thefiremeister

I have a 1988 Cherokee with 4.0L inline 6 200k + miles on it.

The oil pressure appears to be hover around 20psi and may go a little higher when you rev it up.

I'm going to try and put a mechanical gauge on it to see it its really that low but the question is........

Where should the oil pressure be? Whats the normal range for a 4.0L engine?

firstime00XJ

20psi at idle is fine. Mine idles at 20psi and once I'm driving it runs about 40psi. This is at normal operating temperature. When my XJ is cold, it idles at 40psi and drives at 50psi.

mattnwhit

many times it just a faulty sending unit... if you get a new one from autozone chances are its either cracked or going to crack when it does you'll get wierd readings... for example I have 500 miles on my new engine and tranny and my oil pressure is everywhere.. I took it to jeep and they ran a mech test and all checked out fine they said it was just the sending unit...
....on second thought stock jeep guages aren't really all that accurate even it perfect shape...Jeeps are known for going anywhere anytime not high guage quality!

thefiremeister

That makes me feel a little better but I just hate looking at such a low reading.

I'll definitely have to hook up dial type gauge and see what it says.

At least now I'll know what range I should be in.

thefiremeister

Just got done hooking up a manual oil pressure gauge to the engine. It didn't seem to bad to start with.

I started it and it would idle at about 20 or so psi and the when I reved it up it would go up to 35 or so. When I got on it a little more it would almost reach 40 psi. I rattled some but not to bad.

After it warmed up a little the oil pressure wants to hang out around 18 to 20 psi. I had to rev it some ( could not see the tach and the gauge at the same time ) and it would start rattling ( lifters I think ) at about 30 psi.

This seems to be pretty low to me. I'm thinking this is going to implode on me in the near future and ruin a possibly good block.

Any Ideas

firstime00XJ

The rule of thumb for oil pressure is 10psi per 1,000 rpms. The fire trucks that I drive idle at about 20psi and driving above 35mph they are running about 30-40psi. (we've got two Cummins ISM's and a Detroit Diesel 8v92TA) My 4.0L has a tick in it, I believe it to be a lifter, and it also has valvetrain clatter. I wouldn't worry too much about it, unless you see other signs like metal shavings in your oil, your oil pressure drops to 0, or you have a very loud knock.

thefiremeister

I'm trying to anticipate and avoid the very load knock.

if its going to go bad I want to get it fixed before it destroys or seizes the block

firstime00XJ

What Fire Department do you drive for?

bigjustice91

I just rebuilt my 4.0 an it runs 42 oil pressure when drivong, And idles at 38. On a worn motor, thats not uncommon. If it gets down to 10 or below, pull the motor and investigate