Engine Compression Test Questions

[92tanMJ]

Nooby question time! I'm home for the weekend and I'm going to do a compression test tomorrow on my mj, but I have a few questions as this is my first time ever doing one.

 

So they say to run the engine for 20 or so minutes to let it warm up is it okay to do this with a bad head gasket?

  --But they I mean them here: http://troubleshootmyvehicle.com/jeep/4.0L/how-to-test-engine-compression-1

 

Where is the fuse for the fuel pump? Or can I just unplug the injectors? (electrical plug, not remove)

 

And any random advice for me?

[cruiser54]

Unplug the cPS.

 

Sure it's a bad head gasket? You sure don't wanna overheat the engine.

[92tanMJ]

Unplug the CPS.

 

Sure it's a bad head gasket? You sure don't wanna overheat the engine.

 

Yeah, I mean I'm pretty sure. At least the PO said it was, and theres even the 'white milky stuff' on the dipstick. Thats one of the reasons why I'm doing the test. to get a better understand of whats wrong with it.

 

By CPS I'm going to guess crankshaft position sensor? Where is the easiest place to get to that?

[cruiser54]

Top rear of the intake manifold.3 wire plug with 2 wires in it.

 

Photo can be found by clicking on the link in my signature and scrolling down to CPS testing.

[Biotex]

Hey Zack,

If you have water in the oil, i'm not so sure you would want to run the engine. Since oil floats on water, and the pickup is at the bottom of the pan, on initial startup you will be pulling water through the pump and circulating it. Seems to me more damage could occur, and not worth the risk.

 

I'd perform the compression test without starting the engine first if you are dead set on doing the test. With milky oil, I don't see what else could cause that except a head gasket or a cracked block or head. 

 

I'd think if you do the wet test, you'd be able to find out what you need to know.

[92tanMJ]

Top rear of the intake manifold.3 wire plug with 2 wires in it.

 

Photo can be found by clicking on the link in my signature and scrolling down to CPS testing.

Thanks man will do!

 

Hey Zack,

If you have water in the oil, i'm not so sure you would want to run the engine. Since oil floats on water, and the pickup is at the bottom of the pan, on initial startup you will be pulling water through the pump and circulating it. Seems to me more damage could occur, and not worth the risk.

 

I'd perform the compression test without starting the engine first if you are dead set on doing the test. With milky oil, I don't see what else could cause that except a head gasket or a cracked block or head. 

 

I'd think if you do the wet test, you'd be able to find out what you need to know.

Thats what I was thinking but I wasnt sure! Thanks!

[Geonovast]

Unplug the cPS.

 

Sure it's a bad head gasket? You sure don't wanna overheat the engine.

 

He's got a 92, the plug isn't the same as the Renix, but will be in the same spot at least.