checking compression when alone

[turpentyne]

Hi...

I have no help today, and I've gotta check off things wrong with the truck - a 73 with HEI that won't start, just turns over. I want to check compression, but it dawns on me, I have nobody to turn over the engine. Doesn't matter whether I use a gauge or my finger over a spark plug hole. My other hand and feet... well, they just ain't long enough to reach the key!!

Any suggestions?

 

[firebane]

Compression gauage should hold pressure until you remove it. If not then you have a leaky gauge.

 

[friscobob]

Two things to do. Use/get a screw in hose & gauge, use a remote button, clip it to the hot terminal on bat an to solenoid on starter.

 

[turpentyne]

Yeah... was just down at the parts store, and they pointed that out. I didn't realize they held pressure.

Thanks!

 

[chengny]

You are probably aware of this, but I didn't see it mentioned - so just in case.

To get accurate compression readings - which indicate the sealing capabilities of the rings/valves - the engine must have an unrestricted supply of intake air during the test.

If it doesn't, only a minimal amount of air will be drawn into the cylinder on the intake stroke. This creates a very low pressure (vacuum) condition when the piston is at BDC (after the intake stroke).

So, due to the low pressure at BDC, the pressure developed at TDC (on the compression stroke) will be skewed artificially low.

Test results obtained with a restricted intake air supply will not give you an accurate indication of the tightness of the engine. It will comp out lower than it actually is.

While cranking the engine, the throttle plates must be held open. They don't have to be wide open, just stick a screwdriver or a pair of Chanellocks down the carb throat - that's enough.

 

[Camar068]

I had the same thing twice over the past 2 months. The first time it was the hot wire going to the distributor. It was loose and just fell off. Took the pin out of the connector and closed up the gap with a pair of needle nose pliars. The 2nd time it turned out to be either the ECM, wiring from it, or the distributor. I installed one (had one laying around) that didn't use the ECM and it fired right up.

Before I swapped it out, I made sure I had power going to the cap with the ignition on. As simple as that is to check, I'd start there.

Wow....was so busy trying to help...didnt think how old of a post this was. Wish they would post what fixed it.

 

[HotRodPC]

Remote Starter button is always handy to have. You can even make your own fairly cheap. But, as you found out, it's not actually necessary.

Also, to add to Jerry's tibbit about the throttle plates being opened, an accurate test also requires the spark plugs to be pulled. This is how you'd find out if compression is escaping to an adjacent cylinder via cracked head or blown head gasket. 2 dead cylinders side by side indicates a head gasket blown between cylinders.

 

[85c10owner]

I always did my tests with the engine at operating temp, or as close as it could be after taking the time to remove the plugs and screw in the gauge and check each cylinder.