[defdaboss]

Mine read 135 on one side. havent got the chance to do the passenger side yet. is this a good number for a stock engine?

 

[SSandman]

That sounds a little low...but it could be a few things. Leaky tester, not screwed in all the way, engine not warm ect... could cause an incorrect reading. HOw many cranks did you give it, or was that max?

The main thing you want is that there isn't a large difference between cylinders. FWIW, this is how I have always compression tested...

Get motor warm (i know it's a pita with hot headers, but the motor really needs to be warm)

Prop open TB blades (I don't always do this, but for best results it should be done)

crank the motor over and find the highest psi you can get. Take note about how many cranks it takes to get that PSI.

If you get a low PSI, try squirting oil down the cyl bore. If compression goes up afterwards, it's a worn ring/ring seal problem.

 

[defdaboss]

okay then i failed these two. i didnt get it in tight. the tool man told me i wasnt suppose to screw it in too tight so he didnt give me the extra tools to make sure it's enter in tight. and the motor wasnt warm cuz i wanted to avoid burning my hands since it was 9:25pm. i'll try again monday. i gave it about almost 2 second crank

 

[Gregg-O]

With all the plugs out you will hear a steady 'miss' like sound when the engine turns over because of the one cyl. that has the compression tester on it. I used to let it go 5 times, you should have hit peak PSI by then. Hot or cold shouldn't make too much difference, besides, if hot, from when you do the first one to when you are done the last one the engine is going to cool down anyway so doing it cold would eliminate any temp difference.

 

[Buffman]

crank until gauge peaks which is around 5 times. TB blades open. Also need to disable the fuel injectors so you're not spraying fuel down into the cylinders either. two fuses under the hood fuse box do that. Make sure unit is in there tight, but not too tight. 135psi is indeed very low. If consistent it's a good thing, but still low. My 165K LT1 had 200 psi of compression on all cylinders.

 

[SSandman]

Make sure unit is in there tight, but not too tight. .

Yea, you shouldn't need any tools to crank it down, just get it as hand tight as you can get it.

If you are concerned about the low pressure, you can try the oil down the bores. If that doesn't bring it up, you can move to a leakdown test.

I use compression tests a lot at work, but it's not really meant to be a definitive answer as it's not always the most accurate test. It's great when you find a bummed out cylinder, like 100psi when the rest are 190, but a compression test showing a consistent 30psi or less lower than where a good motor should be could mean many things, including just a bad gauge.

And like buffman said, if you were spraying fuel due to not pulling the injector fuses, that will cause a low psi as fuel washes away the oil.

 

[defdaboss]

my teacher who was suppose to teach compression test taught it for a short moment off the book. we didnt get to do a hand on experience. on wednesday we was working on the rear drum of a jetta and i caught a student returning a compression test and i figure i rent these and try it out.

i didnt holding the gas pedal down (throttle open) so i did it all wrong. i'll try it all over again on monday.

i took out one spark plug at a time and reinstall them and do the next cylinder. is it nescessary to remove all plugs?

 

[Buffman]

removing all plugs at once will allow the starter to work easier, but isn't that necessary.