I have a 1991 F-150 with a 351 and an automatic. After the truck warms up whenever you go to put it in gear, or press on the brakes, the oil pressure guage will go all the way back to the Low side and will bounce. However on the highway at regular speeds the oil pressure is right where it is supposed to be with no bounce. I just changed the oil and oil pressure sending unit but it made no difference. Do you think this is just a bad gauge or what else could be causing my problem? Thanks for any help,
Ryan Payne

 

Yea the stock gauges suck on these trucks.

Time fore some nice autometer gauges

 

Thanks for the quick reply, I check them out.
Ryan

 

Well, I found a brand new stock ford oil pressure guage and put it on last night, and the truck is still doing the same thing. Whenever it idles down really low the oil pressure just drops down to the L mark and stays there untill the engine revs back up a little, then it will go back to normal. Does anyone know what else this could be? I just wanted to try all my options before I put a new oil pump in it. I have read where peoples pan gaskets came apart and got pulled up against the screen which caused low oil pressure. However, when mine is running at normal rpm or is revved up a little, the oil pressure is fine, so I dont think that could be the problem. Any ideas will be greatly appreciated.
Thanks, Ryan

 

Hook up an aftermarket gauge just to make sure.

 

You have to see what your oil pressure actually is before you start tearing into the engine.

Get a pressure gauge on there and see what numbers you're actually running before you spend too much money. You might just have a clogged pickup screen or a bad ground in your cluster.

How many miles are on the engine?

 

It has 152000 miles with a new timing chain and gears, timing cover, and water pump. Before I just recently got it to run again it had sat for about 6 years. Thats why I am leaning more toward the clogged pickup screen. Do you think filling it half with oil and half with diesel would help to dissolve the crud if it was a clogged pickup screen? Ill get a real guage and put on it first just to make sure. Thanks for the help,
Ryan

 

No, I don't think that will help. If your screen is clogged you are going to have to replace it.

 

I'd say put a mecanical guage on it and see. My 94 the guage would fall at stop lights from time to time, and it turned out I had low oil pressure. In a weekend, and $120, I was able to put new main, and rod bearings, along with a high pressure/volume pump. Helped out big time. Now at a stop light it has 43 psi. Mine only has 137,000.

 

My 94 5.8l is doin the same exact thing. Below 800 rpm the oil pressure is on low and bounces with the pulse of the pump. Above 800 normal oil pressure.

I just assume worn bearings as well.

 

I had wondered about worn bearings also. But I also have a 93 with a 351 that has 205000 miles with no engine work done to it yet, and it carries great oil pressure no matter what rpm. I am going to rig up a mechanical gauge tomorrow and see what kind of numbers I really get. Ill keep you guys posted.

 

Im not sure about pre 91/92(whenever it changed) but i know that my 93 has a dummy gauge. As long as there are a few PSI it reads the mid/low side of normal. If its moving anywhere but to the L then id suspect a bad ground in the circuit

I'd get a mechanical gauge hooked up to it. i never understood why trucks came with dummy gauges. I have AutoMeter oil pres. and water temp, both mechanical and i love them. Just get some good small diameter high pressure hose made for the oil, because the nylon line included will fail and leak oil.

 

I finally got a mechanical gauge hooked up today. When I first started it on fast idle it read about 40 psi. As it warmed up, at idle it got down to about 20 psi. then I put my foot on the brake and put it in gear, and it dropped down to what I would guess would be 3 or 4 psi. (my gauge is in increments of 20) After that it pretty well stayed right there, if I reved it up just a little it would get up to about 20 psi. But I turned it off pretty quickly since I didnt want to hurt anything. So am I in the market for a new oil pump, or is there anything else to try before I get to that? Do you think I should do main and rod bearings while I am there? The only reason I am a little hesitant to do that is because I know nothing about replacing the bearings, the oil pump I know I could do.

The manual I have says to change the pump I need to remove the radiatior, disconnect the exhaust, unbolt the motor mounts and jack the front of the engine up to drop the pan and pump, does that sound about right for any of you that have done this before? I just wanted to make sure all these steps were neccessary before I got started. Thanks for all your help, Ryan

 

Do the mains and the rods when you are in there. Its not all that hard, basically for the mains, take all the main caps off, than pop the old bearings out of the caps, and put the new ones in. Than, you have to "roll" the upper half out of the block. I used a flat head screw driver to start them, than slide them out. Than put assembally lube on the new one, and push it into place. Thet can only go in one way, cuz the have a notch in them. Than install and tourqu the main caps back down. For the rods, just turn the motor over a bit at a time by hand. Tell two of the rods are on the bottom. Loosen up the caps, and swap the baring as you would the main caps, but than what I did, was push the piston up in order to get more room to swap the top half of those bearings. Than pull it back down, reinstall caps tourqu, abd move on to the next two. Its not hard, just time consiuming. Also, when you pull the oil pan, just take the fan shroud loose, you shouldn't have to do anything else under the hood. But yes pull the exaust, the starter, take the nuts off both motor mounts, jack the motor up and put some 1 inch spacers between the motor mounts and the motor mount pads. and when you get the pan dropped, let it rest on the crossmember,than un bolt the oil pickup tube from the main cap,and unbolt the oil pump from the block and let it fall into the pan. It will be a pain in the *** to get the pump bolted back onto the bock with the pan sitting there. You cannot get the pan out without pullin the oil pump. New oil pump and main and rod bearings will make a huge difference. I just put in the same size that came out o there. Good luck.

 

Just got the oil pump out. It was a heck of a job to say the least. I was glad to find a nice sized chunk of gasket up in the screen of the pickup. It must have fell in there when I changed the timing chain and cover. I could have just cleaned it and put it back together, but I already have one ordered so Ill go ahead and put it in there. Thanks for your help guys,
Ryan