I hate to keep asking questions, but I don't know the answers and you guys do. Sorry. Working on a 77 f100 302. It was in parts when I got it. I got it running but shut it down after about 10 seconds because I did not have a radiator hooked up. This morning I wanted to check compression before I determined the engine is good. I checked all cylinders and got 30 on the gauge at each cylinder. I know it seems odd. I went to auto zone and rented one from them and got the same reading. I must be doing something wrong. It seems to run fine. It cannot be that it runs at 30 psi can it? What am I missing? Neither gauge would even set the reading. Meaning that I had to watch the gauge to see what it read because the highest reading stop did not work. Ideas?

 

Yeah, unless there is something drastically wrong, or a super radical cam installed, 30 PSI is way too low.
Ford doesn't publish actual compression figures, all the manual says is that the lowest and highest cylinders should be within 25% of each other.
I would expect at least 110 PSI on an engine that's in good shape.
You did have the throttle blocked wide open didn't you? Also, all eight plugs out and a fully charged battery are needed to get a valid test.
One possibility if the compression was actually that low would be a timing chain that's jumped 1 tooth. Not sure if it would run though.

 

Compression is tested with engine at normal operating temperature, plugs removed, throttle wide open.

 

The engine must be at normal operating temperature (run for 5-8 minutes usually) and should crank motor 7 to 8 times for each cylinder to get an accurate reading ..

 

With 30psi in all cylinders it would barely run at all. That isn't enough pressure to light off the mixture. It's very unlikely you had two bad gauges, my guess is you didn't spin it long enough for it to build up compression and/or didn't hold the throttle wide open.


Remove ALL the plugs before doing the test. Hold the throttle wide open (allows more air into the cyl), usually 5 rotations will get it to max out, and it won't build more pressure after that. I typically go 8 rotations just to be sure, although if you watch your gauge, after about 5 revolutions, it will max out and additional revolutions will not give you any more pressure.

This procedure should give you better readings.

 

Thanks guys. It hit me before I got any responses that all 8 plugs need to be out. So that's my first mistake. I am not at operating temperature because the radiator is not hooked up. I will do that today. Also, the transmission lines were cut by the PO. I did not know that one line would pump when the engine was running. I should have known. So I need to repair this also before I get it to operating temp. Thanks. I will do these tasks then try again.

 

Thank you for the help. I hooked up the transmission lines and let the engine run for about 10 minutes. I then tested again with the rented compression tester with a FULLY CHARGED BATTERY. I got about 95 for readings on all cylinders. I then tried my 6 month old compression tester and it read 30. So I am happy about the results. Thank you for the help. The key, I think was the FULLY CHARGED BATTERY and a good tester.

 

Yeah, that sounds better .