Should a cold hand crank compression test work?
 

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?

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.:eek:

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)?

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

I prefer cold compression since it gives a realistic example of what you will get in the morning when you go out to start.

Thanks for the help, I'll hook the starter up and crank it over.

All the above is good information.

However, a leak down test ( supposing you have compressed air available ) is the definitive test.
It will determin leak down equaly across all cylinders, unlike a starter, with battery power fluctuation and heat build up, oil pump drag, and other factors.

You will be also able to check bore tapper, ring wear, valve sealing/leakage, determin exhaust or intake valve leaks, head gasket leaks and where, cylinder head cracks.....and, as the commercials say:rolleyes: ...lots more !

I find that, especially on diesels, this will check overall health and without knackering the starter.
With an adapter from Assemmacher Tools, you can also check assembled short blocks, the only way to be sure you have a good piston block assembly before you install the cylinder head.

Down side ? Holding or setting the engine at TDC accurately...AND...be sure the vehicle is not in gear or you left your breaker bar on the crank bolt. :eek:

I have on occasion, have seen readings of 500 plus psi on 25 to 1 c/r engines. ( Heat, plus compressive pressure. )when cranking pressure testing.



.