Hi all,

First let me say this is a great forum, I have found many useful tips/advice from the post here.

Ok, here's the deal. I have a 71 F-250 with a 390 in it. The motor has roughly 40k on it, not entirely sure the details about the rebuild since my pop gave me the truck last year.

My problem is this, the stock oil pressure gauge would read no oil pressure at hot idle intermittently. My dad said it was the sending unit and that it was nothing to worry about. I decided to replace the stock gauges with a gauge pack from Pep Boys which includes a mechanical oil pressure gauge.

When I first start it cold the oil pressure reads about 30 psi and comes down to about 20 psi at hot idle. It reads 40+ psi at highway speed.

The problem is when I drive it for about 15 minutes, when I come to a stop the oil pressure drops down to zero. If I tap the gas it comes back up then drops down to zero at idle again. There is no noise coming from the engine and the water temp doesn't move at all.

BTW I am using 10w-40 oil and a motorcraft oil filter.

Sorry for the long-winded explanation. Any advice would be greatly appreciated.

 

sounds exactly like my truck before i rebuilt the motor. although it ran great, oil pressure was non existant at idle. my mechanic friend told me to try a high volume oil pump first. if that doesn't get the oil pressure up you probably need to rebuild, or at least, replace the main bearings.

mike

 

Had a 302 that did the same thing. Ran great, but had no oil pressure to speak of.

As for the reason, the mains are worn(or the crank) and it is letting too much oil by. There maybe other problems also, but that one will make your oil pressure be nill.

You can try to mask it with a pump, but I dont think you will help it much, or make it last longer. I would try some 20w-50 oil first, then maybe straight 50wt. This will be thicker and help take up the room that is in there. It will help for now, and a bit easier then pulling the pan.

Larry

 

I sure hope it isn't the mains. Wouldn't it make noise under hard acceleration? I don't enjoy the thought of having to pull the pan. I live in an apartment and I'm limited to how much mess I can make

I will try the thicker oil first and see if that helps. It's due for an oil change anyways. I'll let you know if that helps.

Thanks

 

Mains will "rumble", rods "KNOCK". With adequate oil pressure/volume the rods may not knock at cruise speed. The mains will rumble from the crank moving around inside the saddles.

The problem is at idle the pump is running slow enough that it does not put enough oil out to fill the gaps and produce "pressure". Once you rev it up a bit, the pump now moves enough oil to produce pressure(basically moving more oil than can "leak" out between the bearings/crank).

Larry

 

Well it doesn't make any knock or rumble. If it is the mains then should I worry about serious damage? Should I stop driving it until I fix the problem?

I am trying to remain optimistic, I replaced just about everything in the drivetrain except the motor.

 

Pressure smesher, if your not driving like a maniac an fe lasts a long time without much pressure. My old beater 360's oil pump can't build-up enough oil pressure to start on the first go it makes a horrible clatter,clanking n'stuff noise then dies from to much resistance. The next attempt results in rocker/valve clatter but it'll be able to go if I can raz it up in time. Luckily the pressure bypass valve is clogged or something cause after awhile I get good pressure.

 

hehe, not in the truck It has really low rear-end gears, not sure but I think they are around 4.10. It turns 3000 rpm at 65 mph. So I don't think I'll be doing any racing in it.

My 390 starts right up on the first turn since I put in the MSD ignition. Makes no noise from the engine, except a slight exhaust leak.

Could it be the bypass valve is clogged in my case causing the oil pressure to read low??

 

Well, if the valve is stuck in the open position it could be causing it. But I doubt it. Anything is possible though.

Try some thicker oil and drive it easy. It will let you know when things are getting bad

Larry

You probably have 3.50 gears-or worse maybe 3.73's...I have 3.25 and turn 2700 at 65 mph. If you had 4.10 it would be 3500 rpm's. Been there, done that back when I had my 70 F100 with a 302. She would really sing at highway speed

 

Yes, it seems with this truck anything IS possible. I baby it when I drive it. I have my little sport car for going fast

I'll let ya'll know how it goes with the thicker oil.

Hmm, maybe it's 3.73s then, cuz it's actually a little over 3k rpm at 65 mph. It screams pretty loud on the highway, especially with the dual exhaust and glasspaks. I have never been in an automatic that the trans would slow the vehicle down like the truck does. And you answered my next question. I was wondering what rear-end gear it had in it and I didn't want to put the rear up just to ease my curiousity. Thanks

 

I don't want to mudy up the water here BUT. . . .

First off rumble & Knock from engine wear are produced at "no Load conditions". Usualy the mains rumble at idles, especially a cold idles, or a long hot idle.

Worn Rods will rattle at a shift point, just as tranny is about to swap up a gear, or at a point when you reach desired speed and ease off on the throttle so you have neutral throttle condition. At that point easing on & off lightly will make rattle come & go, loud & soft etc. you can "play it" with your foot by breathing on the throttle like you'd tease a turbo to "pump it up" before you jump on it hard. . . . .


I've seen people "rebuild" engines & use same old oil pump w/ big mileage & wear. I've also seen worn/ faulty pressure release valves leak down at idle causing low read out too.

Bad/ faulty/ loose/ gasket-less pick up tubes & dirty screens can cause low idle pressures also.

Before you get upside down:

1 find out what previous owner is calling a "REBUILT ENGINE"

2 who did it & what they did do?

3, before making up your mind based on educated guesses take pan down & look at all alternatives before yanking engine out.

I say this because you say there is neither rumble or knock. And at all speeds/temps except Idle, pressure is fairly respectable. It's only at idle, hot &/or cold you lose track on your guage. . . . FBp

 

If you crawl under the rear, there should be a tag that will denote the ratio. It should be hooked to one of the bolts that holds the center section. It will have 3.73 or 4.10 on the bottom line(If I remember right). That is if no one has taken it off, or changed gears before you got truck.

Larry

PS: depending on the amount of wear, the rods will rattle when idling too. It depends on the clearance and how much oil is getting to it. I've heard them KNOCK like they are coming out the side when idling...but that is an extreme case. Usually you will hear a mild knock at higher rpm, under a light load.

 

Well, seein as the previous owner is my dad, that should be no problem to find out Your right the only way to truly know is to pull the pan.

BTW Freightrain, I put in the 20w-50 as you suggested and the pressure came up to about 5psi at idle. I don't know if that's good news or bad. And as I said before, she purrs like a kitten at idle, no knock or rumble.

There's no tag on the pumpkin bolts, just 30 years of grease and grime