Compression Testing

[Ron CJ8]

Just borrowed a test kit from a friend because I wanted to go over my engine a little (Comp., timing...) just to check a few things out. The instructions say to have the engine running a while to bring it up to temp. Just wondering why it effects the compression? Also, does anyone have any numbers for stock 304. I bought the engine from someone as is for $150. I had to get an intake, carb (edelbrock) and distr. but thats about it. It only smokes a little (whittesh) on startup. Any info would be great.

Thanks

 

[*JOKER*]

Engine warm up

Hiya Ron.

The reason for a warm up is to allow the metals' (i.e. piston rings, etc) to expand. If you notice, your piston rings have a pre-measured gap in them. This is to allow for expansion while providing a good seal. To a slightly lesser degree, a warm up assures that oil coats the cylinder wall, providing a seal between ring and cylinder wall.

Both of these allow for the accurate compression reading of the engine's normal compression. Good luck.

 

[*JOKER*]

Whoops

I forgot to mention:

Your white smoke can be:

1. Excess carbon - pretty common for an older engine (especially that's been running rich. GM sells an upper engine cleaner for about $10. You pour it in, let it sit a few minutes and start it up.

2. Water in fuel. Maybe due to a leaking gas cap gasket. Drain your fuel tank, replace your gas cap.

3. Coolant from leaking head gasket. Check your oil, too. Water in an engine is good way to kill it. Replacing the head gasket isn't a total pain.

4. Cracked head.

 

[Ron CJ8]

Numbers

Joker, Thanks for the reply.

It looks like I will be doing a couple more tests. Here are the numbers I got:

1 145
2 142
3 152
4 155
5 40
6 154
7 155
8 140

I'm still trying to find out what the base line is. My Chilton manual doesn't list the numbers. After reading some more, I'm going to add a tsp. of oil to number 5 and redo the test. Maybe a head gasket or valve?? I do not see any water in oil.

Thanks again.

 

[rickandsandie]

Those numbers look REAL good except for number 5. Looks like it could be a burnt valve, bad valve seat or a cracked head. May explain the white 'smoke' coming out the tailpipe. One way to check for a cracked head is to remove the thermostat and run the engine. Course make sure you've reinstalled the thermostat housing and refilled the radiator. If you can see bubbles in the radiator fill area and/or you can see what looks like oil film in the coolant solution then you've probably got a cracked head. The best thing to do is a cylinder leak down test. If you don't have access to one of these kits, you can hook up compressed air to the spark plug hole. Keep the air pressure at about 100psi. Then listen for escaping air sounds from the carb and/or tailpipe. Look in the radiator fill hole and see if there are air bubbles there. Where ever, will tell you what problem area you have.

Rick Jandrt
81Eight