I have no prior experience compression testing vehicle with a fuel injector so I need advice. So far my plan is to...

1. pull out fuel pump relay
2. start & run engine until it stalls
3. disconnect ignition coil
4. remove spark plugs
5. set throttle to wide-open position

Then test each cylinder with a gauge and look for 120 to 150 PSI pressure.

Are my steps adequate?

Thanks in advance,

2000xj
w/ 4.0L engine

 

It looks like your making this harder than needed..

1, Warm engine up and shut it off..

2, Remove ALL plugs from the cyls.

3, Dissable the ignition (If you want, Dont have to)

4, Block the throttle at WOT (wide Open Throrrle.)

5, Test each cylinder by screwing the gauge into the hole and then cranking engine through several revolutions. Not The Pumping-Up Action on the Gauge!

6, Record each cyl's readings for comparison later...


TIP:
You are trying to create identical cirumstances for each cylinder. And the comparison between each cyl is more important than the actual numbers !!

If a cylinder seems lower than the others by 15% or so, Then try squirting about a teaspoon of oil into the cyl and retest. If the oil improves the reading suspect the rings, if not suspect the valves !!
If two adjacent cyls are low and have the same reading, You have a failed head gasket !!

Compression test are very relative measurements, They are a good way to indicate whether you need further testing with a Leakdown tester...

Hope This Helps..;

Matt

 

Thanks for the detailed tip. I will run my test in a week or two when I buy a guage.

But I'm wondering... I'd feel nervous not shutting fuel from being squirted into cylinders the duration the test. Is is really no problem not shutting it off?

 

It's no Problem, If you want you could pull the relay though....

Just as long as you do the same thing on each cyl...

 

I ran the compression test and got... 160, 160, 170, 170, 160, & 165.

I think Jeep specified compression range is between 120 and 150PSI, with no more than 30PSI difference between any two cylinders. My readings were higher but probably due to an inaccurate test gauge so I consider my readings good.

Again, thanks Matt for your feedback.