Need advice. how to check compression, procedure details

deQuincey

deQuincey

Quousque tandem...?
Site Donor
Messages
8,877
Reaction score
2,777
Location
BIO - 43°15'46.5"N 2°56'03.7"W
Considering a Xmas holidays garage stop

never did the compression test myself

this is what the book says:

1576660124648.png



my understanding:

warm up the engine,
remove one spark plug at a time, (or maybe remove all spark plugs ???)
place the tester in one cylinder
????? throttle ?????
switch on the starter enough time until the reading stops and does not move forward

could you please comment and give details, or tips ?

thanks
 
Usually remove 1 spark plug at a time, remove ignition cables so the engine can't start. Just crank with gas pedal fully pressed (wide open throttle). Crank for a couple of seconds.
It's easier if you have someone who can sit in the driver seat and crank for you while you hold the pressure sensor in place.
 
I have a screw-in compression tester and a remote start switch. You want the throttle wide open to allow the maximum flow into the cylinder. You also want to use the same number of cycles for each cylinder, something like 5. I usually do a couple of reps for each cylinder to make sure the data is reliable.
 
NilsH said:
Usually remove 1 spark plug at a time, remove ignition cables so the engine can't start. Just crank with gas pedal fully pressed (wide open throttle). Crank for a couple of seconds.
It's easier if you have someone who can sit in the driver seat and crank for you while you hold the pressure sensor in place.
Click to expand...


removing ignition cables, ok,
pedal fully pressed, ok, that was my question

then i wonder why one at a a time, if you remove all you will allow rest of cylinders to be free from compression, it seems correct to me, anything against this ?
thank you
 
Mike Goble said:
I have a screw-in compression tester and a remote start switch. You want the throttle wide open to allow the maximum flow into the cylinder. You also want to use the same number of cycles for each cylinder, something like 5. I usually do a couple of reps for each cylinder to make sure the data is reliable.
Click to expand...

all details noted, Mike, thank you
 
deQuincey said:
removing ignition cables, ok,
pedal fully pressed, ok, that was my question

then i wonder why one at a a time, if you remove all you will allow rest of cylinders to be free from compression, it seems correct to me, anything against this ?
thank you
Click to expand...
Removing all spark plugs at once is ok as well, I don't see a problem with it.
Here are instructions for the classic, mechanical motometer with the small recording charts. Cool!
https://www.motometer.de/wp-content/downloads/de/KPS_Betriebsanleitung.pdf
 
DeQ, I'd also recommend doing a leak down test too. On my Golf GTi resto I had comp readings all over the place which was due to my head builder not lapping the valves in. Saved ripping my bottom end apart!
 
I remove all spark plugs so the engine is not drawing in gas from any of the carburetors. Also makes it easier on the starter and battery. Hand tight should be fine as tighter can damaged the o ring.

I always let the engine cool a bit because full hot makes it easier to gall the plug threads and damage the tester o ring seal. When reinstalling plugs, a bit of silver or copper antiseize on the threads will reduce the chance of my damage to the head.

I think compression tests should be combined with leakage tests to give you full info.
 
Mototmeter manual figure 1.............they were able to predict the future....... it looks like my e90.
 
I like to remove all the plugs, which seems to help the engine turn over more freely and keeps that battery "fully charged" longer. As I have an added electric fuel pump, I also disconnect it so it isn't pumping fuel up to the carbs during this process.
 
nosmonkey said:
DeQ, I'd also recommend doing a leak down test too. On my Golf GTi resto I had comp readings all over the place which was due to my head builder not lapping the valves in. Saved ripping my bottom end apart!
Click to expand...


thank you, can you comment on the leak down tester, how to do it, which tools are required, and so on
thanks
 
deQuincey said:
thank you, can you comment on the leak down tester, how to do it, which tools are required, and so on
thanks
Click to expand...


You'll need a leak down tester which you can purchase or possibly borrow from a friend. You'll also need a compressor for the tester..

Here's a decent generic video I used before with tips on locating where a significant leak may be coming from (intake or exhaust valves or the rings). Hope it helps!

 
20% leakdown is 20% of what? PSI has a solid definition and is traceable to the NBS. A leakdown tester has an arbitrarily selected hole that you measure the drop across. The percent leakdown depends entirely on the orifice size. So in my shop I have two testers, one with a big orifice and one with a small orifice. If I want good results I use the large orifice, if I want worse results I use the small orifice.
 
