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View Full Version : Failed Compression Check and Leak Down Test. Valves look ok....?



Tumbler510
05-24-2018, 08:47 AM
I bought a 99 A4 Quattro with the 2.8 V6 knowing that it had a timing belt issue. The PO said the timing belt snapped, he replaced the belt, rollers, and tensioner. He said he couldn't get it to run, so I assumed he had bent valves. He said he had good compression (~152) in every cylinder, which i assumed was not true.
I tried starting the car, and it would spin, and just heard air moving, but nothing started.
I borrowed a compression tester from Autozone, and no cylinder gave any compression.
Then I used my leak down tester, and it was leaking in every cylinder, i even turned the cams until the lobes were closed incase the timing wasnt done correctly. Still leaked.

Last night I pulled one of the heads, and the valves look great, no bending or anything. The head gasket has no breaks either.

What could have cause the no-run condition and the failed leak-down test?

garytightpants
05-24-2018, 11:09 AM
piston/piston ring damage and/or scored cylinder walls can cause such a condition.

Tumbler510
05-24-2018, 11:24 AM
piston/piston ring damage and/or scored cylinder walls can cause such a condition.

I'll get pictures after work today, but the cylinder walls looked good to me.
I am going to test the valves on my bench as well.

It's just odd that it would happen when the timing belt snapped, but be unrelated.

garytightpants
05-24-2018, 01:37 PM
I'll get pictures after work today, but the cylinder walls looked good to me.
I am going to test the valves on my bench as well.

It's just odd that it would happen when the timing belt snapped, but be unrelated.

does the timing belt drive the oil pump...?

offroader1006
05-25-2018, 12:08 PM
does the timing belt drive the oil pump...?
No, the oil pump is crank driven.

Flip the head over on a bench and pour a little mineral spirits on each valve face, see if they're actually sealing tight.

Tumbler510
05-25-2018, 03:09 PM
Yup. Plugged the spark plug holes, leveled it upside down and filled with water. The sprayed compressed air into the exhaust, and all of them bubbled like a siv.
The passenger side bank had every exhaust valve bent pretty badly, so I think the diagnosis of the bent valves is correct.

https://i.imgur.com/yVGxpVB.jpg

a4lownslow
05-26-2018, 05:55 AM
Dude itís an interference engine. If your timing belt breaks while the engine is running kiss your valves goodbye, just like you see.


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