365 GT question re: compression test.

FDH, here's a couple of items to consider when performing a compression test. First the most accurate method is to perform the test with the engine at or close to operating temperature, not always easy to do but the most accurate. The reason is all the components have expanded to there operating dimensions. The thorttle plates should be wide open to provide the most breathing. Most compression tests are performed dry first to establish a base line. By the way record all your readings. The wet test is performed if you have one or more cylinders with low readings. Worn piston rings, poor valve sealing etc. will cause a low reading. If you introduce a small amount of oil, no more than a few CCs, and the compression comes up it's an indication of worn components. Make sure you use the exact amount of oil in each cylinder to gain an accurate reading, you can use a syringe for that, 1 to 2 ccs of motor oil max. Only perform the wet test if you find low readings. Keep in mind valve adjustment can cause low readings as well. If you want to go further you perform a leak down test, which is introducing compressed air into a cylinder with both intake and exhaust valves closed and reading how much air escapes and where it escapes to. A leak down is slightly more involved because you have to rotate the engine to top dead center compression for the cylinder you are testing and you will need a leak down tester. Here's the steps to performing a compression test:
1. Bring the engine up to operating temperature.
2. Disable the ignition system, remove all the spark plug wires, clean around the spark plugs to prevent any trash entering the engine.
3. Remove all the spark plugs
4. Block or have someone open the throttle plates wide open
5. Thread the compression into the spark plug hole
6. Crank the engine at least 5 compression strokes, you can see that while reading the gauge. By the way a properly charged battery will ceratinly help.
7. Read the gauge and record it. On your 365 it should read approximately 150 psi, it may read slightly more or less, but should be consistent with the other cylinders. 2 to 5% difference between cylinders is considered acceptable.

Sorry for being long winded, I hope I didn't miss anything

 

The number of strokes that you have to crank the engine will depend on the type of gauge that you are using. We use a Snap-on gauge that has a fairly long hose (large volume for the engine to fill). So I have to crank the engine for roughly 20 strokes. I count the strokes on the first cylinder for the highest reading and then crank the remaining cylinders the same number.

Brian B.

 

One other thing to do, depending on the car, is to disable the electric fuel pumps.

 

Well done Richard

 

very good info on this important engine test.. this is probably the most important engine test when evaluating any used car but particularly a ferrari. many have been driven hard from time to time [or always] and a good rebuild costs mucho mucho $$$. a word to the wise, no matter how good the compression test is on ALL cylinders, proceed no further until you have done a leak down test on ALL cylinders. this will give you a a true reading of what is going on. also before you do these tests change the oil and run the engine for ten minutes.there are oil additives that can affect the test results and give false positive results. also before throwing out the old oil, take a magnet and swing it around thru the oil. using touch and a strong magnifying glass,see if any foreign objects, no matter how fine, are detcted on the magnet. do this several times and walk away if you detect anything wrong. you can also try straining the oil thru a nylon stocking and testing the residue. if these tests pass muster you have a good chance that the engine i pretty sound. if there is a little off color exaust and all the above tests have passed OK, it is probably the valve guides. treat the engine carefully with heavyer multigrade oil, warming it up well and keeping the revs below 5000rpms and you will probably get good use of the engine.anyhow, replacing valve guides is a lot less $. than doing a complete engine rebuild. hope this is helpful tongascrew