Best way to Compression Test the cylinders
 

Hey guys,

I'm still on the hunt for why the heck my disco won't stay running after it starts up. Has spark, has air, replaced fuel pump, fuel rail, and injectors, I'm totally stumped. Any advice on the easiest way to compression test the cylinders?

You need to get a 'leak-down tester'. Harbor freight or you can 'rent' one at your local parts store.

Define 'won't stay running'...

sorry, dupe thread, let me dig up the old one

You can buy or hire a compression tester, to buy they aren't expensive. They usually have various adaptors to fit various spark plug holes/configurations. You'll need to remove all the spark plugs when testing. Check the RAVE manual for the required cylinder pressure expected. You'll be looking for a similar or slightly less pressure than RAVE states. What you are ultimately trying to compare is one cylinder against the next, i.e. all 8 cylinders should be within 5% pressure of each other with a very max of 10%. Any cylinders outside this spec are suspect, either HG leaks, valve seat leaks or cracks in the head or liners. If two adjacent cylinders show lowish pressure it usually indicates there is a leak of some type between these two cylinders. Good luck.

Just to expand a little bit on previous post.
Start with number 1, insert compression gauge and have someone crank for 3 compression strokes. Write the number down, continue for each cylinder. Remember drivers side is 1,3,5,7(front to back). Pass. Side 2,4,6,8.
When we had a low cylinder, we squirted oil in that one and rechecked, if it rose 2nd time, the rings were suspect. If no change, could be burned valve, blown head gasket, cracked head or even hole in piston. Like previous post, all 8 should be real close.

Pull the fuel pump fuse when you do the compression test. You aren't looking to start the engine, you just need it to crank over. Have someone crank the truck over about 3 times and write down the numbers on the gauge. Leave the gauge on for 10 min or so and come back and see how far the gauge has dropped. Do this for all cylinders and compare your numbers.

Thanks guys, don't I need to disconnect the the ground to the coils? Is there a fuse I can pull? I have the gauge already.

just pull the fuse for the fuel pump or the coil, fuel pump relay is easily spotted

When I tested mine I unplugged the coils from the harness. There is a connector from the wiring harness on the outside edge of each coil pack (so, 2 connectors). It is a royal pain to reach them and avoid dropping those little metal keepers on them. You really have to lay on the intake to do it. If you do that, be sure you have a magnet... they often fall in top of the transmission bellhousing and are hard to reach.

Remove all plugs.
Do each cylinder in turn using the same number of cranks, usually 3-5 or until the pressure doesn't continue to climb, but not more than 5 cranks. If it takes more you have problems.
If you find low pressure in a cylinder install the adjacent cylinder plug(s) and retest. If the pressure goes up from doing that, you likely have a leak between cylinders.
Adding oil to a low pressure cylinder has been covered.

There's no reason to leave the tester connected as they have a check valve in them to prevent the gauge loosing pressure with each cycle of the cylinder.

Compression test alone isn't good enough, also do a leak down test as mentioned. In fact a leak down test is better, but a compression test is a good start if that's all you have on hand. There are instructions on performing a leak down test on the net, including youtube videos.