Should my cylinders compression test out between 140-160 lbs, on the 4.2 litre six?
Submitted by NC19-03320J on Wed, 10/09/2013 - 16:21
Submitted by aj_caruso@yahoo.com on Wed, 10/09/2013 - 15:00
4.2L Compression test ('71 Series II)
Hello all (long time JCNA/JTC member, new on forums).
I recently did a compression test (cold engine) and found 125-135 lbs ...
thought it was low. Should the readings be taken on a warm engine?
Anthony
Submitted by joshbartlett@r… on Tue, 09/24/2013 - 12:17
4.2L Compression test ('71 Series II)
Sounds a little high.... It could be carboned up, or you gauge could be reading high, as they tend to do when they get old.
The more important number is the variation: the rule of thumb is that there should not be more than 10% difference between highest and lowest.
If the car has not been run for a while, you can get more variation than you would get on a daily driver. Put some miles on it and test again, perhaps taking readings with two different gauges.
Best of luck,
Josh
Warm engine, throttles open and slides up and I'll bet that you'll see a jump in the compression test.