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Tue Oct 14, 2014 2:22 pm

The oil pumps in our engines are called positive displacement type which means the faster the engine turns the higher the pressure goes. In order to not blow things up you need a pressure relief valve. The tightness of the valves clearances and the strength of the spring determine the upper end pressure you'll reach. There's no need for oil pressure to reach over about 40 psi at any time. In fact if the oil moves too fast it won't transfer heat away as well. So the big deal with the pumps and oil pressure relief is on the low rpm side. In other words when your engine gets hot and you pull up to the stop light the oil pressure goes too low because of a worn out oil pressure relief plunger or spring and/or your oil pump has excessive clearances. Running a heavy oil can help mask the problems sometimes.

Tue Oct 14, 2014 2:32 pm

my oil pressure falls to about 20psi even at freeway speeds (spec says 28psi is below spec at 2500rpm) but is fine at idle. Oil temps have not exceeded 200F. Is there the remotest chance this could be caused by the relief spring/plunger? Is seems unlikely. Can you explain how valve clearance can affect oil pressure? I have solid lifters and check the clearance every 200 miles or so as top-end of motor was recently rebuilt

Tue Oct 14, 2014 2:49 pm

You might pull the spring(s) and see if they are the correct length or not.

Tue Oct 14, 2014 2:53 pm

I repeat: can the oil pressure relief system cause low oil pressure at highway speeds if the oil is not hot or cold?

Tue Oct 14, 2014 3:11 pm

skimhead wrote: I repeat: can the oil pressure relief system cause low oil pressure at highway speeds if the oil is not hot or cold?

I suppose if it was badly worn or stuck open. Don't get confused about which valves I'm talking about. I'm not talking about your intake and exhaust valves. I don't see how those could affect oil pressure.

Tue Oct 14, 2014 4:51 pm

the relief valve can't /shouldn't cause the low reading on highway,
it's job is done at ambient temp to keep welch plugs in
place, oil filter gasket from blowing & prevent cooler grommets
from letting oil past,+protecting cooler itself from bursting.
Those low numbers are probably crap sender/rubber line
absorbing the true numbers. Could you delete the idiot light
& mount the VDO sensor on a metal stand-pipe to get a
case measurement ?

Tue Oct 14, 2014 5:21 pm

skimhead wrote: I repeat: can the oil pressure relief system cause low oil pressure at highway speeds if the oil is not hot or cold?
Wildthings wrote: You might pull the spring(s) and see if they are the correct length or not.

Sorry, I was referring to the springs in the relief valves. If too short they will dump oil at too low a pressure. Someone may have even installed the wrong springs. Expect to find all kinds of jury rigs on an old rig like this.

Tue Oct 14, 2014 5:34 pm

I'd like to know what weight oil you are running, if the pressure is the same cold as when it's warmed up and if you have tried a different gauge?

Tue Oct 14, 2014 6:16 pm

using Castrol 20/50. Oil pressure cold is 50-60psi then 20psi when warm. I threaded the VDO sender into idiot-lite port and cross-checked thru Tee adapter with PSI gauge. Identical readings as before when cold (60psi) and warm (20psi).

Tue Oct 14, 2014 6:42 pm

skimhead wrote: using Castrol 20/50. Oil pressure cold is 50-60psi then 20psi when warm. I threaded the VDO sender into idiot-lite port and cross-checked thru Tee adapter with PSI gauge (identical readings cold/warm)

The answer is yes. 20-50 is great for a hard working bus in hot weather....but it is thicker than stock listing. It has been found by quite a few ....including me....that if you have an efgicent pump with tight tolerences....the oil pressure generated at highway rpms can force the plunger in the oil pressure relief valve down far enough to cause bypass of the cooler.

This is more of a function usually of the spring in the relief valve not being correct.

But...it should not cause the plunger to drop so low in the bore that excessive oil pressure vents to the drain hole to the case. Your spring would have to be an absolute noodle to drop the piston down that far.
Typically high pressure only pushes the piston down far enough so that a portion of the crown is just past the entrance to the two veryical grooves in the relief plunger bore. The oil snakes past the piston top and vents to the groove under the head and into the two vertical grooves. Downward piston movement should only be a few mm.

If the spring is too weak and the pressure is too high and it forces the piston down a couplemm further....you would get serious....and fluctuating....pressure drops.

If you are not seeing fluctuations. ...I would doubt it is the spring.
Post a picture of the spring. Also check the piston and make sure its the original with the groove under the head.

Also you might check to see if the oil relief hole at the bottom of the relief valve bore near the cap....may have been enlarged. Its a long shot but if a PO had at one time installed something like a melling pump with 30mm gears.....it causes this very problem and typically they enlarge the relief hole to keep the massive oil pressure under control.

If you switch back to a stock pump and also have spring issues you could be venting your oil pressure to the case at highway rpm.

All lonh shots but worth looking at. Ray

Tue Oct 14, 2014 8:03 pm

if the oil pressure falls as the RPM increases that would say bad sender or sucking air to me.

If it falls because the temperature rises as one goes freeway speeds as long as 10 PSI per 1000 RPM is maintained. At freeway speeds that should be 30 - 40 PSI minimum. I run 20-50 Brad Penn and even when it is 250 F in the sump after a long hill climb on a hot 100F+ day I still get 45 - 50 PSI.