2000 Beetle tdi won't hold compression

[tiswamp]

Well I am posting a new problem with my son's 2000 Beetle tdi.

The car was running lousy, poor pickup, hard starting you name it. My son, Son1, tried to find someone to work on it. In our area the pickings are slim. So Son1 asked Son2 to have a go at it. Son2 is a good wrench, but was not that knowledgeable on diesels. After Son1 was unable to find anyone who knew tdi's, Son2 was cajoled into taking it. He did not do a compression test first. He tried to read and find out what he could to diagnose the problem. The following action were taken: injection pump replaced, timing belt replaced, fuel pickup replaced, head gasket replaced. Son2 realized that there were multiple head gasket and matched to the right new head gasket. Car will crank and start briefly but does not stay started.

Son2 did a compression test and found that the compression on all the cylinders would build to approximately 300 after 7 cranks but would not hold there, retreating quickly to zero.

Anyone have any thoughts on what the problem may be????

Your assistance and expertise is greatly valued and appreciated.

[Seatman]

I don't think they would hold compression as such because it's bound to leak back past the rings but it does sound low, maybe he used the wrong head gasket and lowered the compression because they're different thicknesses. Just my thoughts anyway.

[almir]

Originally Posted by tiswamp

Well I am posting a new problem with my son's 2000 Beetle tdi.

The car was running lousy, poor pickup, hard starting you name it. My son, Son1, tried to find someone to work on it. In our area the pickings are slim. So Son1 asked Son2 to have a go at it. Son2 is a good wrench, but was not that knowledgeable on diesels. After Son1 was unable to find anyone who knew tdi's, Son2 was cajoled into taking it. He did not do a compression test first. He tried to read and find out what he could to diagnose the problem. The following action were taken: injection pump replaced, timing belt replaced, fuel pickup replaced, head gasket replaced. Son2 realized that there were multiple head gasket and matched to the right new head gasket. Car will crank and start briefly but does not stay started.

Son2 did a compression test and found that the compression on all the cylinders would build to approximately 300 after 7 cranks but would not hold there, retreating quickly to zero.

Anyone have any thoughts on what the problem may be????

Your assistance and expertise is greatly valued and appreciated.

something is wrong with the compression tester and engine shouldn't be cranked more than 4 revolutions. Once it gets to exhaust stage it will exhale. Compression tester shouldn'tleak on its own unless you press the air relieve valve.

[chittychittybangbang]

Originally Posted by almir

something is wrong with the compression tester and engine shouldn't be cranked more than 4 revolutions. Once it gets to exhaust stage it will exhale. Compression tester shouldn'tleak on its own unless you press the air relieve valve.

That is also a possibility.

FYI, on the TDI engine the service manual says to crank until pressure stops rising. This is usually more than 4 revolutions. Healthy TDI engines are usually 550 psi.

[JHNW]

If air is releasing that quickly you should be able to hear where it's coming from. Air can only be escaping from one of a few places..
worn/broken piston rings - unlikey they'd all be in such a bad state to let so much air past
Cylinder head not torqued down properly?
Injectors not torqued down properly?
Glowplugs not torqued down properly?
Faulty compression tester?

[Turbo Lag]

Originally Posted by almir

something is wrong with the compression tester and engine shouldn't be cranked more than 4 revolutions. Once it gets to exhaust stage it will exhale. Compression tester shouldn'tleak on its own unless you press the air relieve valve.

Second that.

Its been a long time since i done a compress test so correct me if i'm wrong.
If your just want to see if your engine will build the correct pressure the way you did it is right.
Now if you want to see if it will hold pressure you have to set that cylinder your doing so the intake and exhaust valve are closed. (TDC?) Or the pressure will leak thru the valves. Apply shop air to the cylinder and see if it holds. If not then you can hear where it is coming from.

Dose anyone know on these TDIs if the rings softer then the cylinder walls? Is it the Rings that wear out first or the walls?

[JHNW]

It's true you should keep turning the engine until the needle won't go any higher on the tester, the pressure is then kept within the tester until you press the release valve, the compression will have already left the cylinder when the valves opened. It sounds like the tester release valve is faulty.

[almir]

Originally Posted by Turbo Lag

Second that.

Its been a long time since i done a compress test so correct me if i'm wrong.
If your just want to see if your engine will build the correct pressure the way you did it is right.
Now if you want to see if it will hold pressure you have to set that cylinder your doing so the intake and exhaust valve are closed. (TDC?) Or the pressure will leak thru the valves. Apply shop air to the cylinder and see if it holds. If not then you can hear where it is coming from.

Dose anyone know on these TDIs if the rings softer then the cylinder walls? Is it the Rings that wear out first or the walls?

compression test is one thing, and engine leak-down test is another thing, (two different tools (gauges)). With cylinder at the TDC when you apply pressurized air to the glow plug/spark plug opening then you can use leak down tester to see if cylinders are holding pressure and where the air escaping. For an example in a gasoline engine, if you hear air escaping at the exhaust pipe then you know that exhaust valve is burned. If you hear air escaping once you open oil cap then you have ring problem, if air is escaping at the throttle body then your intake valve is burned or not sealing correctly, if you see bubbles in the coolant reservoir then head gasket etc etc. Maybe he is using cylinder leak down test gauge and therefore all this confusion.