Here's the deal.
I bought a rebuilt 355 chevy. I installed a moroso oil pump before I installed the engine. I got the motor installed in my 86 Z. When the motor warmed up, oil pressure was 3 to 5 lbs. idling. When I revved the motor, oil pressure jumped up to 60 lbs. Not happy with that, I pulled the motor down and rebuilt it again. This time I installed a reman. crank and bearings, new rod bearings, new cam bearings, new cam, new "stock" oil pump. After installing the motor, I get the same thing, only oil pressure is lower beacuse I used a stock oil pump. I know there' something I am overlooking! When hot, the motor only puts out 2 lbs. of oil pressure. I have changed guages, and am running a mech. guage. Can anyone shed any light on ths? I had an older mechanic tell me that it's because I don't have a mechanical fuel pump rod installed. Thanks.............

 

I had a friend run into this problem many years ago. The block had been hot tanked, bored, etc.

Come to find out, there is a soft plug (similar to a very small freeze plug) that was inadvertantly left out in the main oil passage under the rear main cap.

This plug (about 1/4") redirects the oil flow toward the oil filter and then up the proper passages.

If I remember correctly, the plug is in the passage next to the main cap bolt hole on the oil filter side of the block.

Take the rear main cap off and look down (or up as the case may be) into the block in the passage next to the main cap bolt hole. If there is no plug (which directs the oil flow toward the filter) then that is probably your problem.

It was my friend's and it was corrected after driving in the soft plug which the machine shop left out.

Someone on here probably has a picture which would better explain this.

This is just my 2¢ worth and the trouble it is to check this, it may be worth your while to get a few more opinions.

jms

 

first thing i ithnk i'd do is screw a mechanical gauge directly into the block and see what you have.

 

The FP rod doesn't go through any oil passages, so it's not from that.

The plug as mentioned above (½" actually) will cause dirty oil to be fed to the bearings, but will not cause low oil pressure. If that plug is missing, oil will go straight from the oil pump to the engine, and simply not be forced to make the detour through the filter; but it won't leak or be restricted or anything like that.

I'd be more inclined to suspect that one or more of the plugs at the front of the rear-to-front oil galleries, behind the timing gear, is left out.

 

According to Lingenfelter Performance, if that small plug is left out under the rear main cap, it can cause an internal oil leak and lose as much as 20 psi!! It could be another plug also, make sure you have all the oil galley plugs in the necessary locations.

 

Leaving out the plug will not cause an oil leak.

Go look at a block. Once you have actually observed a block, and followed each of the passages, what you will have learned, is that there is a vertical passage that leads from the output of the oil pump into the rear main cap, straight up through the casting, all the way up to where the oil pressure sending unit screws into it, next to the dist. About ½" above the regsiter surface where the rear main cap meets the block, you will see a more-or-less horizontal passage that goes off toward the oil filter; and about 1" farther up, yet another somewhat horizontal passage toward the filter. The lower one carries dirty oil from the pump toward the filter, and the upper one carries clean oil back from the filter to the rear cam bearing, where it branches off to the 3 front-to-rear passages that oil everything.

The plug in question goes in between those 2 horizontal passages, about 1" in from the register surface. It separates the dirty oil part of that one vertical drilling from the clean oil portion. As you will notice, there is full oil pressure (minus the pressure drop in the filter... which should be no more than a psi or 2) on both sides of the plug, and that there is no escape from that passage (what might be termed a "leak") regardless of whether the plug is in there or not. When you have observed the block, you will have also discovered that the bottom 1" of that vertical passage is about 5/8" diameter, and the next portion of it is ½" in diameter; and that the plug in question is a ½" plug.

Reading something in a book doesn't make it so.

 

Or.... it could simply be a failed oil pressure sensor. We all know that never happens......