Low oil pressure when engine is hot
 

So, first time posting, been browsing for a couple months now! TONS of great, knowledgable people on here, I'm stumped on my current issue.

A couple months ago I bought 1973 F250 with a rebuilt 390 and it runs great, only 30k miles on rebuild. I use it as my daily driver. I've had a couple electrical problems that were easily fixed, other than that just flushing fluids and replacing filters.

Now, the weather is starting to warm up and I'm now seeing a new problem. I have an aftermarket oil pressure gauge installed by the previous owner. When I start my drive oil pressure is great, as the engine warms up the oil pressure drops but ONLY if I am idling. As soon as I hit the gas pedal, it goes right back up to normal level.

So my question is, since the car is running great, no strange sounds or knocking, is this a normal/ok behavior or a sign of something bad? Thanks,

Andrew

Here is a post that contains the info/fix you need. It's just the design.... No worries! :-X22

Link: https://www.ford-trucks.com/forums/1...-in-390fe.html

How low does it drop? My 360 with roughly 15k miles on a rebuild idles at about 12-15psi hot, the instant the rpms come up it rises, cruises at 55-60 psi.

That's normal for the oil pressure to fluctuate with ambient/engine temps and rpms, my 390FE with 40K on a rebuild reads 20psi @hot idle (800rpm) and 60psi @cruise (2,500rpm).

I may have a problem then. I just noticed my oil psi is at 60 at start up. When then engine warms up it goes down to 40 and stays there no matter what the rpm's.

Is that trash in the oil pick up? I did notice that its time for an oil change, it pretty black and getting thin feeling.

Mine had those same numbers with 15w-40w, a little lower with 10w-30w, what viscosity are you using out of curiosity?

Bama, those numbers are running 15w-40 (Rotella).

I use Rotella too and next change will use 15w-40, summers here almost.

If its getting that good of oil psi, you're fine. Be happy. Most engines I've had run 15-20 psi hot in gear idling. If the oil seems worn, change it. Use some 15w-40.

my 390 has about 400 miles on it since rebuild, cold psi cruising is 55-65psi
cold idle 20 psi
warmed up in gear 4psi, put in nuetral and goes to about 9psi, no noise, no issues clean oil, jump right back up at touch of throttle
using melling high volumn pump, also have the holley jets in oil passage to help slow flow to upper end, crank is turned .010/.010 on mains and rods

Well, thats good news, sounds like this is not a problem. With the engine warm I am getting between 30-35 while accelerating and around 10-15 while idle.

It was just kinda messin with me because the needle moves up and down so fast, I wasn't sure if it was a problem or not.

Thanks everybody :)

I bgt a 1973 Ford F-250 in Dec 2020. Bgt it in GA and drove it back to CT with no issues. Has 117k on 360 engine. I was reading in this forum, that Rotella was recommended for use 15w-40 but I was talked out of it at NAPA. They felt Diesel oil was too high zinc. Whats the right answer??? Also notice for the first time my oil pressure gauge, which always registered just below half is now barely hanging above L. Any ideas of cause? also First time caller,,,thank you all for your help!!

If your engine has the original flat tappets I don't think the Rotella will have too much zinc. The zinc is needed to preserve the lobes on the cam. I'm not sure about the indicated reduced oil pressure. You could try to verify the readings with a known good gauge. Generally more viscous oil will produce higher operating pressures.

To what Ozzie said as well…you cannot have too much zinc in my opinion with the flat tappet cams. I’ve seen plenty of cams get wiped out from running the lower zinc/newer oils. This problem started back in the early 90’s when more and more car makers where going to roller cams. They started to change the additives in the oil and zinc being one of them. If I recall correctly the camshaft makers found the problem.

I use Brad Penn oil. The old Kendall racing oil. It’s high in zinc etc…I run 10w30 in the colder months (don’t drive it in the winter) and when I do the first oil change in spring/early summer I switch to 10w40.

thank you Gentlemen for the quick response!! I will stick with the zinc!! Have a great day