Key point with a leakdown test is that if there is a leak in the system it's far easier to identify. In my case listening to air coming out of both intake and exhaust manifolds.
 
Another aspect of leakdown testing not mentioned yet is the proclivity of the cylinder to migrate to the bottom of the cylinder when you apply air pressure to the top of the piston. Most experienced testers like the piston near the top on the compression stroke. Unless you have a method of locking the crank in position it's headed for the bottom when you turn on he air.
I watched the video and noticed that he didn't re-calibrate the tester once the air was flowing. I always turned on the air then set the primary gauge to 100 before I read the secondary gauge. If you don't correct for the compliance of the regulator you will get bad readings.
 
I agree with Mike. The piston should be at the top for the leak down test. My procedure for holding it there is to put the car in 1st or reverse and put the hand brake on hard.
Chock the wheels as well so you don't run over your foot. It will shock you how much pressure 100 PSI puts on a piston.
I usually do as Mike says and adjust the airflow to 100 PSI so when you do all 6 cyl you have a standard to compare each cylinder to the others.
Also makes it easy to talk in percentages of leak down. 100 PSI base and you get 80 PSI on gauge then you have a 20% leak down.

For compression testing to make the engine easy to turn over remove all the spark plugs and yes the throttle needs to be open at least half way. On my 72 and I'm sure many others they have an orange diagnostic connector near the battery. You can make a push button with two wires and pins on the ends of the wires. Finishing size nails could work. Insert the pins in pin #1 and #9 in the orange diagnostic plug on the fender well. Sorry don't have a photo to show you which pins. I think the wiring diagram shows the pin numbers. I remember I didn't have much trouble finding the pin numbers. They may even be inside the lid of the diagnostic connector. Basically one of those pins is ground and the other goes to the starter motor.
This way you don't have to go around the turn on the key, your ignition and maybe your electric fuel pump. BTW take the fuse out of your electric fuel pump for compression testing if you use the ignition key. You don't want gas washing down the cylinder walls and taking the oil away from your rings which would make your readings even worse.
You can listen carefully at your intake or exhaust while the leak down tester is putting out air and possible hear which valve in that cylinder is the worst.
You can also check to see if your rings are sealing well. Once you get a reading for a cylinder like maybe 80 you can squirt a few squirts of engine oil in the cylinder, crank the engine over a few times and measure again. If the reading improves then some of the 20% leak down was your rings.
Yes, your motor will smoke some when you start it up from the oil in the cylinders but it won't hurt anything and will be burned off before you leave your neighborhood.

I just want to say that a leak down test can be very disturbing to a home mechanic. This is a static number and does give you an idea of leaking or bent valves.
You have to remember that when the engine is running the compression stroke is the only time the valves are under pressure so the leak time is very small and gets shorter and shorter the higher the rpm. So an engine with 20% leak down across many cylinders will actually run pretty well at any driving speeds you would use.
It may not idle as smoothly but the leak down numbers would have to be 40% or more for you to be able to really notice.
So if you do a leak down don't freak out. But if you have one or two cylinders that are 30% or more then the valves in those cylinders probably have some sealing problems.
 
thank you all very much for your responses

it seems that the compression test is the easy part, and the leakdown is more elaborated

i think i will try to put all this together and see how to do a reasonable check of the engine
 
for compression testing, you want to get as much air flowing (and compressing) as possible, so keep the throttle wide open so the "suck" is not just drawing a vacuum. I keep turning the engine until the needle on the gauge is stable(ish). Important to be consistent with this on all 6 cylinders. I also remove all the plugs before I start, partly to enable the engine to spin faster on the starter, and partly as it's easier, especially as I'll probably repeat the test at least once. As you progress, the battery may get a bit flat, so be alert to the sound of the starter as you go

for leak-down testing, I have built a DIY tester from some air tool valves, lines, etc. The orifice plate is a 1mm hole drilled through an epoxy filled connector. I've always managed to balance the piston at TDC when doing this. It doesn't work every time, but I have a 360 degree protractor which I fix on the vibration damper to allow a degree of accuracy when setting to TDC. To build the fitting that screws into the spark-plug holes I smashed the ceramic centre out of a spark plug and glued some compressed air line through the metal part. I built this tester kit when a top-end rebuild on my E24 took a wrong turn and the rings got gummed up with carbon the deposits I'd scraped off the piston crowns (whilst I was there...:mad:). The leak down test clearly diagnosed poor ring sealing, which was then fixed with a slug of ATF in each plug hole.
 
You must log in or register to reply here.
Back
Top