Cylinder compression question!

[Fookie Bookie]

This weekend I'm going to do a compression test on my 4.0. I was wondering if anyone knew what Psi each cylinder should be at?
I looked through the tech specs for the 4.0 on here and can't seem to find anything other than the ratio. I'm in dire need of this info to find a terrible antifreeze disappearance that's been pestering me for some time. :banghead:

 

[kunar]

actual numbers arent nearly as important as consistency between cylinders. up to 10% variance between cylinders is perfectly fine. numbers ideally would be 160-180 but a little lower wouldnt concern me as long as they were relatively consistent

 

[adsm08]

160 to 180 is ideal, 10% loss between the high and low is a problem, and below 90 the engine won't run.

 

[Road Angels]

In this case a leak down will tell you more about what your looking for than any thing, do it with the radiator cap off look for bubbles, also have you pressure checked your radiator, some slow leaks in hot areas wont show up on the ground

 

[Fookie Bookie]

Yeah I'll go ahead and do that pressure check. And I'll also pressure check my radiator as well. I appreciate the help guys. Also instead of me starting a new thread, I'm pretty sure my alternator is going bad. When I drive more that a couple of times a day my voltage meter reads low and dash light and head light dim real bad. And even my turn signal blinks real slow. Do you all think this could be my alt, or something else?

Sent from my HTC One X using Tapatalk 2

 

[RonD]

As said, Compression varies from engine to engine, that's because of crank speed.
The faster the crank spins the higher the compression number, that's just the physics.

So with all spark plugs removed, and good battery and starter, 160-180 would be expected.
You can also prop the throttle open, I don't, some do.


Yes, reads like cable is loose or alternator has lost 1 or 2 fields.

An alternator has 3 "fields" that generate AC power when rotor spins, the AC is converted to DC from each field by a set of diodes.
Either the field or diodes can fail, so you lose full amperage, when 2 fail it's pretty much done except at higher RPM.

Battery voltage with key off should be above 12.4volts, 12.2v or lower means battery needs to be changed, 12.8v is a new battery, 12.5v has 75% life, 12.2v 50%
With engine idling battery voltage should be 1 volt higher, so if it was 12.5v then it should be 13.5v.
1volt above battery voltage is needed to maintain the charge in the battery, not recharge it, just maintain it.
Alternator will put out close to 15volts just after starting to recharge battery after the drain of starting engine.

With engine idling test voltage at battery, then turn on the headlights, blower fan, radio, anything and everything electric, voltage should drop then come back up to 13.5volts