High oil pressure in a 3.9

[djm16]

I am posting this on behalf of a friend who is an experienced engine rebuilder, having done around 500 V8 over a lifetime. He has put together a 3.9 block, and installed a high-volume oil pump (with longer gears, and an extension plate). I have read the previous posts about high oil pressure in a 3.9 without finding a useful conclusion. I also remember the thread where an incorrectly placed locating dowel blocked off the oil flow from the timing cover to the block.

So what happens?
Up to 2,500 rpm the oil pressure acts normal, rising up to the appropriate pressure and staying there. Above 2,500 rpm the oil pressure starts to rise again until it is off the gauge (high quality mechanics gauge). I found the bit about a faulty crankcase breather being a possible cause, but that has been checked. Bear in mind also that I have personally examined the relief valve and can confirm that the control piston slides easily and there is no scoring on it.

Logically one has to conclude that the oil pressure relief valve is working normally, but the flow in the return path to the sump must be restricted in some way so that high flow leads to a significant back pressure. The trouble is that neither of us can figure out where the return path is.

Can anyone point out the return path for me, preferably with a diagram?

[enigmas]

Ultimately there are only a few prime areas that control the degree oil pressure to engine and camshaft bearings.

Bearing shells:
The entry points of oil to and clearances of the bearings (with regard to the shells) and journals.
I presume all bearings are fitted the correct way around, that none of the clearances are 'too minimal' and that too heavy a grade oil is not being used on a fresh engine.

Camshaft Bearings.
I presume all the camshaft bearing inlet holes were lined up correctly...kind of unlikely for an experienced engine builder. Even so the pump pressure relief valve should open at a predetermined over pressure situation.

Oil Pump.
I presume the new taller pump components were assembled correctly and the spacer plate (?) fitted correctly for the longer gears. No transfer ports were covered when this was done?

Relief Valve.
Trying to figure how this works is quite confusing as it doesn't appear to open into a port and directly dump into the sump!

The following photos may/may not assist.

The front timing cover (P76...virtually identical...uses longer pump gears as standard)
Note the ports, 3 shown. The bottom port (suction side) receives oil from the oil pickup. The middle port is blanked off by the front engine block face. The top port is outgoing pressure to engine bearings.


The bottom pump face

Note the ports. The gears turn in opposite directions taking and feeding oil as shown by the arrows.

View to Oil Pump housing in timing cover

Note the large oblong opening at the base of the pump housing. That's the oil entry to the gears...suction side.
Note the small rectangular opening at the base (bottom of image)...thats the oil feed out (under pressure to the oil filter cannister)

The bottom pump cover (where the filter screws on)

Note the rectangular slot at the bottom of the pump cover. Oil enters here under pressure and enters the outer section of the filter cannister. It then continues through the filter and enters the engine block port through the port in the threaded filter securing boss.

Note the small gauze capsule (below to the left) this is the port to the bypass pressure release valve. When pressure is excessive oil via this port forces the bypass piston back against spring pressure and dumps the oil back into the inlet side (but base) of the pump gears...as far as I can discern.

I stand to be corrected (hello John) but as far as I can see this is how it drops excessive pressure. Some form of feed back system.
The return is via the ports in the cresent shaped relief in the 2nd image.



Lastly why not simply fit the original pump gears run the engine and note any change. If there is a normal rise and settling of engine pressure then the culprit is obvious.


PS. Third paragraph...... en.m.wikipedia.org/wiki/Relief_valve starting with... "In some cases, a so-called bypass valve acts..."

[djm16]

Thanks, that is exactly what I needed to know. I will pass the info on. Brilliant pictures BTW, did you take them specially?

"Relief Valve.
Trying to figure how this works is quite confusing as it doesn't appear to open into a port and directly dump into the sump! "

Well it certainly had me confused, but if the relief valve is dumping the excess oil directly back into the intake port, then I can see how the system might get overloaded.

Yes it was my advice too to fit a standard pressure pump and he was supposed to be trying that today. Like me, he tends to want to know why or how this went wrong in the first place.

Thanks once again.

[enigmas]

Dec 5, 2018 12:20:27 GMT djm16 said:

Thanks, that is exactly what I needed to know. I will pass the info on. Brilliant pictures BTW, did you take them specially?

Ah...Yes! It was an intriguing puzzle...and I had all those bits and pieces handy in a tub of spare parts.

[lagain]

I had a similar problem when my engine was rebuilt by some clowns in Middle England !

They fitted standard gears but with stronger springs, so when I started her up the needle went off the end of the guage.
Putting the old springs back solved the problem

[djm16]

You will be pleased to know that the problem went away entirely by refitting an oil pump with standard gears. With any luck, when someone else encounters the same problem, they will see this post and be spared the hours of consternation encountered by my friend. Thanks once again.

Note, YMMV. There are several variations on the oil pump to a V8 which will bolt onto the timing cover, but with subtly different internal passages. So I cannot presume that all V8 oil pumps are prone to this problem.

[enigmas]

It would be interesting to see where the differences are between the 'versions' of oil pumps and timing cover housings. I'm sure that it is something quite straight forward like the pressure relief port being obscured.

* I'd be taking a close look at the extension plate...if that and the longer gears are the only changes.

[petervdvelde]

Hi Vince,

I fitted a kit with larger gears and an extension plate to my daily P5B and had no problems with it. The problem could be caused by combining a 3.9 block with an early timing cover. Djm s friend probably has not used the 3.9 timing cover as this already has the large oil pump gears. He may have used an early cover due to the oil filter position as there are a fair bit of differences. As far as i know, these larger gears were first used in the SD1 engine


Peter

[enigmas]

Hi Peter...the timing cover I presented above is from a P76 V8 (1973) and it has the larger pump gears from original manufacture.
The base of the pump (cover plate with the oil filter boss) is also P76 but my coupe fitted with a P76 engine has a 'Buick' base plate from a GM 215ci engine (the die caste version of the Rover 3.5 litre) that changes the angle of the oil filter cannister. There were no issues when I fitted it 26 years ago to the present day. So the obstruction/area of concern I'm sure is something miniscule. Comparing both base plates I'm sure would reveal the problem area.

PS. The issue has to be with the operation of the relief valve/ports/passages (within the pump) as clearly the pump delivers pressure to the engine bearings but can't reduce its output with increasing rpm.