The V8 Owners Forum -

need some help interpreting the results of a compression test on my 3.9 v8, a hot test was carried out yesterday, all cylinders were between 85 & 93 psi!!! obviously I was a bit gutted with these figures, so, after a bit of sulking I surfed the net for info and decided to do another test this morning. I just did one cylinder, without warming the engine. after 5 revs the pressure was 85psi, after 20 revs the pressure was 100psi!?!? repeated the test on the same cylinder but added 2 teaspoons of engine oil to the cylinder( eyedropper perfect for this!) and got exactly the same result!?!? Generally the engine seems quite good in use, starts quite easily considering it has a chokeless Holley carb(4160), the fuel mixture is a little rich at idle so plugs are alittle sooty, but no traces of oil. The engine is in a little kitcar(striker) so I imagine it would still go like sh*t even if it was well knackered.There's no misfires and it doesn't use any oil. Because the battery is pretty small, the engine cranks pretty slowly but usually fires up within 4 or 5 revs. the engine is quite old, early 90's I think but I don't have any history on it. it's a 9.35 ratio. any ideas anyone?

Are you doing the compression test properly as it doesn't sound like it. All the plugs need to be out so the engine spins freely and the throttle needs to be held wide open and crank the engine until the gauge stops rising. Oil down the bores doesn't always work on a rover unless you use a fair bit. The reason being that the pistons are dished so the oil just sits in the dish and not down by the rings where it needs to be. Your figures are low but they will be if you aren't doing the above but they are within 10% just of each other.

There must be a bug going round that is eating pistons rings!

Here is another post running on the same subject:-

http://www.v-8.org.uk/forum/viewtopic.php?t=2672

katanaman wrote:Are you doing the compression test properly as it doesn't sound like it. All the plugs need to be out so the engine spins freely and the throttle needs to be held wide open and crank the engine until the gauge stops rising. Oil down the bores doesn't always work on a rover unless you use a fair bit. The reason being that the pistons are dished so the oil just sits in the dish and not down by the rings where it needs to be. Your figures are low but they will be if you aren't doing the above but they are within 10% just of each other.

thanks for the reply, I did have all the plugs out and throttle wide open, when I retested the pressure stopped rising after 20 revs, I did read somewhere that a low cranking speed may give low readings. I think I might just buy a tyre inflator compressor, regulate it to 100 psi and stick it on one cylinder at a time and look/listen for any leaks, If Ican't see/hear anything then surely there aren't any substantial leaks?
cheers
Paul K

tumchiehead wrote: I did read somewhere that a low cranking speed may give low readings.

Yes indeed. I tested mine once when it had a knackered starter (worn brushes/commutator), and the results were all low and all over the place. Sometimes a cylinder would read low, but then high the next time you tested it if the starter felt like co-operating.

Once I'd replaced the starter they were all consistently high (~10 bar/150psi)

Jeff

Have you indepently verified that the compression tester is working properly?

Chris.