So I want to do a compression test on my 4.0 to make sure my head is ok. I understand the 2001 had a weaker casting and with 155k miles on it when I bought it, I just want the peace of mind that it's ok. It runs fine, idles maybe a bit rough sometimes at a stop, but seems to have plenty of power and such. My only concern is when I calculated my gas mileage, it came out to 11.5, which seems low. I've done all the normal tune up stuff, and that was after the new exhaust and cat. I'll check again next tank, but I'm pretty sure I didn't screw up the math.

Anyways, I tried finding a "compression testing for dummies" write up, but my search-fu skills were weak, so I'm hoping somebody can give me the run down on this. The FSM says disconnect spark and fuel, and then crank it 3-4 times and get a reading. So I've ordered a gauge and am waiting for it to show up.

I'm assuming this is how I do this:

1. Disconnect and remove coil pack rail thingy.
2. Remove all spark plugs.
3. Turn off fuel, I'm assuming there's a fuse or something that turns off the pump?
4. Screw in gauge into each cylinder one at a time and crank 3-4 times, get reading.

Do I need someone to crank it while I look at the gauge, or can I do this myself? And as long as the readings are within 10-15% of each other, and not below like 100, I should be fine?

Sorry for the wall of text, I just want to make sure I don't screw up my toy doing this.

 

"that is correct" as they said in Billy Madison

 

That's sounds close enough to me. Block the throttle fully open for the test. The screw-in type gauge gives better readings. JMO.

 

You want to make sure it is warmed up also.
You do not need anyone to watch the guage,it will hold the highest pressure until you release it.

 

Thanks for the tip about the throttle, I'll make sure to do that as well.
Yup, a screw in gauge is what I ordered.

Ok, cool, thanks for confirming that!

 

I know its been 4 years but how were your results?