Leak down #'s off....compression good....

[Soterik]

Hi....need some opinions on a car I'm looking at.

Compression #'s are good all the way across, within 5 lbs of each other.

Leak down #'s are off. 3 cylinders at 90% to 95%, 3 cylinders at 60% to 70%.

Owner is going to have a valve adjustment done to see if that brings the numbers up.

But... just for my edification, it's usually rings or valves at this point right?

So, if the numbers don't come up...it means a "top end". How much? $4k?

Any input?

thanks,
Eric

[Flat Six, inc.]

Carbon on the valves or worn guides will do that.....you can usually get the leak down to come around by tapping on the rocker while the cylinder is pressurized......

Typically it means the top end is a little tired.

[MarkS]

I personally look real hard at leakdown #'s even if compression is good. 30-40% is pretty bad. My bet valves/guides 80%, ring related 20%. You did not state what kind of P engine but guessing air cooled and 4K should cover it. As for carbon between valve face and seat maybe but I have never been so lucky.

[Steve@Rennsport]

Hi Eric,

The great thing about leakdown testing is that it not only displays cylinder pressure values, it allows the person to find the source of any leakage.

As you know, the majority of such leakage comes from the valves and rings so its imperative that the technician notes where he hears any air leaking from. If this isn't done, the tests lack any context and are thus, useless.

Valve leaks are relatively cheap to repair but excess air escaping from the oil tank filler tube indicates problems that can be much more expensive to resolve.

IMHO, the leakdown tests should be re-done as I'd suspect a problem with the procedure, if the compression figures are good.

[Eminence Gris]

I agree with Steve's last line.

I've seen too many bad leakdowns to trust them totally. the procedure loosens carbon and it can pin a valve slightly open. Run the car and do the leakdown again. I bet the result is different.

[911quest]

Are all three of the suspect cylinders on the same bank?

[Soterik]

Guys,

Thanks for the replies. Not sure if the cylinders all on the same bank.

At this point the car is rescheduled for another compression test, leakdown, and also for a valve adjust (not necessarily in that order). It's going to be done at a different shop as well.

Have also asked current owner to drive the car a bit before this next appt., which shouldn't be an issue given the current weather we are having here in Seattle.

thanks,
Eric

[kenikh]

Make sure to decoke the valves and pistons using the "water method". Get her warmed up and running at 4000 RPM and use a spray bottle to inject water into the carb/MFI throats - don't add so much as to kill it - but this will usually break up the carbon and let things seat better.

[304065]

Hey Eric,

BELIEVE the test.

It's tempting to rationalize the numbers but unless you have affirmative evidence the leakdown was incorrectly performed, the data are the data.

I went through the same thing with my '66. Leakdown showed 45%. I scratched my head like a monkey and thought, "Could it be carbon?" I poured Seafoam in, which had no effect on the leakdown but killed every mosquito in the County.

"Hmm, something must be wrong with my testing technique." Not really. Something was wrong with the ENGINE.

[Soterik]

John,

So you tried Seafoam eh??? I haven't got the guts to try that on any of my cars yet....did use it in the gas on one of my trucks though.

I'm interested in this car I'm looking at, but want to make sure what I've got in front of me, and will use the numbers as a point of negotiation on the price.

Both the seller and I agreed to use a different shop to confirm the numbers. Body, paint, and original interior on this car are great, which are all more important to me at the outset.

Guys,...thanks again!!