Most mechanics don't really use a spoon for a spoonful of oil, they squirt it in and spin the motor before rechecking compression. By your reasoning, every time fuel enters the chamber, the compression is raised. The wet test is to look for a significant increase in compression, not just a minor increase in psi. Large amounts of carbon deposits will also raise the compression ratio falsely. These are basic engine troubleshooting concepts that every ASE mechanic knows.

 

The accepted method when doing a leak down test on airplanes is to find TDC for that cyl, put 80# of air into the gague with the regulator on the unit and read the second gague. If you get 76 to 78 on gague #2, thats as close as it will ever get to perfection. You mwill never see 80# in and 80# on the cyl. Often we rock the prop back and forth to seat the valves better and rings in the bottom of the grove.

This will only tell you the condition of the cylinders/ rings and not berrings, nor camshaft.

 

A teaspoon of fuel enters the cylinder and if its that much its not going to run.

Leak down is the only way to go.

 

You will always have some leak down in any engine. If all the cylinders are within 10% or so of each other and no cylinder leaks any more than say 20% or more. You are probably o.k. with the rings, intake gasket, head gasket and valves. Just follow the instructions with the kit and my method above.

Since you are taking the engine out anyway, drop the oil pan, take the caps off the crank bearings and do a visual inspection of the journals. Then, check the crank bearing clearances. It's easy to do. Be sure to install the caps in the same place they came off. When reinstalling the oil pan put on new front and rear main seals and a new pan gasket.

If the leak down check is o.k. and the crank bearings and journals check out o.k. and your engine seems to be running well I wouldn't bother with the cost of rebuilding it.

BTW...the reason you have to crank the engine over at least 4 times to do a compression check is to assure the cylinder you are checking passes through the compression stroke (intake, compression, power, exhaust). Sometime between 1 and 4 cranks it will pass through that compression stroke and you will get a reading. It has nothing to do with building up more pressure.

 

The engine doesn't run during a diagnostic compression test.

 

NO shyt the comment was directed at this statement

 

Over how long a period of time?

 

The leak down is instant. Thats why it's called "compression differential ". The cyl's wont hold air like a balloon.
I'm not sure why we use 80# of air in the cyl, except that much more than that it becomes dangrous. I've got a 520 ci 6 cylinder eng on my airplane and when performing a differential compression test you have hang on to the prop. If not on TDC when air pressure is applied it will rotate with grate force and could split your head open. I guess I'm warning you to be carefull when applying the air pressure.

 

Ok thanks, that helps me understand fully on how to do and what to look for on the leak down test.

I hope it doesn't turn into a heart breaker.

Thank you all, Marty

 

You really don't need to hold it at TDC for any length of time. With a socket and a breaker bar on the balancer even though the air at 90 psi wants to push the piston down you can hold it quite easily and even control the piston travel.

Be sure to index the engine through the firing order in the clockwise direction when in front of the engine looking at it.

 

 

90 psi on a 4 inch diameter piston will generate about 1000 pounds of force pushing on the crank. If the piston is at TDC, there is no 'moment arm' on the crank, so it won't turn. If it is off by just 5-10 degrees...look out! It is best to fix the crank in position (long breaker bar or ratchet with socket on the damper bolt and restrained by frame rail, etc.) so that the crank can't spin.

 

there is some truth in this statement however if the op would have said he was pulling the mill anyway, could have saved a lot of opinions.....hell with the mill out of the car and pulling the heads anyway..what's wrong with checking the ridge and maybe re ringing or changing slugs to tailor a cam pick???..........

 

I just did the leak down check a month ago. Believe me it's quite easy to hold the crank and piston at TDC with 90 psi with very little effort even as it moves off of TDC. As soon as the piston starts to move off of TDC the volume increases, thus the pressure decreases significantly and quickly.

I used a 15" bar over a socket wrench.

 

That's true....once you've shut off the air line.

 

To the OP: 7T1vette brings up a point to remember. There is a push/pull of the valve **** to grab the 90 psi and stop the flow of air from your compressor. At TDC the volume is very small and as the volume increases as the piston starts to move, that very small volume of 90 psi increases significantly. I didn't use the shut off but you probably should since it is a feature of the kit.

 

This is very informative and helpful.

I ordered the leak down tester. I will follow all instructions closely.

Thanks again guys, Marty