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Go Back   TDIClub Forums > TDI Model Specific Discussions Areas > VW MKIV-A4 TDIs (VE and PD)

VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old July 17th, 2007, 12:27   #1
Herm TDI
 
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Default low (or loss of) compression

I have an A-4 Jetta that has experieanced a sudden loss of compression on #1 cylinder. Befor I go further here is a list of the diagnostic checks that have been compleated:

1. Verification of TDC (no TB slippage) ( I checked and confirmed, as of this moring)
2. Positive fuel delivery to each injector ( I did this test this moring)
3. Fuel injectors spray pattern and pop-off test
4. Swap the #1 injector from another ALH engine
5. Compression test ( Cyl #1 reads extreamly low)

Here are the results of the compression test:
Cyl #1 = 160 PSI
Cyl #2 = 480 PSI
Cyl #3 = 485 PSI
Cyl #4 = 480 PSI

I could not find (or see) any visual indication of a defect or malfunction within the valve-train components. The lobes of the cam-shaft and the surface of the cam followers look good.
This visual inspection indicates (to me) that the cause of the loss of compression is from another area of the engine. The engine does start (rough idle...smokes)
No harsh or metalic knocking noises as the engine runs it just idles rough.
I have not found any oil in the coolant or loss of coolant.
My instincts lead me towards the head gasket or a possiable valve problem.

What do you other "wrenches" think....anything I've over-looked?
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Old July 17th, 2007, 13:51   #2
DPM
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Is there much blowby?
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Old July 17th, 2007, 14:00   #3
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Quote:
Originally Posted by DPM
Is there much blowby?
Thats a good question...I'll check the air flow at the CCV puck on the valve cover....this would indicate a cracked piston.
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Old July 17th, 2007, 14:41   #4
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I just checked the blow-by flow at the valve cover CCV puck.
Things don't look good. I used my A-3 TDI as a referance check on the blow-by.

The A-4 has MUCH more blow-by than my A-3

These engines are the same displacement and simular compression ratios. Even the compression ratings are the same (for both engines).
All things being equal...the blow-by should also be the same or close to each other.
But that is not the case in this situation. When I removed the CCV puck from the A-4 the "blow-by" flow is much higher compaired to the blow-by on my A-3.

So the only way to tell (for sure) will be to remove the cylinder head.

Could a failed head gasket cause the excessive blow-by ? (crossing my fingers...hope ? )

Last edited by Herm TDI; July 17th, 2007 at 14:53.
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Old July 17th, 2007, 16:19   #5
imo000
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Put a little auto trans oil into the cylinder just before a compression test. If the number goes up then it's an indication that the rings are leaking. If it doesn't then it's probalby a burnt vavle or a cracked head.

I hope this helps!
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Old July 18th, 2007, 10:44   #6
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If this were a bike I'd say a broken ring. Had a piece of a single ring piston ring break and the only symptom was low power. The missing piece must have blown right out the exhaust. I know a diesel is much different from a 2 stroke dirt bike, but compression is compression. Also figured you're looking for some goofy thoughts that aren't normal, so I figure I'm in.
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Old July 18th, 2007, 11:41   #7
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My FOD experience resulted in loss of compression on only one cylinder (#3).

the top two rings were siezed.
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Old July 18th, 2007, 12:54   #8
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Thanks guys, for your input.
I now have the cylinder head removed and it looks like the #1 exhaust valve is leaking badly.

Question is (considering the options):
1. Should I replace the valve & valve guide and then lap the valve seat?
2. Go for a rebuilt cylinder head?
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Old July 18th, 2007, 15:58   #9
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how many miles? valve guides on these heads are pretty weak. you'd probably want to do them all especially the exhaust.
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Old July 18th, 2007, 16:16   #10
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Quote:
Originally Posted by runonbeer
how many miles? valve guides on these heads are pretty weak. you'd probably want to do them all especially the exhaust.
Could a leaking exhaust valve cause the excessive blow by?
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Old July 18th, 2007, 16:41   #11
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How should I know!??? just kidding, perhaps if the guide was really worn it would result in blow by. still wondering if there's anything going on with the rings...
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Old July 18th, 2007, 17:19   #12
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Are there any other symptoms of blow by that Herm might see? On my Volvo 122 (yes, I'm that old) it forced oil past the rear main seal. Drip, drip.
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Old July 18th, 2007, 17:31   #13
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Seems to me that a bad exhaust valve WOULD cause low compression. Don't see how it could add to blowby. Is there an engine/head machine shop that could give you an estimate on rebuilding the head? If one valve is bad - all should be done.
If you don't see a crack in the piston odds are good that you need to fix stuck/broken rings. THAT means an engine rebuild and perhaps boring and oversize pistons, etc.
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Old July 19th, 2007, 13:07   #14
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Leaking valve (either intake or exhaust) would certainly result in LOW compression in diesel engine (compression ignition). May be other issues as well, but you have most likely found the culprit.
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Old July 19th, 2007, 14:40   #15
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Quote:
Originally Posted by Fourdiesel
Seems to me that a bad exhaust valve WOULD cause low compression. (snip) If you don't see a crack in the piston odds are good that you need to fix stuck/broken rings. (snip)
__. That's the first thing that popped into my head "I'd bet that there are two problems with this engine". Any chance that the valve lowered the compression, causing poor combustion resulting in coked up rings (or something like that???)
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