Engine compression testing with coil rail?

BobK

BobK

TJ Enthusiast
Supporting Member
Joined
Apr 11, 2019
Messages
386
Location
Parker, TX
As part of my refurbishment of this high mileage 4.0 inline 6 (208,500 miles), I changed out the spark plugs and the coil rail (see pic of old plugs below)

I removed the coil rail first and took each plug out one by one and used a Harbor Freight compression tester to test each cylinder, but since the coil rail was off all I could do was crank the engine for a few seconds. I'm unsure if that produces a reliable result or not. But when I was all done, the engine roared to life and sounded more powerful than before the new plugs and coil rail.

By the way, the plugs I used are the Autolite XP Iridium Spark Plug, gapped to the manufacturers .035

Four of the cylinders reported about 95 lbs of compression, one at 105 and one at 110.



IMG_0483.jpeg
 
If it runs fine forget about it. 95 is low. You should have the throttle plate full open when you do the test. Just carefully bridge the starter next time to spin it instead of using the key.
 
pagrey said:
Just carefully bridge the starter next time to spin it instead of using the key.
Click to expand...

I've been putting off a compression test because I thought I needed a second set of hands. What do you do, just run a jumper cable from the battery to the starter? Hold it long enough to take a gauge reading?

Edit: or just activate the relay somehow?
 
thinkmassive said:
I've been putting off a compression test because I thought I needed a second set of hands. What do you do, just run a jumper cable from the battery to the starter? Hold it long enough to take a gauge reading?

Edit: or just activate the relay somehow?
Click to expand...
I would pull the small connector at the starter and use a metal object like a screwdriver to short the small tab on the starter to the power cable at the starter. It'll spark a bit and run the starter. Basically you just need to get 12V to that little terminal on the starter. They are only about 1" apart.
 
You should get an accurate reading with just 3 or 4 crank rotations per cylinder. Each "puff" on the gauge is 1 rotation.😤
 
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