Oil pressure question

I

Ianrhall

Senior Member
44 flathead pickup

When I first start up oil pressure is at max, comes down as expected ad it warms up. If I drive around locally,stop start and maybe the odd 40-50mph cruise it is about 1/4-1/2 on the gauge. But if I go on a longer run at "higher" speed the needle will go right down to minimum, if I then slow right down for a while then speed up again the pressure will be restored back to a 1/4. Any thing to be concerned about? Would changing to 20/50 oil help?
Ian
 
You need to hook up a known good mechanical gauge and see what it says. The dash gauges are just an "indication", not real trustworthy.

That said, I had a truck that was very sensitive to throttle. Oil pressure would drop on even the slightest amount of gas, then pick up if I let up a bit. It threw a rod about 100 miles later....
 
Ian, You didn't say what you currently use, but I assume it is 10-30 which should be fine. Your symptom sounds like your oil pickup screen is partially clogged from years of non-detergent oil. Next time you change the oil, look to see if you have the truck style pan cleanout, which will make for an easy job. If not, try fishing from the normal drain hole checking for sludge. If this is the case, you'd be wise to remove the pan for a proper cleanup.
 
I am yet to change the oil, oil looks pretty clean (and quite thin) on the dipstick
What do you suggest, take pan off and clean out the screen, all accessible with motor in?
Ian
 
Check to see if you have that very large cleanout hole first. Just a guess, maybe 6" in diameter, that exposes the pump pickup. If not, you likely have just the 1 3/4" drain plug, from which you can at least determine if you do have excessive sludge.

Pan removal is a job that is easier with the engine out, but can certainly be done without pulling the engine. With pan off, you can do a first class cleanout from topside, with removal of the intake manifold.
 
If you do the cleanout, don't just scoop out the sludge, hook up your parts washer pump with a longer hose to rinse out all that remains.
 
Why not drop the pan? On trucks it's usually pretty easy, and you can check a lot of stuff quickly -- oil pump, bearings, etc. and do a good cleanup if needed.
 
Sounds to me like a week bypass spring in the bypass valve. Early pumps
didn't have a valve in the pump. Up until 1948 the blocks have a bypass in
the front of the valve valley. If there is a valve in the pump and the valley
the weakest spring will determine the oil pressure. To thin of oil will also
cause low oil pressure. Castrol 20/50 has been used by many people to
overcome low oil pressure. G.M.
 
I think I'll pull the pan off and have a look. It's not causing a problem just a little un nerving to see the gauge at 0 after a few miles!!!
Ian
 
Ianrhall said:
I think I'll pull the pan off and have a look. It's not causing a problem just a little un nerving to see the gauge at 0 after a few miles!!!
Ian
Click to expand...

I would say it will cause a problem in time if the oil pressure
reading is true. G.M.
 
if oil seems too thin, smell it to be sure its not thinned with gasoline from a bad fuel pump
 
I did suspect that too because I've had issues with fuel getting past the valve in the bottom of the float bowl(correct name escapes me) however the oil doesn't smell of fuel at all
I'm going to be draining the oil and removing the pan soon. How does the oil pan seal around the front pulley?
I intend to use 20/50 when I refill, is that ok?
Cold wet weather putting a stop to driving in the uk now
Ian
 
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