I'm not sure which part of my setup is failing me but something doesn't seem right.

First of all, this is on my 87 300SDL that was running good and not smoking before I started to remove components.

I have the starter, intake manifold, radiator, and several other components removed so I cannot start the engine to warm it up.

I went and got my Harbor Freight 93644 compression tester this morning.

Removed all of my GP's and selected the correct adapter.

I tested the number 1 cylinder first and after about 5 crankshaft rotations I had almost no pressure, nothing reading on the gauge, just a bit of air when I hit the release valve.

I then moved to the number 2 cylinder and turned the crank about 10 times. This time I got nothing at all, no reading on the gauge, no air when I hit the release valve.

Then I got side tracked before getting to the other cylinders but I'm guessing that the engine either needs to be warmed up prior to doing this or I need to crank it around alot more than 10 times. According to the instructions it needs to be turned 8 times but I'm thinking that might mean 8 compression strokes, not just 8 revolutions of the crank.

What do you all think?

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1987 300SDL 167k
1992 Volvo 740 140k
1990 Volvo 740 250k
1989 Volvo 240 269k

Anyone want to trade an old Volvo for an '87 300sdl?

You cannot do a compression test without the engine turning at cranking speed.

The rings have a gap at their ends and all the compressed air in the cylinder will leak out of this gap in about three seconds. At cranking speed, the air doesn't have the time to leak past the gap so you'll get a "proper" reading.

At engine idle speed, the air has even less time to leak past the gap and the compression readings would be higher. But, you can't do a compression test with the engine running.

8-10 revolutions of the engine, at cranking speed, is sufficient for a proper reading. When you observe the gauge, it will jump up on every revolution.......a little less each time.......until it no longer increases. That's the reading for that cylinder.

So, get the starter back on before you do anymore testing.

You can. Just cover the port on the IP with a rag to abosrb the fuel.
Running compression is done quite often in gas and mechanical diesels.

.......no kidding.........learn something new every day.........and you can do it on gas as well??..........shut the fuel on the one cylinder??

What's the difference in readings as compared to crank speed..........I'm quite curious on that.........if you've ever done it.........

Detroit Diesel prefers running (Dynamic) compression insted of a traditional static compression test.

On gassers, use a test sparkplug (A sparkplug with a clip welded on it to attach on a ground) while testing compression so you don't fry the coil. You can't stop fuel on carburator engines.

On mechanical diesels (Like ours with external injectors and injection pump), remove the injection line and injector. Cover the IP port with a rag so fuel is not spit everywhere. DO NOT use the glowplug type adaptor and run the engine with a normal 0-500 or 0-1000psi testor. Running the engine with that and a working injector will pop the line/gauge.

HERE is a good page on g@sser testing.

Interesting thanks for that, Ive been around Jimmys and large diesels a long time but have never seen a compression test done on one.

How accurate is a "cold" test on our engines. (using the starter)?

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

That's excellent info.......thanks for the link.