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Volkswagen Jetta, Golf, GTI: 1993-1999, Cabrio 1995-2002 (A3) Compression Test Results


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Last Post: Nov 25, 2013 6:28 PM Last Post By: tcovenant2000
Gene54

Posts: 169
Registered: 06/24/04
Compression Test Results
Posted: Nov 22, 2013 7:18 AM
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bq. I finally got some time to to pressure test the cooling system and a compression test.
The cooling system test resulted in finding one of the hose joints
behind the alternator/ AC compressor bracket was leaking. Since it was
also accessible I pulled the water pump and found the previous owner
installed one with the plastic impeller that also was hitting the back
of the housing. So the first job was fix the leak and replace the pump
with the metal impeller version.
Today I did the compression check and got some interesting and maybe
troubling results. I took 4 readings for each cylinder so here's the
averages:
#1=140
#2=131.75
#3=135.5
#4=121.5
The Bentley Manual says 140 is the low end of the specs. While #2 and #3 look a little low, they aren't too terrible, but #4 could be a real
problem. I didn't get to squirt some oil into the cylinder, so I'm not totally sure
whether its a valve or ring.
Any of the ABA gurus here lend me information on what these numbers are
telling me? Are they too far out of spec for an ABA and is #4 low enough
that it indicates I need to pull the head?
tcovenant2000

Posts: 1,762
Registered: 12/09/03
Re: Compression Test Results
Posted: Nov 22, 2013 6:17 PM   in response to: Gene54 in response to: Gene54
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What, if any, symptoms?
The numbers aren't great, but not terrible either. I'm more concerned that this isn't valve or ring, but head gasket. Could be you have a leak from cylinder to coolant system. Have you tested coolant system for combustion gases?

Thomas
Gene54

Posts: 169
Registered: 06/24/04
Re: Compression Test Results
Posted: Nov 24, 2013 9:20 AM   in response to: tcovenant2000 in response to: tcovenant2000
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Hi Thomas, hope all is going well.

The symptoms are a little vague. The motor is running a little rough. After it reaches operating temperture it feels like it idles a bit rough with an occational "thump" like you can feel a miss. What makes it a bit more confusing is when you come to a stop (like at a light) it feels nice and smooth, but as it sits at idle it starts runing rough again. I've done all the usual fixes, replacing the cap, rotor, wires, and spark plugs with Bosch. As towards the condition of the head gasket, the oil isn't milky like there's coolant getting into it and I've been keeping an eye on the coolant sphere and there doesn't seem to be loosing any coolent.

One thought I'm having is could it be the injector sealing o-rings?

Gene
tcovenant2000

Posts: 1,762
Registered: 12/09/03
Re: Compression Test Results
Posted: Nov 24, 2013 11:03 AM   in response to: Gene54 in response to: Gene54
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If you're not losing coolant it's probably fine.
I would guess vacuum leak somewhere for your symptoms. However, injector seals are a good place to look. A leaky one would cause loss of compression. If you did the leak-down test with oil that would also help point out a bad injector seal. In addition, a bad injector seal would also cause a vacuum leak.
You can do a check of the seals by warming up the car and letting it idle, then spraying starter fluid around the seals themselves. If you notice a change in idle then you've found a leak. This test is best done with a dwell meter or duty cycle meter connected to the Lambda test port. The duty cycle should be stable, varying only 5% or so but staying in a fixed range. If you spray the starter fluid around the seals and you see a sudden drop in duty cycle (indicating that the O2 sensor noticed the exhaust getting richer) that would be a sure sign of an injector seal.
It might be easiest to simply replace the seals anyway. If they are bad...new ones will be like magic to your car! Don't forget that the injectors seats need to come out and they have seals on them as well.

Thomas
Gene54

Posts: 169
Registered: 06/24/04
Re: Compression Test Results
Posted: Nov 24, 2013 1:58 PM   in response to: tcovenant2000 in response to: tcovenant2000
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I'll probably have a look at the injector seals sometime this week. Does VAG have a "block" that monitors one of those parameters?

I've found the injector seals are ID, 7.5mm or 7.52mm, with a thickness 3.5mm or 3.53mm and there's a difference in the number of o-rings needed on each injector. Haven't yet found the specs of the seat seals.

Is this correct? If so I should be able to grab the viton o-ring at any autoparts store?

Gene
tcovenant2000

Posts: 1,762
Registered: 12/09/03
Re: Compression Test Results
Posted: Nov 25, 2013 6:28 PM   in response to: Gene54 in response to: Gene54
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I know nothing about Vag-Com. I'm pretty much a CIS guy who knows something about compression, engines, leaks, seals, etc which is why I answered originally.
Yes, you should be able to find the seals at any Oring place if you have the specs. Viton is what you want. The color of the oring doesn't matter.
There may not be any seat seals. I don't know on these later engines if they have them or not. Again my experience is mostly with A-1 and A-2 chassis.

Thomas

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