Question engine compression testing specs

[wirelessf]

I recently did an engine compression test and the max pressure was 230psi -/+ 10psi all cylinders. What is the compression specs for the v8 engine? It does not list it on my chiltons book.

[Burger Steak & Eggs]

From the 2003 manual:

At 250 rpm,

STD: 192 psi or more
MIN: 142 psi

Difference of pressure between each cylinder = 14 psi or less.

[Hi Volt]

Was that 10% or 10 psi ? if it was 10 psi then the range is within spec., 10% is kind of a large variance ( 23 psi in your case). The actual max. compression value will depend on a number of things such as engine temp, cranking speed, open or closed throttle, but the variance between cylinders is very important as it can point out a potenial weak cylinder(s). Low overall pressure ( below factory spec) is usually a plainly worn out engine, however a low cylinder or two can point to valve issues, cam issues, head gasket leakage etc.

I tested my 4.7 ( over 210,000 mi.) and the highest was 231 and lowest was 224. The numbers are above the listed factory spec. ( which is good) but more importantly the variance was fairly tight.

If you did have some cylinders bordering on the 10% range, might be an idea to re-run the test and ensure that the compression tester fully seals, even a little leak where it threads in the plug hole will cause a lower reading.

[Burger Steak & Eggs]

Compression tests can be fooled when a poor ring seal keeps a lot of oil around the ring pack. The oil can temporarily seal around the rings well enough at low cranking speeds to artificially inflate the readings (i.e. make the compression test result better than it actually is).

I prefer leakdown tests. In leakdown tests, shop air is blown into a closed combustion chamber, and an air flowmeter measures leakage. This is less likely to be fooled by retained oil because it will tend to push oil back out of the rings for a more realistic measurement of leakage.

[wirelessf]

I relooked over my test results and all are within specs. Lowest was 225psi.