Engine compression levels

dustin

UAIOE
I have had a vehicle engine compression test done on a car we are thinking of purchasing used. The car is a 1996 AUDI A4 2.8L V-6. The seller stated the mechanic which looked at it today said 5 cylinders were between 120 and 115. I take that as an average of 118.

BUT there was one cylinder which had a reading of "90ish". The mechanic told the seller each cylinder should be around 100.

The seller is supposed to be faxing me the results today so I can have the exact numbers shortly but the nubmers I have are what the buyer told me.

I asked a buddy and he said I should take the average of 5 cylinders (118) then get 25 percent of that (29.5) and take the low number cylinder (90) and subtract the low number cylinder from the average of the other cylinders and it shouldnt be OVER the 25 percent.


Does anyone agree this is a good method to determine if the engine compression is correct? Or is there a better way?

I'm curious as to if it matters depending on the manufacturer or if there is a general rule of thumb.

I've been on audiworld.com and havent found any specs yet as to what the engine compression readings should be.
 

dustin

UAIOE
Ok here are the hard readings from a DRY compression test:

Cylinder PSI

#1 150
#2 99
#3 150
#4 120
#5 120
#6 150
 

Nickel

curiouser and curiouser
I actually don't agree...you haven't seen the car, and one low compression reading isn't a fatal blow. If something needs to be fixed, I'm willing to pay for it. We're getting the car for a steal, and the mechanic says that it's a good car. This is the mechanic speaking (yes, the very one who in awhile will try to tell us we need :gossip: replaced b/c that's his bread and butter), not the seller. The seller is Italian, and so freakin' polite and honest it's almost sad.
 

Ken King

A little rusty but not crusty
PREMO Member
Nickel said:
I actually don't agree...you haven't seen the car, and one low compression reading isn't a fatal blow. If something needs to be fixed, I'm willing to pay for it. We're getting the car for a steal, and the mechanic says that it's a good car. This is the mechanic speaking (yes, the very one who in awhile will try to tell us we need :gossip: replaced b/c that's his bread and butter), not the seller. The seller is Italian, and so freakin' polite and honest it's almost sad.
3 of six cylinders reading low, one extremely low, for me indicates a significant problem and possibly a hole in a piston. If the deal you are getting is such that an engine replacement/rebuild still makes it a good buy then by all means do it. I see it as getting someone else's headache.

At this point the only way to know for sure what is wrong is to tear it down and if you can't do the work yourself you are getting into a lot of shop time which equals dollars, followed by replacement parts, and the related rebuilding work to be able to put it back together, again big dollars. Any idea as to what a long block replacement would cost?
 

dustin

UAIOE
The car runs fine. Doesnt have any smoke whatsoever of any color. No leaks. If anything its probably just dirty valves. If there was a hole in the piston that would mean that the cylinder would have been running too lean which would also have caused a check engine light. There is no check engine light (the light functions properly upon initial startup).

I was just wondering if anyone had a method to the madness on determing the cylinder compression reading ratio?
 

willie

Well-Known Member
How many miles on the engine? The readings look like worn rings on a hi mileage engine. A squirt of oil in the cylinder before the test will tell you if it's valves or rings. It could have a lot of good miles left on it with those readings but performance won't be up to par.
 
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