Leak Down Test

[czech09]

I am thinking about buying a car I found on dsmtrader, well I asked the seller for compression #s and he told me this:

A compression test isn't the best way to tell if a motor is good or not. A leak down will tell you exactly where a problem is weither it be the valves or the rings. It leaked down at less than 4% in all 4 cylinders about 2 weeks ago.

What does the less than 4% mean ? Thanks.

 

[DSMeclipse4G63]

I am thinking about buying a car I found on dsmtrader, well I asked the seller for compression #s and he told me this:

A compression test isn't the best way to tell if a motor is good or not. A leak down will tell you exactly where a problem is weither it be the valves or the rings. It leaked down at less than 4% in all 4 cylinders about 2 weeks ago.

What does the less than 4% mean ? Thanks.

did he mention any hissing in the crankcase, exhaustr or intake? less the 4% is pretty good. A cold motor or motor that isnt up tpo full operating temp wont seal fully cause the rings havent expanded yet. Oh and tell him he is crazy to tell you that compression numbers arent good they are the first step BEFORe you do a leak down. If you dont have good compression then there is no point to do a leak down.
Andrew

 

[1fast97gsx]

didnt you already post this in like 2 other posts as well???

 

[czech09]

yes I deleted them both b/c got now answer and people kept jacking the thread with their own questions thanks

 

[DSMJim]

4% is very low and almost impossible to acheive even with a new motor. A compression test will tell you a lot, a leak down other info. One does not replace the other both have their purpose.

Leakdown of 15-20% or less is pretty well acceptable.

 

[czech09]

his other response:

I always do one everyonce in a while cause on a turbo car it's very difficult to know if you have bad leak down. We have had 300zx with holes in the piston and they felt fine under load. The customer brought it in cause of a rough idle, but under load he said it felt fine. I do it just to know when or if I should tear down the motor and rebuild it before it can't be fixed.


Sounds confusing/weird to me...under load ?I am guessing that means under boost ?

 

[RDS]

If he had ANY hole in a piston the car would run like a heap of junk, and you then have the possibility of blowing your sump off/up with unburnt fuel going into the crankcase

 

[DSMJim]

If he had ANY hole in a piston the car would run like a heap of junk, and you then have the possibility of blowing your sump off/up with unburnt fuel going into the crankcase

Not on a 6 cylinder. I was fixing a 6 cylinder maxima that had 0 compression in one cylinder 50 in the other, and the rest were 240 acrosst he board. The car idled nicely with a little popping sound in the exhaust and drove well too. It had descent power too all the way to redline. I didn't think it could have possible holed pistons with that type of power and drivability, however I was wrong, two holed pistons.