When I do a compression check on my 79 f100 all I need to disconnect is one spark plug at a time, and the main wire at the top of the distributor correct. No need to disconnect anything else for safety?

 

Pull the wire that runs from the center of the distributor cap to the end of the coil.

I've done the compression test "one at a time", but have heard it's better to pull all the plugs out. Either way, the "range of results" is what is most important. There shouldn't be more than 20% difference between the highest and the lowest. For instance, the highest might be 150, and the lowest might be 120, and the others in-between.

My book doesn't define what's "too low", it's more important that all cylinders be relatively close

 

So just that one wire, do I need to pull the spark plug wires for safety or no?

 

Nope, just the wire to the coil.

 

If you have someone to help "man the key" sort of speak, you can pull a plug, slap the tester in and fire her up for a few seconds....that's all it takes for a good reading.

By yerself, yes pull the coil wire.

 

maybe I'm old school, but I was always taught NEVER to fire it up with the compression tester on it, and to get the most accurate readings turn the same number of cycles per cylinder. You will see the compression rise steeply in the first couple of turns, go up a little more by about the 4th cycle - that is where you want to read. Generally it's best to remove all the plugs before testing. Like I said, that may be outmoded nowadays - but that's what I was taught.

 

Dave, X2 plus lock the throttle open.

 

Houston Dave and Fuzzybob: Ever see..or use.. one of these before?

 

Sorry for the double pictures....

 

mine has a screw-in connector for the spark plug hole...

 

Watching the way and how fast the needle rises on its way to the highest PSI is part of the diagnostic. For consistency or accuracy, the highest number reached across all cylinders is noted at the exact same number of pumps. Say, 5. This is where the remote starter buttons are sweet.

What you're looking for is also if a particular cylinder lags behind or is "lazy" or erratic compared to the others in how fast it reaches the peak PSI for example. They should all come up smoothly and evenly from the first pump.

All spark plugs have to be removed before starting the test carburetor choke and throttle plates blocked open. I doubt the RPM would be consistent for a good test with plugs installed, the idea is to isolate and test each cylinder one at a time time uniformly. Can simply ground the coil wire, or remove the primary wire connection. The engine should also be at normal operating temperature, but this one is ignored a lot of times. If the numbers are good or acceptable on a cold engine, they would only be higher on a warm engine is the thinking.

It's important that the battery is in good shape and fully charged before starting the test, otherwise the cranking RPM will be high when testing the first few cylinders and start to fade at the last couple cylinders.

 

And.....please enlighten me as to why?

 

My understanding is that a minor head gasket leak between cylinders won't be caught if you don't pull both plugs.

 

Interesting concept, thanks.

Wouldn't a leak down test show the same...mebbe a better test?

I've had more faith in leak down tests than a compression check anyway...but I'm not discounting the fact they both have their place.

 

So, I guess I get no answer to my question as to why one should "NEVER fire it up with the compression tester on it".........Mebbe 4 days is not enough time?

But, thanks CelticOne for the reponse.