compression testing

[85md]

Going to test the compression on the stock 85 Gt. The ford manual reads to test it warm. No more than 25% variance between highest and lowest numbers. What should the highest number be? Thanks, Paul

[richpet]

More important than highest is whether all the readings are close to the same. I honestly forget the top number I got when I did mine last, but I seem to remember around 180? If it runs decent and they are similar across the board you should be good. If you are low in one, put a bit of oil in throught he plug hole and do it again. This will let you know if the pressure is being lost through the ring pack or not.

[MurPHy]

I get 140-150 psi on my engine.

Make sure to do it with the engine at operating temperature, with the fuel system shut off, the throttle wide open, and all sparkplugs out. Crank the engine the same amount of compression strokes for each cylinder when you test them. Put a battery charger on the battery too, to keep the battery from draining down during testing and skewing the results.

If one cylinder is excessively low (eg 70psi on one and 140 on all others), pour a tablespoon of oil into that cylinder and retest. Pressure goes up = bad piston rings. Stays the same = do a leakdown test for further diagnostic data.

As for the 25% figure, that is quite a bit of variance to be honest with you. A very healthy engine will show less than 10-15% variance between cylinders. Post up the numbers you get after you do the test.

[Chris Wood]

Be sure to "squirt" the oil towards the center of the engine block (furthest away from spark plug hole) to coat the cylinder wall. If pressure goes up, then your problem is the rings, if not then it is either the valves or the compression chamber (head gasket, crack etc.).

[85md]

alright..thanks...Will be doing it in the next severral days