Lame'O compression test question......?

[howslowcanyougo]

History: 98 Talon, Tsi, AWD, Modded.

Had a SBR Bastard 20G on last year and the turbo blewup after about 90 days. SBR pulled it apart and CLAIMED the "something must have broke free" in my motor to cause the serious damage to the Exhaust side turbine. I never had it apart myself, so I'm still not sure what really happend. We ended up splitting the difference on the repair, So I bought Tom's FP Big T28, and sold the newly rebuilt "Bastard" right here on the style board for 1/2 price of new.

I finally got around to running a compression test tonight to try show me if my motor is really screwed up or not before I throw on this good T-28.

According the FAQ, the motor should be at operating temp for the test, <which it was'nt as the turbos off from it> and it hasn't run in about 4 months now.

Anyway the results I got were as follows.

On cyls 2,3,4, the VERY first "pop" of cranking the moter I got 120, and the 2nd rotation it went to 150 and held there. But on cyl #1 the very first pop gave me only 20 then the 2nd gave me 150 and held?

I ran the test about 2-3 times on each cyl and got the same results each time.

The one thing I didn't do was hold the throttle open during this testing. still not really sure why you are suppose to either?

Please forgive my lameness at running compression tests, but what would results like these indicate to you guys?

The only one that could be suspect is cyl 1, but the question is to what degree?


Thanks in Advance........


Howie

[tim]

It would probably be better to do a leak down test if you cant get the car up to operating temps first. A leak down test will give you more information too if there is actually a problem.

And yes, you are supposed to hold the throttle plate wide open when you do a compression test.

[howslowcanyougo]

Originally Posted by tim

And yes, you are supposed to hold the throttle plate wide open when you do a compression test.

Why?

[tim]

It allows the max amount of air into the cylinders. I think that is the reason. That is what I was told a long time ago and I have done it that way ever since.

Maybe someone else has better reasoning, I guess I have never really thought of it.

[howslowcanyougo]

Originally Posted by tim

It allows the max amount of air into the cylinders. I think that is the reason. That is what I was told a long time ago and I have done it that way ever since.

Maybe someone else has better reasoning, I guess I have never really thought of it.

So then my readings with the throttle not open should really be higher than 150 that I just got then or?

[Halon]

I've always done mine with the throttle plate all the way open. Never really dug into the 'why' aspect, just did it like that because that's how I learned to do it. Whatever way you do it, I'd say just be consistent across the board. Don't do 2 cylinders one way, and the other 2 the other, or one month do it one way, and a couple months later change your method. Just be consistent is what I'd say.

As for only having 20 on the first crank and then jumping up, well the only thing I can think of is, maybe the piston was already like 3/4 of the way up when you started the test, so there for the first crank only gave a little bit of pressure compared to the other which maybe had a full crank in on the first 'pop'. I've never really payed much attention to the psi number on the very first crank, because I'm usally by myself, and so I'm just sitting in the car, cranking it over a few times, then get out and see what the gauge is at.