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Sun Jul 31, 2011 7:33 am

Can some please tell me the right way to check compression on a 1970 bug. Does engine need to be hot, are all plugs taken out before test is done how many times does engine need to crank to get proper reading

Sun Jul 31, 2011 8:23 am

I do not reccomend compression gauge testing on V.W. especially on the 1/2 inch reach spark plug head's .
V.W. is infamous for crackin head's between the plugg hole's and valve seat .
This is a combination problem of many variables but the one re occurring variable is removal of plugg's from a warmed up engine sportin aluminum head's .
I prefer instead to turn the engine by hand and feeling the compression .
Feel a low cylinder check the valve's to see if adjustment of the valve is to tight and holding a valve open allowing for compression to bleed off .
You wont get accurate result's from compression testing on a cold motor anyhow ?
If mechanical failure is the culprit causeing low compression you can either feel it by hand or it will runn well enough to indicate a miss in a particular cylinder that you can both look at the plug and or hear with a stethoscope .
In other words there are different way's of determineing if you have a badd hole or naught that dont require tortureing plugg hole thread's .
Sean

Mon Aug 01, 2011 7:11 am

Do this:

Remove all four spark plugs. The compression tester usually has a few different hoses/fittings for different sized spark plugs; Choose the ones that fit your spark plug holes. Attach the gauge/hose assembly to one spark plug hole (just threads in, hand tight. There should be a rubber washer on the end to make a seal against the cylinder head). With a fully charged battery (it matters), crank the engine with the key with the accelerator fully depressed (wide open throttle). Crank the engine until you hear 5-6 compression strokes (you will hear it). After that, look at the gauge and see what it reads. Record the reading for that cylinder, taking note of which cylinder as well. Repeat the process for the remaining cylinders. A fully charged battery is imperitive, because cranking speed factors into the compression. Once you have these numbers for each cylinder, post the results.

Also, make sure to remove the center coil wire from the coil. We don't want stray sparks!

Do it on a cold engine first, as candy said above, the plugs can rip threads out of aluminum heads if removed when hot.

Mon Aug 01, 2011 11:18 am

Paul Windisch wrote: ... crank the engine with the key with the accelerator fully depressed (wide open throttle).

What happens when the accelerator is not fully depressed when cranking? I just did my first compression test this weekend but left out that step. :oops:

My results were 100 in #1, 110 in #2 and 120 in both #3 & #4. While all readings were 100+, from what I read the spread between all the cyclinders should be closer. Within 10% or something, right?

Mon Aug 01, 2011 1:40 pm

Not on a cold engine ?
Yeah Im kind of suprised tho that #3 isnt the low one ?
The compression on number one might come up if you back the valve adjustment off a lil .
Maybe naught ?
Bugg's are known for havin a cold piston pinn slapp kind of hard to get accurate result's from compression testing unless the engine is thoroughly warmed up before you beginn .
Wich leave's you trying to carefully manipulate pullin plugg's on a hot aluminum head motor .
Wich is bogus in of the fact that these car's are infamous for spittin the spark pluggs out of them and crackin the head's between the plug hole thread's and valve seat .
Wich might explain why number 1 is low ? LMAO Im just brutal this mornin never mind me ?
Do as you please ? :twisted: LMAO
Ohhh and it's 10 PSI difference between cylinders on compression testing your Dad's old Chevy ??? As in metaphor !
Sean

Thu Aug 04, 2011 4:30 am

slione wrote: Paul Windisch wrote: ... crank the engine with the key with the accelerator fully depressed (wide open throttle).

What happens when the accelerator is not fully depressed when cranking? I just did my first compression test this weekend but left out that step. :oops:

My results were 100 in #1, 110 in #2 and 120 in both #3 & #4. While all readings were 100+, from what I read the spread between all the cyclinders should be closer. Within 10% or something, right?

It will not show as much compression sometimes if the throttle is not wide open, that's all. On a warmed up engine, the compression will be slightly higher than what you have now, your engine should be okay.

Thu Aug 04, 2011 6:27 am

Here is the best rap I have seen on doing a compression test...

Key words are warm engine, wide open throttle, fully charged battery, oil squirt....

You don't have to do the FI stuff since you don't have it anyway.

http://www.ratwell.com/technical/FAQ/FAQContent.html#0101

http://www.ratwell.com/technical/FAQ/FAQContent.html#0105