I don't think that those bad o-rings can cause such a press drop, on my '88 i've changed them last month because the big one was leaking and the other two on the bolt were totally flat and hardened, but the oil press before and after didn't have any change at all.

Thanks for all the replies. No resolution yet, but I must be getting closer!!

I purged the air out of the line to the mechanical gauge but to no avail.

Is there anything I could have missed when installing the cam/timing gear? The bearings are new and alignment checked before cam installation(NEW cam) Is there a shim/thrust-washer between the camshaft and timing gear that would keep the oil from being pumped out the front of the camshaft that would account for such low pressure? One of those wavy type of shims/spacers sometimes seen that are used more for tensioning/spacing? (I'm searching for ANYTHING that I could possibly have missed!!)

Any sugestions on where I can find an exploded view on-line of the 4.0L ??

i am certainly no expert but sounds like the pump to me. the oil pump is a positive displacement gear pump meaning how ever much goes in has to come out. hyraulics 101. the springs in the pump are what help regulate the pressure based on the compression rate of the spring. softer spring lower pressure and visa versa.

There is a washer under the bolt that holds the sprocket and a pin with a spring.

how much pressure did you have before the rebuild?

Engine was a basket-case when I got it. No idea how it ran/why it was torn-down (guy who owned it died and I got it from his son)

I remember putting the cam gear bolt/washer on as I lost the spring/tensioning pin for a day or two so the memory on that one is vivid.
Is there some type of seal/shim that goes between the camshaft and the gear that would restrict the oil flow to keep the pressure up?

As I've said, this is my first 4.0l rebuild so I'm in new territory here. I've rebuilt Chevy, Ford, Dodge.... and never faced a problem like this...

I've got a new pump coming in today so we'll see how that goes.

Thanks All !!

Not that i know of.

ditto, solid cam, oil doesn't go through it. you said it's a high volume pump. there is a difference in high volume and high pressure. was there a gasket on the mounting surface? perhaps the pump is working but leaking on the pressure side.

Here are my thoughts, and I have rebuilt the 4.0 x2.

First if you have mic'd all you surfaces and all bearings are new, and everything is torque properly. You should have good oiling. That being said did you tighten the oil pump to the proper torque specs.


In one situation where I rebuilt the bottom end of a 4.0 I recieved a defective oil pump from Advance Auto parts. It demonstrated the same symptoms you are discussing here. At speed oil pressure was fine, once I let off throttle oil pressure dropped down to 3psi. Of course by the time I figured out the problem I had wiped out my reman crank and had to start all over again.


Be warned do not run the engine with low oil presrure you will cause damage to the motor.