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General Information => Troubleshooting => Topic started by: AudiVWguy on December 10, 2008, 12:54:35 am

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: AudiVWguy on December 10, 2008, 12:54:35 am

I was wanting to do a compression test soon. The Bently doesn't say about the engine temp, but the case could be made for cold because that would be the worst case senario. I assume I should disconnect the glow plug system too.
How many seconds do you normally crank?
Thanks,
-JB

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: Vincent Waldon on December 10, 2008, 09:26:56 am

I'm pretty sure I've seen one of the Bentleys mention "normal operating temperature"  by the compression specs, so that's what I've always used when comparing numbers to the book.

Having said that, I've seen two diesels now with markedly different pressures when cold as opposed to warm... so if cold starts are the issue I test the pressures while warm, let it sit overnight, and then run it again for comparison.

Oh, and per a recent thread... best idea is to leave the heat shields in.  :wink:

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: the caveman on December 10, 2008, 10:57:01 am

And just crank it directly at the starter to save the glowplugs and definitley keep the heat shields in.

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: rabbitman on December 10, 2008, 04:29:58 pm

I assume your talking mk1? My bentley says get it up to operating temp before checking compression.

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: Vincent Waldon on December 10, 2008, 08:02:01 pm

Quote from: "rabbitman"

I assume your talking mk1? My bentley says get it up to operating temp before checking compression.

Then that must have been where I read it from. :wink:

So many VWs, so many Bentleys.

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: AudiVWguy on December 10, 2008, 11:15:32 pm

Yeah, maybe these Bently guys put their books together while they're sittin at the bar.  :D
My book gives specs in two locations, neither talks about normal operating temp.
Sorry, yes its an MK1.
-JB

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: Vincent Waldon on December 10, 2008, 11:44:15 pm

OK, did a little reading... pg 15a-1 of the MK3 Bentley under "Compression Test":

"For the most accurate test results, the battery should be fully charged and the engine should be warm".

Yea verily, thus it is spoken.   :wink:

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: Smokey Eddy on December 11, 2008, 12:25:49 am

I have a tester from a friend but the fittings wont fit the injector hole or glow plug hole. I don't really want to buy the adapter :P so if anyone is in abbotsford and wants to hang out and talk VW with me I'd love to. In addition to knowing what the comp. on my new engine is in a month or two  :D pm me!

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: Methanolab on December 15, 2008, 08:47:24 pm

My Bentley for the 1983 Quantum TD says oil at 86 deg F.

But help...

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

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: AudiVWguy on December 15, 2008, 10:55:04 pm

Sounds like the rings aren't seating. What kind of oil are you running? Maybe it's too good for the rings to seat.

Title: Compression Test-- is the engine cold or hot for accuracy?
Post by: Methanolab on December 16, 2008, 01:37:33 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.