Air Cooled VW Compression Test - COMPLETE GUIDE

[@kentbarnard1000]

This was an awesome video! I have never done a compression test, and now I've saved this for when I have to. Right now my issue on my 79 FI bus is that he wont idle, unless the brake assist hose is pulled out. Then the idle is rough. Worked great up until about a week ago. Have checked and cleaned all the wires, teste the 3 & 4 FI with a noid, and evertyhing is working. Just haven't tried fuel pressure test. Have you got a video for that? :)

[@ericknoblauch9195]

My father had a 1600 dual port in a 1971 VW. The compression in the #3 cylinder would drop below 115 every 30,000 miles. The problem was always a valve, and never was a sealing or ring problem. The engine also sucked the exhaust valves in the same cylinder down into the crankcase. It happened three times.

[@MaineMachinist]

Something very wrong was going on in that situation….

[@Aircooledbliss]

very helpful video, why does the throttle need to be in a wide open position? many thanks!

[@MaineMachinist]

To allow the engine to draw the maximum amount of air during the test Thanks for watching!

[@Aircooledbliss]

@@MaineMachinist thanks for clarifying man ✌️

[@italo195]

Nice vid, I am fairly new to air-cooled VWs and been learning about them lately, I'm planning to do a compression test on a stock 1600 dual port Would elevation affect the results of this test? My logic says so, that they should be lower, but I might be wrong... I live at around 9000ft or 2800m

[@fakhroo5]

thank u

[@MaineMachinist]

Thanks for watching!

[@vintage76vipergreenBeetle]

Could you do a leak down test on that engine?

[@MaineMachinist]

Yes you can, and I will probably make a video showing that sometime soon

[@vintage76vipergreenBeetle]

@@MaineMachinist Did you hone those 87mm cylinders before installing them? You got great compression for 87mm P&C.

[@MaineMachinist]

@@vintage76vipergreenBeetle yes! In fact they were a used set. They got a real quick hone on the Sunnen machine and a set of Total Seal rings

[@vintage76vipergreenBeetle]

@@MaineMachinist No wonder your reading were so good. You did the right thing with those 87mm P&C. I read that even new cylinders need a light honing to work well. A high quality set of rings helps too.

[@MaineMachinist]

@@vintage76vipergreenBeetle yes, even brand new sets of AAs especially can use a very very quick sweep with the hone. But even the Mahles aren’t perfect anymore.

[@jlibb099]

VERY interesting

[@MaineMachinist]

Thanks , Jeff. What did you like about it?

[@jlibb099]

@@MaineMachinist The fact you tested compression twice, once hot and then cold. Never had done it that way.

[@MaineMachinist]

@@jlibb099 in my opinion it really does give you a more complete picture of what’s going on

[@MaineMachinist]

@@jlibb099 thanks for watching!

[@oogie-boogie]

i had a sand rail motor that had 300psi in each cylinder,, i was like what! naaaah i checked it 3 times and yep, it was a 1835

[@JONWICK-oo6mt]

a friend just put together a vw motor and it won't start when turning over instead of sucking in the carburetor it blows out what has he done wrong?

[@MaineMachinist]

Do you mean the carburetor makes a popping sound or it’s coughing? Or are you saying if you took the carburetor off and put your hand over the intake manifold that it would push air OUT against your hand instead of sucking air IN

[@JONWICK-oo6mt]

with the carb on it pushes air out instead of sucking could the distributor drive gear be installed wrong by the way thanks for your help@@MaineMachinist

[@MaineMachinist]

@@JONWICK-oo6mt first thing take the carburetor right off and crank it over and see what it’s doing

[@MaineMachinist]

@@JONWICK-oo6mt if it’s a dual port take the center section of the manifold off and see if both sides are doing the same thing

[@JONWICK-oo6mt]

we finally found out what was happening tore the motor completely down and found one of those plastic bushings that hold the wrist pns in had fallen down and was keeping the valve open.Put it back together and it runs thanks for all your help @@MaineMachinist

[@paulcostinett886]

I don't understand why I need to know the average or the variance across the engine for each test. Problems are on a cylinder by cylinder basis. And since you numbers are so good, why would I want to know what the variance or average is for a bad engine? If the readings are out of tolerance per cylinder cold, and out of tolerance per cylinder hot, what use are those other two stats? I'm not nitpicking, I just don't understand why you bother to figure those up? The "wild changes" will appear from test to test.

[@MaineMachinist]

If you don’t feel the need to test cold and hot, or with an average reading, then don’t do it. I prefer to gather as much information as possible. If the compression suddenly drops when it’s hot, this could indicate a crack somewhere that has now expanded, or another issue. It takes such little time to do a thorough test, I just don’t see what it hurts, and there’s a chance you’ll notice a pattern.

[@paulcostinett886]

@@MaineMachinist I understand getting data. And please don't think I'm criticising the technique. I'm actually trying to understand it thoroughly. I need to test a 1600DP this weekend. What I don't understand is how the variance is useful to you in determining a problem. Variance is about dispersal across the numbers. Does the manual state a variance as equal to a cylinder by cylinder loss of pressure? If cyl 1 is 125 psi cold then 85 psi hot, with all other cylinders within range, how does that variance help in determining the issue? In other words, how are YOU using that statistic so that I can use it. What does the variance specifically tell you?

[@anthonyrstrawbridge]

​@@paulcostinett886I think he said it may be a clue or a reason to investigate further. And that the compression test is generally inaccurate and unreliable. You might consider using a differential pressure cylinder leakage tester in the future. ✌🏻👶🏼🚬

[@toddwoods1167]

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