compression testing cylinder ?

[Blazin'4x4]

Is the first second or third reading I get the proper one?

Cylider 4 reads 30psi on first stroke, 90 on the second and tops out at 115 psi.

Basically need some general instructions on compression testing.

 

[munepit]

When I did mine, they were all around 110psi. I turned my motor over a few strokes, like 5 or 6. I don't think you get a good reading with one one stroke of the motor. You need a few strokes, I think. Also, I heard you need the TBI or carb held at WOT, wide open throttle. But you go a few strokes. As long as they are all close is what matters. I had 7 at 110psi and 1 at 0psi, mine was easy to tell what cylinder was bad. Don't just go by what I said, I am no machanic. Good luck.


Is it a stock motor? What yr.? Any mods? What is wrong?

 

[tRustyK5]

Remove all the plugs so you have the best crank speed possible. Carb or TBI at WOT. Usually 4 or 5 cranks, until you get the highest number. The important part is that they're all within 10% of each other.

Rene

 

[4X4HIGH]

To properly do a compression test it should be done with the engine at operating temp, carb or TBI held at WOT and all spark plugs removed. The first crank should be 75% of the eventual total which should be after about 4-5 cranks. The biggest thing to worry about is the difference between the lowest and highest numbers not wanting to be more than about a 10% difference.

 

[76zimmer]

On F.I. should you disable the fuel supply too?

 

[magik235]

Remove all the plugs so you have the best crank speed possible. Carb or TBI at WOT. Usually 4 or 5 cranks, until you get the highest number. The important part is that they're all within 10% of each other.

Rene

This is the format I use.

 

[K85 Octane]

unfortunately though, a compression test does not tell you everything
I had 7/8 cylinders @ 140 and the last cylinder at 145. Sounds good right, or at least even and problem free-ish.

Ugg, no.
#7 cylinder was sucking in coolant and blowing it out the reservoir bottle. Pulled the motor, and I had prolly 2-3 shot glasses of coolant sitting on top of the piston.
ugg
207 block 2 bolt, already bored 30 over, bad cylinder, slap job engine work w/ miss-matched bolts=
new engine

this is where a leakdown test may have shown me the problem

 

[4X4HIGH]

unfortunately though, a compression test does not tell you everything
I had 7/8 cylinders @ 140 and the last cylinder at 145. Sounds good right, or at least even and problem free-ish.

Ugg, no.
#7 cylinder was sucking in coolant and blowing it out the reservoir bottle. Pulled the motor, and I had prolly 2-3 shot glasses of coolant sitting on top of the piston.
ugg
207 block 2 bolt, already bored 30 over, bad cylinder, slap job engine work w/ miss-matched bolts=
new engine

this is where a leakdown test may have shown me the problem

Well in your case a compression test showed that your compression was good, you just happened to have a different issue that needed a different test to be performed. You have to use the proper test based on the issue at hand. You can have good compression (as you've found out) but still have a different problem.

 

[Blazin'4x4]

My problem consists of a very shakey and a serious lack of power 305 while in gear. idles great, throttle response and fireup good, timing has been checked repetively.

Sounds like I did a good compression test (115 psi all around) but I still notice white smoke out the exuast. which led me to believe the rings may be shot. Also noticed oil accumulation on the passenger side collector.

Head gasket or cracked head?

 

[4X4HIGH]

My problem consists of a very shakey and a serious lack of power 305 while in gear. idles great, throttle response and fireup good, timing has been checked repetively.

Sounds like I did a good compression test (115 psi all around) but I still notice white smoke out the exuast. which led me to believe the rings may be shot. Also noticed oil accumulation on the passenger side collector.

Head gasket or cracked head?

White smoke out of the exhaust is water so you're looking at either a cracked head or a blown head gasket.

 

[folkenheath]

unfortunately though, a compression test does not tell you everything
I had 7/8 cylinders @ 140 and the last cylinder at 145. Sounds good right, or at least even and problem free-ish.

Ugg, no.
#7 cylinder was sucking in coolant and blowing it out the reservoir bottle. Pulled the motor, and I had prolly 2-3 shot glasses of coolant sitting on top of the piston.
ugg
207 block 2 bolt, already bored 30 over, bad cylinder, slap job engine work w/ miss-matched bolts=
new engine

this is where a leakdown test may have shown me the problem

Yep, I had the same problem, compression was consistent, but I knew there was a problem. So I borrowed a friends leakdown, and two cylinders were at 50% leakdown! The head gasket between those two cylinders was leaking. Turns out the head was not flat by .006". Had them resurfaced .010" and all was good. Also, you can use your compression tester to test that a little bit. Mine uses quick connects on the gauge. So you can hook your air hose to the compression hose which is in the spark plug hole. And with both valves closed (@TDC), apply about 100 psi to the cylinder with your compressor. Then listen, at the intake, at the exhaust, at the breather, in the radiator, etc. If you hear air leaking (or bubbling), then that's not good. Because if you can hear it, it's most likely too much. I've heard valves leaking out the exhaust or intake, rings seaping air out the breather (that one you might hear a minute amount on a good engine), bubbling in the radiator from a bad head gasket, etc.