naidoo2610

Just had a compression and leak down test done on my 91 964 with ~149k miles. has a few oil leaks but nothing major. I had the tests done to see if I need to prepare myself for a reseal/rebuild.

Here are the numbers I got and wanted to get input on how to interpret the numbers and to find out if a reseal/rebuild is necessary:

Compression test:
Cyl #1-4: 160lbs
Cyl #5: 180lbs
Cyl #6: 175lbs

Leak down test:
Cyl 1: 5%
Cyl 2: 4%
Cyl 3: 5%
Cyl 4: 6%
Cyl 5: 5%
Cyl 6: 5%

Thanks for taking the time to read and provide input.

hawk911

based on the information I had received in my last go-round with a mechanic and the forum here, 5% leak is normal, as are your compression numbers. I'm not sure the exact compression number, but I thought 180 was very close to normal.

ras62

Those are the numbers of a fit and very healthy engine! Sort the leaks and sleep easy.

darth

Compression #'s on 1-4 are low. More than 10% difference is unacceptable.

FeralComprehension

Unless you *want* to rebuild, run it.

How is your oil consumption?

Indycam

A long time ago I had a Mercedes Benz
http://www.google.com/imgres?q=67+22...w=1280&bih=681
with zero compression in one cylinder . It was no big deal . I bought it that way and sold it that way .

If all the cylinders were at 100 psi , zero % difference , then it would be fine ?

darth

"If all the cylinders were at 100 psi , zero % difference , then it would be fine ? " Correct. Rule of thumb is 10% difference is undesireable. In other words, if any of the other cylinders were less than 90 psi or more than 110 psi there would be a concern. "with zero compression in one cylinder . It was no big deal " Full combustion would not be occurring and the unburnt fuel would be washing down the cylinder walls of lubricant and accelerating wear between the cylinder and piston.

ja78911sc

Leak test #'s look good. 160 even on four cylinders? Was the compression test performed on a warm motor?

Indycam

So one cylinder at 100 psi means rebuild but all 6 at 100 psi means no rebuild ? 100 psi is good and bad .

You test a motor one week , one cylinder at 100 psi and all the others are at 125 and you say the motor needs a rebuild , next week you test and all the cylinders are at 100 psi and the motor is fine and needs no rebuild ?

A cylinder with no compression does not breath in or out when you disable the intake valve .

berni29

Hi

No, 100 psi would be considered low and even if all six readings were identical it would be unacceptable. It would pass the 10% difference rule of thumb test though. Your numbers would not worry me or put me off buying your car of themselves were you selling.

Berni

darth

"100 psi is good and bad ." I thought you picked 100 to keep the math simple, in order to demonstrate the 10% rule, but yes a reading of 100 is suspect low. I might question measuring technique or the compression guage.

"You test a motor one week , one cylinder at 100 psi and all the others are at 125 and you say the motor needs a rebuild , next week you test and all the cylinders are at 100 psi and the motor is fine and needs no rebuild ? " Again, I might question measuring technique or the compression guage if readings aren't consistant.

"A cylinder with no compression does not breath in or out when you disable the intake valve . " Is the valve disabled open or closed? Either way, I wouldn't want a piston rattling around in an engine doing nothing, at least not for any length of time.

naidoo2610

Thanks all for the great response and input

naidoo2610

"Compression #'s on 1-4 are low. More than 10% difference is unacceptable. "
Does this mean that a rebuild may be necessary?

"How is your oil consumption? "
its hard to tell, I only do about 2k miles a year and I change the oil every year, probably half a quart during the year, at most 1 quart.

"160 even on four cylinders? Was the compression test performed on a warm motor? "
Yep 160 on 1 through 4. I believe the engine was cool for the compression test.

Thanks again for the input

Indycam

No . It does not mean that .

You need to go for a good long drive , up and down some mountains and then test again .

darth

"Compression #'s on 1-4 are low. More than 10% difference is unacceptable. "
Does this mean that a rebuild may be necessary?

Not necessarily. But it does warrant closer examination. The leakdown #'s contradict the compression readings. Usually they go hand in hand. In other words, cylinders with low compression typically have corresponding hi leakdown numbers. Unbalanced engine cylinders, which have wildly differing compression readings manifests into a rough idle.