Newly rebuilt engine -low oil pressure

(Stewart Martel) #1

My 4.2 has been fully rebuilt, new liners, pistons, rings, bearings, crack tested, measured, regrinded etc - professionally stripped and reassembled by reputable machine shop.
Later oil pump fitted from XJ6
Conrods from an XJ6 (invoice says they resized conrod tunnels)
No external oil leaks.
Low oil pressure on start up - around 15PSI?
My mechanic has checked the oil pressure release value and that didn’t help.
He is going to remove the sump to have a look for anything obvious.
He’s very competent and knows what he is doing so i guess i’m asking for any tips for my own benefit - any ideas?


(Huff) #2

Stew- check first the (external, no need to pull the sump) oil bypass valve for bits of junk interfering with its closing, especially after a rebuild. My $.02 Huff 69 2+2

(69 FHC ) #3

First of all use a known good mechanical gauge to verify the oil pressure. You’ll have to buy an adaptor to go from the threads in the oil filter head to the threads on the ending unit.

IIRC, in the sump there is a pipe carrying he output of the oil pump that has o-rings. a missing o-ring or mis installed pipe (if that’s even possible) might cause a low reading.

(Huff) #4

John is right- it is easy enough to put a mechanical gauge in line, even if it is temporary.

He is right again- also pay heed to those o-rings! They can be (2) different sizes per side of the pump/pipe and they were probably a goodly part of my downfall, along the way. H

(Stewart Martel) #5

thanks John - he has already installed a known good mechanical gauge.

(William H Wayman 1970 S2 DHC) #6

Did he replace the chain tensoner? The old orifice needs to be transfered from the old one.
The shim can also create a leak. Very confusing. Show him these photos.

(Stewart Martel) #7

thanks William, i’ll check, i know the chain was replaced…

(Puddinhead) #8

20-50 oil ?

'66 FHC

(Liam O) #9

As others have said, assuming the reading is accurate and confirmed, the oil pump O-rings were my first thought.

Also, the gasket(s) between the pump assembly and the block. Perhaps a fatigue crack in the pipes? I assume the pickup is seating in oil and not sucking air, although I don’t think you’d get the sump on otherwise.

If the sump is removed, and the pressure relief valve is shown to be good and functional, I would insist on a new pump. Using an old one from an XJ6 just sounds like false economy. (I know it may have measured as unworn and with life left, but a new one is not expensive.)

(Liam O) #10

Further to my post of a moment ago…The O-rings.

They get hard as a rock and don’t do the job. If the pipes slipped in at all easily to the pump body without resistance, then a good seal is not happening. You really must use new O rings and use the right one in each case.

(Stewart Martel) #11

So all fixed… now reading 60PSI at cold. there was a journal (plug?) left out near the distributor. I guess it was missed when the engine shop reassembled the block?