89 SHO Compression Test Results

[SHOmont]

I decided to do a compression test on the front 3 cylinders since they’re easily accessible, in my quest to find a miss…….

#4 is 170, #5 is 210 and #6 is 170PSI

Within the spec of 171 to 199, BUT more than a 10% diff between the highest and lowest.

Thoughts?

 

[sperold]

I don't think there is anything to worry about. with the numbers you have.
Check to ensure your plug wiring is correct as many publications have it wrong on cylinders 5 and 6.

A big cause of misses is the plug well having oil at the bottom.
Use a variety of short and long extensions and swivels and get the back plugs out to see if they have oil on the outside and on the inside too.

The coil unit is usually good if it is the ford original.

If there is something wrong with the wires, you can usually see it at night in total darkness.

 

[NoSlo]

Also, you'll get temporarily high compression readings if you let oil leak down into the cylinder. A test to see if it is rings or valves is indeed to squirt oil into the cylinder to seal around rings.