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#60
by
racer_x
on 17 Mar, 2005 20:19
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Racer-X (and others) That is interesting the A2 Diesel Bentley advises against wet compression test where other sources advise it. Is the A2 Bentley manual an official Volkswagen of America service manual? FWIW, the A1 Bentley is not an official VWoA service manual, and I know of several errors, and in general although better than most manuals, it is not the "end all" of reference literature.
Yeah, I think the A2 Bentley is in the same "official status" as the A1 Bentley. Technically it's an "aftermarket manual," but it was produced with assistance from VW and uses many pictures and borrows a lot of information directly from the official VW service manuals.
But I also think that the paperwork that came with my diesel compression guage had some very dire warnings about "wet" compression tests. Somthing like, "Never put oil or fuel in the cylinders when performing a compression test with this guage. The warranty doesn't cover damage from 'wet' comperssion tests and if you are injured or killed, don't blame us and be sure your heirs know that it's not our fault if you screw up that way." I might not have the language exactly right, but I'm paraphrasing it pretty close.
And other diesel engines I've worked on have warnings similar to the one in the A2 Bentley about "wet" compression tests.
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#61
by
srivett
on 17 Mar, 2005 20:46
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I asked an owner of the factory bentley about this and he posted the compression test page for me. It is inconclusive as it doesn't say to put oil in and it doesn't say not to either. Check the very last picture.
http://www.cardomain.com/memberpage/471729/8Steve
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#62
by
dieselpower
on 17 Mar, 2005 22:48
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hey,
i've heard that because oil can't be compressed, if you do a wet test on a high compression diesel it can pressurize the cylinder and actually push the tester out with a huge force. im not sure that that could even happen with using just the force of the starter motor...but thats what i heard somewhere anyway.
good luck
Mark
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#63
by
jtanguay
on 17 Mar, 2005 23:32
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oil can be compressed a bit can't it??? :? I know water can't though.
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#64
by
phredde
on 18 Mar, 2005 00:47
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No liquid can be compressed as I understand it.
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#65
by
racer_x
on 18 Mar, 2005 05:20
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One other thing to keep in mind, on a diesel, 1 Tablespoon is more than 80% of the volume above the piston at TDC. On a VW gasser, it's closer to 25-30% of the volume, and in larger gas engines, it's even less of a percentage of the compressed volume. So the increase in compression from the added non-compressible fluid in the cylinder is much less in a lower compression gas engine.
If all that oil stays above the piston, in a diesel, it could multiply the compression pressure by a factor of as much as 5 or 6.
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#66
by
lord_verminaard
on 18 Mar, 2005 05:50
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Well anyway- why in the world would his compression be consistently low across all four cylinders? I've seen quite a few compression tests, and even on a Chevy 350 with a zillion miles, the cylinder pressure will fluctuate more than that. And if there is a bad cylinder, the pressure will be a good bit lower. Just doesnt make sense that his readings are nearly exact in all four cylinders.....
Out of curiosity, did you use a thicker head gasket when you put the new head on? I'd be about 90% sure that there is nothing wrong with the valves sealing... even if there were, the compression readings would not be THAT consistent..
Somthin just aint stirring the Kool-Aid here....
Brendan
84 Scirocco 8v
00 Camaro L36 M49
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#67
by
fspGTD
on 18 Mar, 2005 10:55
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Racer_x is right about 1 Tablespoon being a large quantity of our IDI VW Diesels' combustion chambers. I did a calculation of the combustion chamber size to verify this. 1600cc / 4 cyl / (23 + 1) = 16.66 cc combustion chamber size. Searching for the conversion of 1 Tablespoon to cc, I found a few different conversion rates on the internet, clustered aroundd 1 Tablespoon = 15 cc. Let's use the precise rate I found of 1T = 14.786cc. In that case, 1 Tablespoon is about 88% of the combustion chamber size! I agree with you guys that this wet test is fishy. Filling the combustion chamber with 88% liquid implys that it is expected to see a significant increase in compression numbers during the wet test, regardless of the condition of the rings. Looks like I shouldn't be too quick to condemn the rings.
Brendan, I also am kinda baffled as to what could explain the even lowness of the dry compression results. I used the thinnest, 1-notch head gasket, which was called for by piston projections. (The Piston projection numbers have been posted earlier in this thread.) It almost seems to imply... something is out of spec that is uniform to all cylinders... IE: cam timing or valve clearances. I did have the intake valve clearances set on the tight end of the specs... I could try changing that to the middle of the specs (cost of 4 adjustment shims is not bad...) and see if it makes a difference. I could also re-check my cam timing while I'm under the valve cover. It could be worth a shot, to try and rule valve timing out.
