compression concerns

[mikeesho1]

Ok l so i did a compression test to see if i had any bent valves all cyclinders held compression. One cyclinder is 12 psi lower then the rest so i put some oil in the cyclinder and seen on 2 psi differance. Second cyclinder on the left as your facing the engine.

All cylinders where reading 62 psi. Exept the one it read 50 psi.

The way i tested them was i took the cams off because i did not the pistons hitting the valves i wanted to make sure all the valves where in the closed position. Then i took the connector off the coil pack and cranked the motor over by turning the key. I dont know if i did it right but i got compression readings.

I guess my question is how bad is a 12 psi differance and what would happen if i just put everything back together and kept driving it like this. Is it possible to change the pistons out with the motor in the car. If so how would i go about it.

 

[SrKegler]

I wouldn't worry about the 12 psi difference, I would worry about the low compression. Service limit is around 120 psi. If the car runs, apparently there was something done wrong on your compression test.

 

[19Eclipse90]

You tested way wrong for a compression test. Most importantly without cams, there's no way for the cylinder to pull in air to compress. Basically what you measured is the compression of an unspecified volume of atmospheric air that was trapped in the cylinder when you pulled the cams off and the valves snapped shut.

What you need to do is a leak down test: http://www.dsmtuners.com/forums/articles-engine-fuel/338152-compression-leak-down-testing.html.

 

[mikeesho1]

The reason for a compression test is beacause my timing belt broke at an idle in my driveway. I wanted to see if i had any bent valves. I dont have the timing kit yet so i figured i would get a rough estimates. So what u are saying is to warm the engine before test. Thanks for the info it will really help and hopefully the reading will change when the motor is warmed up. How do i go about doing a compression test while car is running. Just take one spark plug out then start the motor for a few seconds take the reading and move on to the next.

 

[Calan]

You tested way wrong for a compression test. Most importantly without cams, there's no way for the cylinder to pull in air to compress. Basically what you measured is the compression of an unspecified volume of atmospheric air that was trapped in the cylinder when you pulled the cams off and the valves snapped shut.

What you need to do is a leak down test: http://www.dsmtuners.com/forums/articles-engine-fuel/338152-compression-leak-down-testing.html.

Not to mention you were making the rings work "in reverse" by applying a vacuum to the top of the pistons each time they traveled downward. I've never really thought about what effect that would have on the rings seating in the piston lands or working carbon loose, but it can't be great.

I think you need to take a few steps back, relax, and do some research.

How do i go about doing a compression test while car is running. Just take one spark plug out then start the motor for a few seconds take the reading and move on to the next.

Did you even GLANCE at the link Brian just gave you? You don't do a compression test with the motor running. (Well you can, but that's beyond this discussion).

Besides that, you need to do a leak-down test at this point; not a compression test ... as Brian also stated.

 

[19Eclipse90]

Granted, that you got any results at all with this make-shift compression test might suggest that your valves are fine, but it would still be very wise to do the leak-down test.

 

[g4ebguy90]

Maybe he could do the compression test. Then let each cyl sit. To see how much air leaks out over time... But............ yeh. I remember using a rather expensive gauge set to check cyl leakage. He probaly don't have one. He could just pump air into the cyl I guess then see where it comes out

 

[Calan]

Maybe he could do the compression test. Then let each cyl sit. To see how much air leaks out over time...

Uh... what?

**********

A leak-down tester can be bought new from Harbor Freight for $30. Or you can get loaners from most auto parts stores for nothing.

 

[g4ebguy90]

Uh... what?

**********

A leak-down tester can be bought new from Harbor Freight for $30. Or you can get loaners from most auto parts stores for nothing.

Yeh you know if his valves are bent, the air that's *still* in the cyl. Will slowly leak out of the cyl. If it's bad compression will drop. You see what I'm saying. If his valves are good the gauge should hardly even drop in psi. If they are bent, compression should drop alot.

I'm trying to think what he can test kinda right now, seeing how all he has is a compression tester.

 

[Calan]

Yeh you know if his valves are bent, the air that's *still* in the cyl. Will slowly leak out of the cyl. If it's bad compression will drop. You see what I'm saying. If his valves are good the gauge should hardly even drop in psi. If they are bent, compression should drop alot.

Um... no.

Compression testers have a check valve in them. You can completely disconnect the gauge from the engine and toss it on the bench, and it will still read good pressure.

What he needs to do is a proper leak-down test, as described in the article that Brian linked to in the 3rd post.

 

[Lone]

I purchased a Laser that had the timing belt skip on the intake side while they were backing out of the drive way. All of the intake valves were bent and the guides were cracked. I knew something was wrong when I pulled the valve cover and some of the rockers had come off.

I can't imagine it not bending a valve, but my experience is limited.

 

[g4ebguy90]

Um... no.

Compression testers have a check valve in them. You can completely disconnect the gauge from the engine and toss it on the bench, and it will still read good pressure.

What he needs to do is a proper leak-down test, as described in the article that Brian linked to in the 3rd post.

*Most compression testers have a check valve* . I had to bust out 3 of my college level books for that one. I swear I read in one of them that to let the gauge sit and then record readings. But I guess I was wrong.
Think it was in a haynes manual.

 

[19Eclipse90]

Yeh you know if his valves are bent, the air that's *still* in the cyl. Will slowly leak out of the cyl. If it's bad compression will drop. You see what I'm saying. If his valves are good the gauge should hardly even drop in psi. If they are bent, compression should drop alot.

I'm trying to think what he can test kinda right now, seeing how all he has is a compression tester.

The fact remains that you cannot (to my knowledge) do a compression test without the cams installed. If you are wanting to test for bent valves, you want to do a leak-down test. Doing so without the cams installed would be opportune since the valves need to be closed anyway. If the the cylinder doesn't hold pressure, you should be able to identify very easily where the air is escaping to.

This puts things about as simply as can be:

In very basic terms, a compression test tells you about your engine's ability to generate cylinder pressure, and a leak down test tells you about it's ability to hold pressure.

*Most compression testers have a check valve* . I had to bust out 3 of my college level books for that one. I swear I read in one of them that to let the gauge sit and then record readings. But I guess I was wrong.
Think it was in a haynes manual.

Let's say compression testers don't read peak values, but instead always reflect actual cylinder pressure. Imagine that you crank the engine over a set amount of times and obtain your reading -- how do you know the position of the pistons and valves when you stop cranking? Either the intake or exhaust valves could be open, meaning you will have 0 compression, or the piston is mid-compression and (assuming an otherwise sealed cylinder) the compression will only be a fraction of the peak value. What good would that be?

 

[g4ebguy90]

Your right. Makes perfect sense. I just don't know what book I read that out of. But it was deff wrong.