compression test on rebuild

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Methanolab
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compression test on rebuild

Postby Methanolab » Mon Dec 15, 2008 8:44 pm

I have about 4 months and 1200 miles on a rebuild. Bored out .20 over with new pistons. I just did a compression test and all four cylinders are around 255 psi - nowhere near 400-490 psi. I got a chance to put a little oil in one cylinder and retest it. It went up to 550 and was spiking over 6 or 700 psi. It almost started on the oil and compression. Am I good and still breaking in the motor or what? Any ideas? Thanks, merry Christmas

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Vincent Waldon
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Postby Vincent Waldon » Mon Dec 15, 2008 11:50 pm

My ideas would be:

- given your mileage and compression my Internet diagnosis is that your rings didn't break in and instead glazed over.

- you've proven why VW specifically says *not* to perform the wet compression test on a diesel!! :wink:


A couple of questions that may provide some clues:

- what did you use to hone the bores, and what angle were the hone lines ?
- how did you treat the engine during the first hour or two ?
- what kind of oil have you been using ?
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libbybapa
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Postby libbybapa » Tue Dec 16, 2008 1:04 am

Did you baby it? If so, then that's the problem.

Andrew

Methanolab
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Postby Methanolab » Tue Dec 16, 2008 1:35 am

It was probably sixty some degrees when I did the compression test. There is some confusion over hot vs. cold test. My Bentley says 86 deg F.

A good machine shop did the bore and hone. They are familiar with VWs and diesels. They assured me that they had the proper setup, stones, and grit, and experience for a diesel. The work looked good when it was done.

I have been using the cheapo Coastal Premium Fleet 15-40.

The first thing that happened after the first start-up was that I couldn't get it in gear. I was out of time and had to leave town so I had a garage replace the busted CV. I assume this required little running of the motor. I got it towed to the shop then drove it home a couple miles, not babied.

The first thing the machine shop told me to do was warm up the motor and then drain the oil, look for chunks and put in new oil.

After that I pulled out and drove it 200 miles medium hard.

Its been running and starting real good. I am up to about 41.5 mpg, about where it was before it died before I rebuilt it. It runs real rough and coughs a lot of smoke at start up for a few second when the weather dips below freezing. But then it runs great. I have also had fuel seeping out of one of the injectors between the top and bottom half so I thought this might be causing a problem with the spray pattern causing the rough running for a few seconds on sub-freezing start-up.

I sure hope the cylinders did not glaze. Please tell me it will be fine in a few thousand miles or that all I have to do is dump in some boraxo : )

I do have some quickseat powder that the shop told me not to use. I could try lubing the cylinders then blowing a little of that into the injector holes and cranking.

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rabbit_man
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Postby rabbit_man » Tue Dec 16, 2008 3:19 am

I just read somewere that for the best break-in your suppozed to warm it up normal and then run it hard otherwise the sharpness from the honing gets worn smooth and full break-in never happens and a rebuild is necessary.

- you've proven why VW specifically says *not* to perform the wet compression test on a diesel!!

I'll have to double check but I'm almost sure the 77-84 diesel bentley says how and why to do a wet test. :P

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Vincent Waldon
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Postby Vincent Waldon » Tue Dec 16, 2008 10:04 am

rabbit_man wrote:
- you've proven why VW specifically says *not* to perform the wet compression test on a diesel!!

I'll have to double check but I'm almost sure the 77-84 diesel bentley says how and why to do a wet test. :P


Yup, as I recall that's the case: MK1 manual says do it, MK2 and MK3 manuals definitely say "don't".

Lots of people say they've never had a problem either... but the comment that it "almost started" makes me think the Bentley folks changed the instructions over time on purpose. :cry: :lol:

The good news is that it probably told you what you need to know... the rings are likely the issue.

Sounds like the prep work was top-notch, too.
Vince

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Postby tawney » Tue Dec 16, 2008 3:40 pm

Do a test with the engine warmed up; if it's starting in cold weather your compression can't really be that bad.

You're letting it turn over until you get the maximum reading? I never saw one as low as 255.

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Methanolab
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Postby Methanolab » Tue Dec 16, 2008 6:31 pm

Yea and I tested each cylinder twice letting the motor turn for about ten seconds. I talked to a guy at a diesel injector shop and he said it is probably a bad compression gauge or a timing problem. He suggested checking again hot, trying a different gauge, and running the snot out of it for the next few thousand miles. Thanks, I'll get back to you all.

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Quantum-man
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Postby Quantum-man » Tue Dec 16, 2008 7:06 pm

If all values are very close; then I'd want to be sure it's not the comp gauge under estimating. Perhaps try a different tester before more radical work...
Someone tell me the use of a wet test? IMO in most instances the compression would go up, perhaps to rediculous levels, except when it shows up a duff valve seating, am I right??
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libbybapa
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Postby libbybapa » Wed Dec 17, 2008 11:13 am

The wet compression will do exactly as you suggest. If the value goes up significantly you can assume rings, if not, then it would be valves, a crack or gasket. I am not recommending the wet test.

Andrew

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libbybapa
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Postby libbybapa » Wed Dec 17, 2008 11:19 am

Methanolab wrote:I talked to a guy at a diesel injector shop and he said it is probably a bad compression gauge or a timing problem. He suggested checking again hot, trying a different gauge, and running the snot out of it for the next few thousand miles. Thanks, I'll get back to you all.


I would certainly try another tester. I don't see how timing would significantly affect the compression. Pump timing is irrelevant, and valve timing would cause piston valve contact prior to altering compression significantly. The valve lash being incorrect (bad hyd lifters or oil pressure - improper shims for solid lifters) would affect compression, tho. Cranking speed will also affect compression readings. Is it really spinning fast?

When you run the snot out of it, make sure it is up to temp first and then do multiple high load runs letting off the go pedal every few seconds. Whenever I start a fresh rebuild, I make sure it is up to temp and then actually work to seat the rings. Diesel is such a good lubricant that it is very easy to glaze the cylinders.

Andrew


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