"B. Schiefelbein" wrote:
> I have tried methods one and two above (both wet and dry tests) and the
> results vary greatly depending on whether the test is conducted with the
> engine running (cyl 1 = 85 psi, 2 = 90 psi, 3 = 85 psi, 4 = 90 psi) or with
> plugs in and using the starter motor to turn the engine (cyl 1 = 150 psi, 2
> = 150 psi, 3 = 150 psi, 4 = 145 psi).
>
> 1. Is there a correct method?
I have always used the engine off. I don't remove all plugs
either, I just take one out at a time, but that may be wrong.
> 2. What readings should I be looking for?
Generally, the main thing you look for is uniformity. Those
results look excellent.
> 3. Regardless of the readings, why is there such a difference between the
> two methods outlined above.
I would guess that it's because, with the engine idling,
there is a very strong vacuum in the manifold. Since the air
isn't as dense, when it's sucked into the cylinder there
just isn't as much air in there. So when the piston compresses
it, the total pressure isn't as high as when it gets a full
charge to compress.
When doing it with the engine off, you do it with
the throttle wide open, for much the same reason.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
"Oh. Make that two no nos." - Ajax
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