Compression Test - Am I doing this right?

[djdropit]

I went ahead and did a compression test because I was seeing white smoke to help verify a blown head gasket before I rip the thing apart. I just wanted to make sure I did it right.


1. Removed all Spark Plugs and the Fuel Relay
2. Screwed in the compression gauge adapter to Spark plug (1 through 6)
3. Plugged on the compression gauge
4. Held the throttle wide open
5. Turned the engine over 6 times

I did steps 2-5 for plugs 1-6 and these were the results:

1 - 160 psi
2 - 165 psi
3 - 125 psi
4 - 150 psi
5 - 160 psi
6 - 80 psi

I plugged it out of 6 and redid it, and the second time I got:
6 - 95 psi

I'm pretty sure it's blown, unless anyone sees fault in my testing.

Thanks,
DropIt

[Cabriolet]

looks like you might have a bad valve in #3 and blown gasket in #6. its time for a full top end rebuild for you.

[djdropit]

When I pull the head off how do I tell whether the valves are bad or not?

[JTapper]

Maybe you should do a leakdown test.

Compression tests are a waste of time.

Do a leakdown test. You'll be able to tell whether it's the rings, exhaust valve, or intake valve pretty easily.

[slammin.e28]

Do a leakdown test. You'll be able to tell whether it's the rings, exhaust valve, or intake valve pretty easily.

This x1000.

[downernsp]

I agrre with 2 last post also when you do a leak down. Un cap the radiator cap if water start to spill out for sure blown head gasket..

[jlevie]

White smoke from the exhaust means a bad head gasket and further diagnostics aren't needed. But if you want further proof, a leak down test is more likely to work than a compression test. The definitive test is to pressurize each cylinder with 180psi air for several minutes while monitoring cooling system pressure with a sensitive gauge. A rise in cooling system pressure is proof of a leaking head gasket.

The disappointing compression results are more likely to be from valve or ring problems. The valves can be attended to once the head is off. A leak down test before pulling the head may tell if there are ring problems. But if the valves are leaking you won't be able to assess the condition of the rings until the head is fixed.

[djdropit]

Great advice everyone, I really appreciate. When doing a Leak down test, can it be done with the valve cover off? If not I'm having trouble picturing how I would be at TDC for each valve pair.

White smoke from the exhaust means a bad head gasket and further diagnostics aren't needed. But if you want further proof, a leak down test is more likely to work than a compression test. The definitive test is to pressurize each cylinder with 180psi air for several minutes while monitoring cooling system pressure with a sensitive gauge. A rise in cooling system pressure is proof of a leaking head gasket.

The disappointing compression results are more likely to be from valve or ring problems. The valves can be attended to once the head is off. A leak down test before pulling the head may tell if there are ring problems. But if the valves are leaking you won't be able to assess the condition of the rings until the head is fixed.

J - I was under the impression that the leak down test would show air leaking into various places in the system (exhaust valve would hiss into the exhaust, intake valve would hiss into the intake manifold, as you indicate the head gasket would leak into the coolant). Is there not a way to listen for bad piston rings?

-DropIt

[jlevie]

Great advice everyone, I really appreciate. When doing a Leak down test, can it be done with the valve cover off? If not I'm having trouble picturing how I would be at TDC for each valve pair.

I always do a leak down test with the valve cover off so I can make sure that the cylinder I'm doing is at TDC on the compression stroke.

J - I was under the impression that the leak down test would show air leaking into various places in the system (exhaust valve would hiss into the exhaust, intake valve would hiss into the intake manifold, as you indicate the head gasket would leak into the coolant). Is there not a way to listen for bad piston rings?

Air will leak at the various places and you can pretty well identify where leaks are from where the sound is coming from. But if a valve is leaking badly enough to produce the compression numbers you have, the pressure will probably never get high enough to assess the conditions of the rings, and vice versa.

My bet in this case is valves just based on the compression numbers. While there are conditions that can cause the rings on a cylinder or two to bad simple wear usually has a similar affect of the other cylinders.