How do I do a compression test?

[PsYcHo GN]

I'm going to purchase a compression gauge tomorrow to do a compresson test on my engine to see if it's a head gasket issue that is causing excessive blow-by.

How do I do a compression test on these 3.8 engines? What needs to be disconnected? How do I get the most accurate readings?

Thanks in advance!

 

[boostingbuick]

The best way to test the engines health is to do a leak down test. However to do a comp. test is to disconnect the ignition module , open the throttle and remove all the plugs. Install the tester and crank the engine for 6 comp. strokes. You will hear it. They should all be within 20 psi from lowest to highest.

 

[PsYcHo GN]

The best way to test the engines health is to do a leak down test.

How do you do a leak down test? Also, is the test gauge different?

Thanks!

 

[Boost231]

How do you do a leak down test? Also, is the test gauge different?

Thanks!

yes its a different gauge. just do a compression test first and see what you find then do a leak down test if you need to. when i lost a head gasket the compression test showed it. sometimes it wont.

 

[boostingbuick]

Differnt tester. And harder to do. You will need to get the cylinder your testing to top dead center. very tricky. And then you fill the cylinder with air and see how much leaks out, and where. But gives you a good understanding if what shape the engine is in.

 

[PsYcHo GN]

Thanks for the info guys!

I'll do a compression test first and see what I find. If it tests fine, then I'll plan to do a leak down test in the near future. If it doesn't test fine, then I'll have to replace the headgasket(s) before something worse happens.

 

[iwantatta]

Somebody had previously posted using something like this also for headgasket issues.

http://www.napaonline.com/MasterPag...&Description=Leak+Detector+Kit+/+Engine+Block

As far as compression testing, not sure if the buicks are different but on my subaru i pulled the ignition fuse before testing. Was the easiest way for me to cut off the fuel.

Cranked 3 times and then took a reading. Unfortunately for me, one cylinder had no pressure. Turned out to be a headgasket. By then, my Check Engine Light came on.

 

[Chuck Leeper]

And why must the piston be at TDC? Ring leakage will show at the top of the bore as well as the bottom.
Leakage thru gaskets, valves, guides, will show regardless of piston position.

 

[S2V6RACER]

And why must the piston be at TDC? Ring leakage will show at the top of the bore as well as the bottom.
Leakage thru gaskets, valves, guides, will show regardless of piston position.

If not at exact TDC the piston will push down and a valve will open. You would have to back off the rockers to do this at BDC. Good question.

I have done it both ways, with a shoddy engine built by a local engine builder. You could tell it was a ring seal issue as the numbers were better at the bottom then the top. Come to find out there was a .006 out of round on that one.

A leak down can also show slight problems with coolant system pressurization. If you leave the test on for a while you will hear bubbles in the radiator.

I prefer the leak down method, but if it is bad, a compression test will show a problem first.