I could also consider getting the car back ready to drive, jump starting my motor (which is a lot easier with the rebuilt starter!
), and driving it somewhere to get a leakdown test, to try and conclusively detect whether the rings are bad. Thanks for helping me work through this guys!
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#68
by
Mark(The Miser)UK
on 18 Mar, 2005 12:00
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Re: cranking RPM with and without compression... Keep in mind that the energy used to push the piston up against compression during the compression stroke is energy that is returned to the crankshaft during the power stroke - assuming there were no blowby at all and also no loss of energy from compression heat conducted away. So the corrolary is that if energy loss from heat conduction was negligible and the cylinder was sealing pretty well, then we shouldn't expect a big change in cranking RPM by relieving compression in 1 or more cylinders.
Ah yes there is energy input from the compressed cylinder air...
But this only reduces the energy required. This is why a cranked engine goes wah wah wah wah where it speeds up and slows down (4 times a revolution) I have not done a comparison with my diesel but if you can compare cranking with sudden turn off of fuel solenoid with the key; the engine rotation doesnt peeter away but stops within a rev(ish) I guess the larger the flywheel the more even the cranking speed for any particular cranking speed. Many times I have done this on gassers...Including last week when I jump started my 1970 Bedford with 15ft cables (couldnt get my Quantum closer) There was no battery on the Bedford ; it had been stood since last August and last filled with petroleum in the previous September (2003). Cranking speed soon dropped down even with my 'Q' fast idling. I remembered my old trick and removed two spark plugs. Cranking speed felt like it doubled and the old girl fired up..
Why have I ruled out fuel injection as a cause of the low compression: fuel injection and injection timing is not a factor used during the compression test, but yet the dry compression results, at 300-320, came out much lower than they should have been (400-500). I also know thereis fuel getting to the injectors when cranking (as it occasionally sputters and there is a smell of diesel and occasional smoke puff out the exhaust), and the car runs like timing is set right and everything else is fine when it is warmed up and idling and running.
Sorry Jake I was thinking of the link of poor starting fuel supply to poor starting; and not as the cause of poor compression... with fuel ratios at idle alledgedly only requiring nearly 200:1 this may be too weak for starting hence a cranking fuel volume check might show something.
:twisted: Remember all we want to achieve here is to cure your hard starting so my occasional lateral interject is only an attempt to solve a mystery assuming its not simply a ring thing :twisted:
:idea: Yes or no on the wet compression test?... I say yes! Chances of igniting a drop of oil in a cold engine that is hard to start with a spray of fuel and glow is um er ziltch :wink:
Worried about using up over 50% of compression chamber? Then use a gauge with a reservoir of air between the two valves on the ends of the gauge hose. (a fatter or longer pipe with a teaspoon of extra capacity would do) :idea:
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#69
by
racer_x
on 19 Mar, 2005 15:54
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Racer_x is right about 1 Tablespoon being a large quantity of our IDI VW Diesels' combustion chambers. I did a calculation of the combustion chamber size to verify this. 1600cc / 4 cyl / (23 + 1) = 16.66 cc combustion chamber size.
Well, if you want to be precise, it's 1588cc / 4 cylinder / (23 - 1) = 18cc in the combustion chamber.
There's 396cc of displacement in the cylinder and 18cc above the piston. So compression is ( 396 + 18 ) : 18 = 414 : 18 = 23:1.
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#70
by
Mark(The Miser)UK
on 19 Mar, 2005 18:20
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Racer_x is right about 1 Tablespoon being a large quantity of our IDI VW Diesels' combustion chambers. I did a calculation of the combustion chamber size to verify this. 1600cc / 4 cyl / (23 + 1) = 16.66 cc combustion chamber size.
Well, if you want to be precise, it's 1588cc / 4 cylinder / (23 - 1) = 18cc in the combustion chamber.
There's 396cc of displacement in the cylinder and 18cc above the piston. So compression is (396+18) : 18 = 414 : 18 = 23:1.
:shock: Yep your math
is correct although not easy for the non mathematical to follow due to lack of steps and strictly speaking a shortage of parentheses (brackets)... I assume that the Bentley is referring to the American tablespoon which is a pesky 2/3 of our Limey one and so about 9cc. or 50% of the chamber size. Why not use one of your puny teaspoons or about 3cc. 
