Low oil pressure in my 78 Bronco

jbdkz · Jul 10, 2007

I have a 78 Bronco, the odometer reads 34,000 but I bet the true mileage on it is about 134,000. It has a 351 Cleveland engine. I'm not sure if the engine has ever been rebuilt, replaced, or is the original. When the engine is cold the engine oil pressure is within the high normal section of the guage on the dashboard. As the engine heats up the oil pressure starts to drop but is still within the normal limits on the guage until I sit at a stop light for awhile, then it drops below the normal pressure range on the guage. Once I get the engine up to speed while driving again the oil pressure goes into the normal range on the low side. I am using 10W30 motor oil as specified in the Haynes manual.

I am thinking there is somthing wrong with the oil pump but am hoping the problem is something else.

Justshootme84 · Jul 10, 2007

The original motor in the 78 Bronco would be the 351M, as the Cleveland motors were never offered on the Bronco. Regardless, a few possible items on the low oil pressure would be the oil pressure sending unit on the engine. Second and more likely is worn main bearings. You could try a heavier oil like 20W-50, but that won't help much for the bearings. When you change the oil filter, take the old one and cut it in half. Look for any metal filings or sludge. The other common problem is a clogged pick-up tube for the oil pump. The tube can get clogged with sludge or gunk and will starve the oil pump. That would lead to a "sudden death" symptom where the pump completely fails, and you will hear the knocking. Some guys have installed a mechanical oil pressure gauge in place of the stock one, which is only an "idiot' gauge that can't read below 20psi. JSM84

jbdkz · Jul 10, 2007

Justshootme84 said:
The original motor in the 78 Bronco would be the 351M, as the Cleveland motors were never offered on the Bronco. Regardless, a few possible items on the low oil pressure would be the oil pressure sending unit on the engine. Second and more likely is worn main bearings. You could try a heavier oil like 20W-50, but that won't help much for the bearings. When you change the oil filter, take the old one and cut it in half. Look for any metal filings or sludge. The other common problem is a clogged pick-up tube for the oil pump. The tube can get clogged with sludge or gunk and will starve the oil pump. That would lead to a "sudden death" symptom where the pump completely fails, and you will hear the knocking. Some guys have installed a mechanical oil pressure gauge in place of the stock one, which is only an "idiot' gauge that can't read below 20psi. JSM84

Your right its a 351M, I'll start by installing a mechanical oil pressure gauge and see what it reads. Can you drop the oil pan and replace the oil pump on a 78 without lifting the motor? The Haynes manual says you have to remove the radiator, starter motor, and front support insulators and jack up the engine so you can put some wood blocks in place to do it.

Justshootme84 · Jul 10, 2007

Right, it's a PITA to reomve the pan or even change the oil pump, due to the engien crossmember. You'll need to raise the motor enough to unbolt the oil pump and pick-up tube, drop them into the pan and fish them out if changing those. Small hands come in handy then.

bidibronco · Jul 13, 2007

Given my Bronco is a 94 not a 78 I have done the unbolt motor mounts, connect cherry picker to motor, put jack under tranny and had a buddy lift one slowly while I lifted the other slowly to change the pan gasket. If you do it, don't be like me and get the cork gasket to replace it. Get a real gasket. I think if you get that far then getting the oil pump and pickup looked at it'll be fine. Just don't forget to always have some kind of stand so you're not relying on the hydrolics of the jack and cherry picker. Just so no one gets hurt. Safety is paramount...

jbdkz · Jul 16, 2007

Justshootme84 said:
Right, it's a PITA to reomve the pan or even change the oil pump, due to the engien crossmember. You'll need to raise the motor enough to unbolt the oil pump and pick-up tube, drop them into the pan and fish them out if changing those. Small hands come in handy then.

Any idea where the Oil Pressure sender is located on a 351M?

78 Bronco · Jul 24, 2007

The electrical sending unit is sticking up on the back of the block just behind the intake. The wire that connects to it slides over a threaded stud that sticks out the top. If you decide to put a manual gage in it you can connect it in one of two places. One is a plug in the block just over the oil filter and the other is a connection point just under electrical sending unit.

devo · Aug 20, 2007

I use lucas oil stabilizer and it seems to keep the oil pressure in the good

it's usaully not the pump it's in the bearings and the lucas helps keep the oil in the bearing helping to keep oil pressure up, or you can go threw the work and still have low oil pressure your call

good luck

pdeddy · Aug 20, 2007

I changed the oil pan gaskets on my '79 a while back and avoided the step, "unbolt the motor mounts and insert a 2x4 block" by using a hylift jack I had on the front bumper and extending the front suspension enough that the oil pan was free of the front axle and slid out. My bronco at the time was already on ramps in the front for more clearance working underneath and I was able to use the hylift jack to extend the springs. The front tires never left the ramps and I had enough room to remove the oil pan. Just a suggestion for you. If you're running taller tires than 33" (that's what I have) you may be comfortable working underneath without ramps and you wouldn't have to jack up the front bumper as high to extend your springs to get the room to remove the oil pan.

MariposaBronco · Oct 20, 2007

jbdkz said:
I have a 78 Bronco, the odometer reads 34,000 but I bet the true mileage on it is about 134,000. It has a 351 Cleveland engine. I'm not sure if the engine has ever been rebuilt, replaced, or is the original. When the engine is cold the engine oil pressure is within the high normal section of the guage on the dashboard. As the engine heats up the oil pressure starts to drop but is still within the normal limits on the guage until I sit at a stop light for awhile, then it drops below the normal pressure range on the guage. Once I get the engine up to speed while driving again the oil pressure goes into the normal range on the low side. I am using 10W30 motor oil as specified in the Haynes manual.
I am thinking there is somthing wrong with the oil pump but am hoping the problem is something else.

In my experience, Ford gauges are kind-of faulty in a sense ( not very accurate). my '79 broncos gauge was reading similar to yours. when i hooked up a mechanical gauge to it. the oil pressure was fine. HOWEVER, the fact that it does read low (below) normal range! would suggest that your engine has worn main & rod & cam bearings, which would be normal for the miles you stated. my 400ci equipped bronco has 173000 miles on it. I'm the second owner we have seen a lot of great times. a year or so ago I broke the trac-lok carrier and it has been sitting for a while. awaiting a rebuild. NEVER SALE YOUR BRONCO, YOU'LL REGRET IT. I know I'll never sale mine!

JHH

Fritz180 · May 21, 2008

I know its been some time before anyone has posted to this thread but I know when I replaced my oem oil pressure gauge it was reading between 20-25psi while drive and almost 0 at idle. and it was acually causing my truck to overheat. What I did was put a high volume oil pump in it and now driving down the road I'm at 75psi and about 25-50psi (depending on Phoenix temperature) at idle, with no over heating problem

jbdkz · Jun 5, 2008

Fritz180 said:
I know its been some time before anyone has posted to this thread but I know when I replaced my oem oil pressure gauge it was reading between 20-25psi while drive and almost 0 at idle. and it was acually causing my truck to overheat. What I did was put a high volume oil pump in it and now driving down the road I'm at 75psi and about 25-50psi (depending on Phoenix temperature) at idle, with no over heating problem

The problem was the oil pressure sender unit, I replaced it and the oil pressure gauge is reading within normal range on a dash gauge. I also installed an aftermarket oil pressure gauge to verify that the pressure is correct.

Thanks to all for your input!

Low oil pressure in my 78 Bronco

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Rest in Peace Friend! Never forgotten..

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