low (or loss of) compression

Herm TDI

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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?
 

Herm TDI

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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 ? )
 
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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!
 

Souzafone

Veteran Member
Joined
Nov 4, 2004
Location
Freetown,Ma.
TDI
'99 Jetta A4, Whitish
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.
 

Herm TDI

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Richmond, Maine...The far side of Witsend
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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?
 

runonbeer

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Apr 15, 2002
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'00 Golf 02M, '10 Golf 02E, '02 UTE 02M
how many miles? valve guides on these heads are pretty weak. you'd probably want to do them all especially the exhaust.
 

runonbeer

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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...
 

IndigoBlueWagon

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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.
 

Fourdiesel

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May 19, 2003
Location
SW Wash. USA
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'04 Touareg V10 TDI
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.
 

jettawreck

Top Post Dawg
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Aug 2, 2004
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Northern Minnesota-55744
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2001 Jetta and 2003 Jetta
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.
 

mrGutWrench

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Aug 29, 2002
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Carrboro, NC
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'03 Jetta Wagon, 5-speed, 563K Miles (July '23)
Fourdiesel said:
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???)
 

Fourdiesel

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'04 Touareg V10 TDI
After more thought -- There is a CHANCE that the "larger than normal" blow-by is not very significant in the scheme of things. It may indicate "normal" wear - depending on how many miles on the motor and the oil used and the regularity of changes.
If the valves are fixed - and I think they should ALL be ground / adjusted, the compression may pull back into the "normal" range on all cylinders. If they don't, then ring/piston work is in order.
Do your best to try to 'free up' the rings w/ comething like GM's "top engine cleaner" while the head is off. This is an excellent solvent to free stuck/varnished rings - even when the cylinders are no longer very "cylindrical".
 
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Herm TDI

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Fourdiesel said:
...Do your best to try to 'free up' the rings w/ comething like GM's "top engine cleaner" while the head is off. This is an excellent solvent to free stuck/varnished rings - even when the cylinders are no longer very "cylindrical".
This is a good idea...I have a TSB from "Saturn" on this same product and according to the TSB will work at clearing the rings. Having the cylinder head off will ensure that I sdon't have any issues with residual solvent in the combustion chamber. I'll be changing the engine oil anyway so cleaning rings makes a lot of sence (to me).
 

Franko6

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Sw Missouri
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Jetta, 99, Silver`
It seems I have gotten involved with this one... When I get the head,g I'll do my usual analysis. I'll make a picture essay.

Btw: For those whom I have led to be misunderstanding the Bentley Book's valve guide clearance measurement of .051" max that is a ROCK measure.
In other words, if you move the valve approx .250" off of its seat, and dial indicate the total lash back and forth from the edge of the vavle head, it should not exceed .051". To me, that is still quite a bit of slack. In any case, I'm posting this retraction of mine on the 101 section.
 

hevster1

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98 NB
Blow by is compression getting past the rings. It has nothing to do with the valves. While the head is off why wouldn't you pull #1 piston and check for damage/problems? When doing any head I would strongly suggest doing ALL the valves over. Replacing guides and seats is expensive and may get very near the price of a rebuilt head. I am not sure on these but if the seats are machineable I would do that and replace all the exhaust valves and machine the intakes after checking the guides. If it is very high mileage i might just replace them all. I would also replace the valve springs. Better safe than sorry. The guides may be able to be knurled if out of spec but replacing is the best way to do it.
 

mrchill

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These engines blow by a lot anyway. Also, a bad exhaust valve CAN increase blow by, while decreasing power. The question is, how did this happen. Make certain your piston isnt damaged in any way. They may look fine at first, but be sure. I have seen a few missing some or much of thier mass near the ring lands. They often fill with soot and hide the damage. As far as the head goes, in the absence of a culprit, I'd consider a full head rebuild. It isnt much more at that point, and you will be certain of its condition.
 

professor

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Jan 8, 2001
Location
Edina, Mn USA
Next time, remove the glow plug and put air pressure (125#) in. Piston must be exactly at TDC. See where the air comes out; easily heard at exhaust, intake, coolant fill, or through oil fill. That way you know where the compression is going before you start.
 

hevster1

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Columbia NJ
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98 NB
professor said:
Next time, remove the glow plug and put air pressure (125#) in. Piston must be exactly at TDC. See where the air comes out; easily heard at exhaust, intake, coolant fill, or through oil fill. That way you know where the compression is going before you start.
A leak down test is a good idea. He already has the head off though and while a leak down test will give you an idea it could also mask if it has another problem. In other words his valve was leaking heavily so it could mask the rings being bad.
In what way would a bad valve increase blow by mr chill? You just decreased the cylinder pressure by having a burnt valve.
 

mrchill

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The gases are constantly going both ways. Depending on the rest of the condition, and engine speed, the combustion can leak into the crankcase. It also will leak past the guide in the exhaust valve area as this condition usually starts with heat which then causes excessive clearance at the guide, culminating in cylinder pressure going past the leaing valve and the guide and into the cylinder head at the spring. It also can overheat the intake charge as the combusted gases leak past the valve, into the cylinder and when the intake valve opens, into the intake for the rest of the cylinders.
 

MOGolf

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underneath something
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This mention and discussion of blow-by seems to have forgotten that the perception of blow-by by feel of air out the oil filler cap, or CCV vent, is skewed by the vacuum pump exhausting into the cylinder head cover/valve train area. This does not happen on an A3 but does on the A4. Comparison is questionable.
 

hevster1

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mrchill said:
The gases are constantly going both ways. Depending on the rest of the condition, and engine speed, the combustion can leak into the crankcase. It also will leak past the guide in the exhaust valve area as this condition usually starts with heat which then causes excessive clearance at the guide, culminating in cylinder pressure going past the leaing valve and the guide and into the cylinder head at the spring. It also can overheat the intake charge as the combusted gases leak past the valve, into the cylinder and when the intake valve opens, into the intake for the rest of the cylinders.
OK but that is not blow by per definition. Also in order for cylinder pressure to go up the guide as suggested the valve seals would blow off. At that point due to vacuum on the reverse of the stroke, it would suck oil down like there was no tomorrow and smoke badly.
 

PDJetta

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Souzafone said:
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.
My '85 Jetta TD began missing on idle once and I pulled the injector and gave it a compression test. 450 psi on three and about 200 psi on one cylinder. I tore the engine apart and when I pushed the low compression cylinder's piston out, the top ring fell off. It was broken right in the middle (oposite of the ring gap) and its face that contacts the cylinder wall was all black, indicating it was broken for quite some time. I also noticed an occassional slight ticking sound (that was very hard to distinguish from normal diesel clatter) when the engine ran and that must have been the broken ring.

Its possible, and the ring may still be on the piston.

--Nate

--Nate
 
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