:twisted: If this wet test is going to work properly we need to drive the car half onto the side walk inorder to be sure of spreading the oil over the entire compression ring including the gap! else oil is all in a 'corner':twisted:
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#71
by
dieseltech
on 19 Mar, 2005 19:04
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:lol: :lol: :lol: this place is great for a good laugh sometimes
Racer_x is right about 1 Tablespoon being a large quantity of our IDI VW Diesels' combustion chambers. I did a calculation of the combustion chamber size to verify this. 1600cc / 4 cyl / (23 + 1) = 16.66 cc combustion chamber size.
Well, if you want to be precise, it's 1588cc / 4 cylinder / (23 - 1) = 18cc in the combustion chamber.
There's 396cc of displacement in the cylinder and 18cc above the piston. So compression is (396+18) : 18 = 414 : 18 = 23:1.
:shock: Yep your math is correct although not easy for the non mathematical to follow due to lack of steps and strictly speaking a shortage of parentheses (brackets)... I assume that the Bentley is referring to the American tablespoon which is a pesky 2/3 of our Limey one and so about 9cc. or 50% of the chamber size. Why not use one of your puny teaspoons or about 3cc.
:twisted: If this wet test is going to work properly we n
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#72
by
fspGTD
on 23 Mar, 2005 08:51
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I checked a few things out with my GTD motor the other day. It looks like the engine builder set up a couple of the intake valves and one exhaust valve (checked on a cold engine, at least) too tight, so I installed slightly thinner shims bringing them all into specs (although generally towards the tight end of the specs.)
I also found that the cam timing was off a little bit. The cam was retarded a bit (I figure by about 2 degrees, since I measured it being off at the flywheel TDC reference by about 4mm, which I'd estimate sits at about a 9" diameter. You can do the math.) I think this was off because previously I didn't leave the cam locked when tightening the cam pulley bolt like is required to prevent things from moving when tightening the bolt. So I re-timed the cam, following the bentley procedure of leaving the cam locked when tightening the cam pulley bolt this time. (We should try and remember: lock cam when tightening the pulley bolt, unlock cam when loosening pulley bolt).
In theory, the earlier-closing intake valves should help build low-RPM compression. It should be interesting to see if what I did makes any noticeable difference.
I cranked the engine over some to prime the high pressure fuel lines and clear some oil out of the combustion chambers from the wet test, although wasn't able to start it, I'm hoping just because the battery was down (have I mentioned this is a really small battery, and I don't think I charged it since doing the wet compression test.) So I set the battery on a trickle-charger, and that be ready to make a run at starting the motor next time I get to working on the car. I hope it works!
PS - good catch, Racer_x, on the combustion chamber volume calculation error.
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#73
by
VWRacer
on 23 Mar, 2005 10:54
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Racer_x is right about 1 Tablespoon being a large quantity of our IDI VW Diesels' combustion chambers. I did a calculation of the combustion chamber size to verify this. 1600cc / 4 cyl / (23 + 1) = 16.66 cc combustion chamber size.
Well, if you want to be precise, it's 1588cc / 4 cylinder / (23 - 1) = 18cc in the combustion chamber.
There's 396cc of displacement in the cylinder and 18cc above the piston. So compression is (396+18) : 18 = 414 : 18 = 23:1.
:shock: Yep your math is correct although not easy for the non mathematical to follow due to lack of steps and strictly speaking a shortage of parentheses (brackets)... I assume that the Bentley is referring to the American tablespoon which is a pesky 2/3 of our Limey one and so about 9cc. or 50% of the chamber size. Why not use one of your puny teaspoons or about 3cc.
:twisted: If this wet test is going to work properly we need to drive the car half onto the side walk inorder to be sure of spreading the oil over the entire compression ring including the gap! else oil is all in a 'corner':twisted:
Puny? Hah!
Actually, our respective volumetric measurements have a close and intertwined history.
Back in the 18th century there were two gallons in common use in both Britain and North America...the wine and the imperial gallon. The wine gallon was based on a 7" wide by 6" tall cylinder and was the basis of taxation (smaller gallons made for more tax revenues...), while the imperial version was based on 10 pounds of water (a wine gallon weighs about 8.33 lbs).
When the USA went their own way from Britain public sentiment was pretty much against anything with the "imperial" ring to it, so the fledgling government chose the wine gallon as a standard measure of volume. Later it was formalized to
exactly 231 cubic inches. (A cylinder 7" wide and 6" tall is 3.5^2 x 3.14159 x 6 = 230.9 cubic inches.)
In any case, the wine gallon was always volume-based and fractionally divided (the imperial gallon was
weight based). A gallon has 4 quarts, each of which has 4 cups of 16 tablespoons (a 4th of a 4th).
Pretty cool, eh?
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#74
by
Mark(The Miser)UK
on 23 Mar, 2005 11:18
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