 2: 90 3: 70 4: 70

Vacuum test: Idle 20. At idle the needle bounces quickly between 20 and 22.
High RPM 18

Needle is always steady throughout the test, except at idle

When I let increase throttle quickly it drops to 0 then goes to 25, then back to 20.

Last edited by old31; 12-08-2019 at 05:12 PM.
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Old 12-08-2019, 09:27 PM   #2
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Default Re: Compression test and Vacuum test readings need help

Looking at the following guidelines on this page:

Quote:
Gauge will indicate loose valve stem guides when gauge pointer pulsates rapidly at idling speed. This rapid pulsation disappears with the increasing speed of the engine.

Gauge will indicate a choked muffler when engine is speeded up several times in succession and the gauge pointer drops slowly. A clear muffler is indicated by a quick drop to zero.
So the drop to 0 seems like it's normal behavior, and the pulsating at idle could indicate loose valve stem guides.
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Old 12-08-2019, 10:39 PM   #3
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Default Re: Compression test and Vacuum test readings need help

https://www.racingjunk.com/news/2019...rmance-issues/
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Old 12-09-2019, 11:26 AM   #4
Jim Brierley
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Default Re: Compression test and Vacuum test readings need help

3 and 4 being lower could mean the head gasket is leaking between the two. Try re-torquing the head first, may need a new gasket? A leak-down test would be more conclusive. If that is the problem, it could also cause the vacuum readings you have.
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Old 12-09-2019, 11:31 AM   #5
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Default Re: Compression test and Vacuum test readings need help

I would check compression on cylinders 3 and 4 with oil squirted in cylinder bore,if compression comes up an appreciable amount could be scored cylinder or rings.If there is very little difference possibly worn valve guides,tappet clearance or valves not seating,could also be head gasket between 3 and 4.process of elimination.
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Old 12-09-2019, 11:48 PM   #6
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Default Re: Compression test and Vacuum test readings need help

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Mike, great article.
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Old 12-09-2019, 11:54 PM   #7
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Default Re: Compression test and Vacuum test readings need help

The head was retoruqed before the test, both hot and cold.

I will redo the test and squirt oil in the cylinders and report back.
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Old 12-10-2019, 09:54 AM   #8
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Default Re: Compression test and Vacuum test readings need help

Follow up:

With oil number 1.90 number 2.90 number 3. 75 number 4. 70
No oil. 100, 90,70, 70.

What does that tell you?
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Old 12-10-2019, 10:02 AM   #9
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Default Re: Compression test and Vacuum test readings need help

Tells you that whatever's causing the disparity in compression numbers between 1/2 and 3/4, it ain't the piston rings.
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Old 12-10-2019, 11:10 AM   #10
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Default Re: Compression test and Vacuum test readings need help

A leak down test will tell you exactly what is going on!! Air coming from carb=intake valve, air coming from exhaust = exhaust valve, air leaking from oil fill= rings, air leaking into an adjacent cylinder= head gasket. If you don't have a leak down gauge I 'am sure a local auto parts store would loan or rent you one.
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Old 12-10-2019, 11:53 AM   #11
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Default Re: Compression test and Vacuum test readings need help

To my knowledge none of the national auto parts chains stock loaner leak-down kits, but you can get one at HF for $30.

Edit: Never mind, apparently the kit is discontinued and I couldn't find a local store that still had one. Now is the time to get this kit if you want it and can find it.

Last edited by alexiskai; 12-10-2019 at 12:01 PM.
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Old 12-10-2019, 12:27 PM   #12
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Default Re: Compression test and Vacuum test readings need help

It is undoubtedly valves. You can just force compressed air into the spark plug hole and listen for where the air leaks out, likely into the exhaust pipe.
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Old 12-10-2019, 12:57 PM   #13
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Default Re: Compression test and Vacuum test readings need help

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Originally Posted by alexiskai View Post
To my knowledge none of the national auto parts chains stock loaner leak-down kits, but you can get one at HF for $30.

Edit: Never mind, apparently the kit is discontinued and I couldn't find a local store that still had one. Now is the time to get this kit if you want it and can find it.
Check Amazon.com. They have numerous Leak Down Testers, from this inexpensive one on up. https://www.amazon.com/GOTOTOP-Compr...00636497&psc=1
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Old 12-10-2019, 02:10 PM   #14
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Default Re: Compression test and Vacuum test readings need help

$23.97 with coupon

https://www.harborfreight.com/cylind...ak+down+tester
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Old 12-16-2019, 02:44 AM   #15
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Default Re: Compression test and Vacuum test readings need help

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Originally Posted by Jim Brierley View Post
It is undoubtedly valves. You can just force compressed air into the spark plug hole and listen for where the air leaks out, likely into the exhaust pipe.
The first time I became aware of leak down testing I was a teenager. I will never forget, a aircraft mechanic was performing it and I was about to walk into the propeller arch when the propeller spun, OMG it would have made hamburger of me. We had along talk after that, with only 60 psi that he was putting into the cylinder I asked what if I could hold it " being young dumb and strong" he answered no way! No way to put it in gear and choke the wheels like in a model A.
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Old 12-16-2019, 04:38 PM   #16
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Default Re: Compression test and Vacuum test readings need help

If your compression tester is the kind with a flexable hose and the check valve is at the spark plug end and the gauge can come off you'r in business. Rig up some fittings to put a male air hose fitting where the gauge was and regulate the PSI down to 35 and the rest a matter of getting the hole to be tested up to dead center on the firing stroke. Have the throttle open to hear intake leaks and the oil filler cap off for exhaust leaks.
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