What is the correct procedure for doing a compression test on a 1980 242, with B21F engine with K-jet ? Should the engine be cold, hot (operating temp), or somewhere in between ? Which wires from ignition should be disconnected and any fuses pulled to stop fuel flow ? How long to crank the engine to get a proper reading ? Thanks.

Mike

For most cars you would want to remove the fuel injection fuse/relay and disconnect the coil so that you don't keep injecting raw fuel which is bad for the pistons and also don't burn the ignition coil.

Like Kyle said, with a few additions:

the remote starter connection is usually found behind the intake manifold at the firewall - a single female spade terminal. Jumper it to the battery pos. terminal to crank the engine. Since the key is off, the fuel pumps won't run. But due to the ballast resistor ignition setup, you will get spark at the coil while cranking, so disconnect the #15 coil terminal.

Block the throttle about 1/4 open (or more) to let air flow into the test cylinder. Warm-to-hot engine is best, it simulates operating conditions.

Screw-in gauge is preferred. Good cylinders should come up to 99% of final pressure in three strokes: 75%-95%-99%..then the needle will just flicker on subsequent strokes.

Main thing is that all cylinders are fairly close, say within 20psi. Should all be around 140-170 psi depending on mileage and wear. One bad cylinder? - almost always points to a leaking valve, usually the exhaust. If all cylinders are low, but fairly even, it suggests worn rings. At that point a "wet" test may be worthwhile to confirm. Squirt a couple of cc's of oil in there, trying to get it to the high side of the piston (intake side) so it can run down and "seal" the rings all around. Put a rag over the oiled cyl plug hole and crank the engine a few turns - oil will spray out. Then do your test. Good cyl's will come up a little, ones with bad rings, maybe a lot.

I've tried leakdown testing - great in theory, but I found them very time-consuming and hard to perform correctly, and results were quite variable and didn't add much useful info to a good compression test. Maybe just my lack of skill.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

As usual, I agree with Volvodad (being another one), but learned to block the throttle wide open.

I also pull off one of the coil primary wires to kill the ignition—which on our Bosch FI cars will also prevent any fuel pumping when cranking. No need to mess with FI fuses or relay.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

Bruce - I agree with blocking the throttle wide open; it can't hurt. But with only the cylinder under test trying to pull vacuum on the manifold (all others have open spark plug holes) I've found that even a closed throttle makes only a small difference.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

Bob, That makes excellent sense. I've just been following an old rule without giving it any thought.
Thanks,

Bruce
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

Thanks muchly to all of the guidance and advice.

Mike

Pull all of the s/plugs out.....
Key off.....
Remote starter switch connected to the starter.....
Compression guage installed.....
Crank it for several revolutions....(you'll see the guage needle climb and then level off somewhat)
Write it down and go to the next one.

I like to attach a battery charger when doing this so the battery juice will stay up.

You might add a few squirts of oil in each cylinder and then repeat the test to see if the readings come up. If the readings go up then the rings might be worn.
called the dry/wet compression test.

The delta should be less than 20% (I think thats the number) between high and low cyl.

In the performance of a compression test, I do not recall ever pulling any fuses, some times I would disconnect the coil. I usually used a starter button device to bypass the switch, this made it a one-man job.
About 4 to 6 turn-overs yielded full compression
This on the old Saabs and Volkswagens, MG and Triumph,etc.
I don't think the engine temperature mattered - usually the problem was a burned valve or two, sometimes a badly blown head gasket.
The leak-down test is considered to be better, more accurate.
One can also pull one ignition cable at a time and measure the RPM drop at an idle.