Compression test questions

[SGB]

You would think I would have done this before in my many years of 914 ownership, but NO! So, I got a few qwerstions-
Firstest, what I learned from old posts-
1. WOT required.
2. about 8 revolutions needed.
3. Can check valve face sealing by loosening slightly to ensure full closure.
4. Engine should be at operating temp.

My questions-
1. What about that raw gas?
2. why so many revolutions? What does fewer tell me?
3. If valve has been too tight, wont faces be scoured (worst case) or carboned (best case)?
4. How the heck can I do this on a hot motor? Can cold engine figures be compared to anything?

Thanks!

[Dave at Pelican Parts]

Remove all four spark plugs, as well.

Unplug the FI wiring harness to disable the FI, so no raw fuel in the combustion chamber. If you run carbs, unplug the fuel pump and let the motor run the float bowls dry.

Eight to ten revolutions can show you if a cylinder "pumps up" or if it gives you full pressure on one or two revs. As I recall, it's normal to have the pressure build up over a couple of revs--taking much longer is a cause for some concern.

If there is crud on the valve seat or the valve, it will not seal completely and you will get compression leaking out. The same is true for a compression test or for a running engine.

Cold figures can be used, but are not as reliable as warm figures. You can run the check on a hot engine the same way you do on a cold one--but wear work gloves and work fast!

--DD

[John Rogers]

As Dave and you have noted, the engine has to be warm, operating temp is best. Warm the engine and get the tools ready and then shut it off. Pull the fuse or relay for the fuel pump, it is easier than unplugging the harness. Pull all the plugs and block the throttle wide open. Test one cylinder at a time and when you crank watch how the pressure jumps up. A good, stock engine should come close to full pressure in a couple or three cranks. A hot rodded engine with lots of cam overlap will take more revs and come up slower. When done, make sure you unblock the throttle before starting....

[lapuwali]

Also note that this is one of the most unreliable of the common tests done. The relative numbers cylinder to cylinder have some bearing to reality, but the absolute numbers are just as much a reflection of how good your starter and battery are as they are an indication of how well the cylinder is sealing.