I have a 91 f 150 with a 351W. After startup cold it has about 50psi. After it warms up, at idle it will drop to almost nothing. However, when driving at highway rpm the oil psi is fine. I have put a new sending unit, gauge, and oil pump on it with no change. In the oil pickup screen I found some pretty good sized chunks of gasket, so I was sure I found the problem, but after I cleaned it out and put it back together it made no change. I am really running out of ideas on what to try, the motor has 152000 miles on it, so I was hoping to get some more life out of the bearings and bottom end. I am going to try 30w oil, instead of 10w 30 since that is what it has ran most of its life, and take the teflon tape off the sending unit, and take the valve covers off and check the passageways. But my engine really didnt have any crud in it. Does anyone have any more ideas of what could be causing this problem. Sorry for the long post, but I am really running out of ideas and patience. Thanks for any help,
Ryan

 

My 351 did the same thing until I rebuilt it. Sounds like the bearings are wore. Thicker oil will help for awhile.

 

You already identified the problem right there. It's getting a little tired, that's all. If it still has good pressure at cruising speed and on cold startup, I wouldn't worry too much about it being low at hot idle. You might want to go to 20W50 oil, that could help a bit. But the rule of thumb is 10 psi for every 1000 rpm as a safety factor, so being on the low side at hot idle is still within that margin. Have you checked it with an actual mechanical gauge, as the factory electronic gauges aren't terribly accurate.

 

Thanks for the help guys. I guess I really am about ready for a rebuild then. I just kind of hated to admit it. haha. Ill try the thicker oil for awhile, this is far from a daily driver anyway. When I do get ready would you guys recommend me taking my block to a machine shop and have it and the heads rebuilt, or just getting a rebuilt long block? If you recommend the long block, where would you recommend I get one? There is a local place called wholesale engine distributors that sells them, but I havent heard anything about their reputation. Id really like to do all the work except the machining myself just because I think it would be an interesting project, but Ive never been that deep into an engine before and Id hate to mess something up.
Thanks for the help, Ryan

 

get a mechanical gauge on there. the stock gauge is not an accurate way to gauge whats going on.

also what oil filter are you running?

 

I tried a mechancal gauge on it, and the psi. really is dropping off. I dont really trust the stock gauge either. Im running a Motorcraft FL1A filter. Thanks, Ryan

 

Yep, he's about had it.

I'd recommend lucas oil additive instead of the thicker oil. I've had good luck with it in the past for old engines.

 

You could drop the pan and replace the rod and main bearings and oil pump only, with the engine in the truck. As long as it is not knocking. That will get you some time to make decisions on the type of rebuild you want to do.

 

Yes, replacing the bearings would likely bring the oil pressure up to a usable level. Don't do it without micing the crank though.

 

^Agreed, you can check the crankshaft endplay which will show how the thrust bearing health is; .004-.007 is ideal.

 

I had two different engines with the same symptoms and in both cases I replaced the rod bearings and oil pump. This cured the problem both times.
I just pulled the pan, removed one rod cap and checked the part number on the back of the rod bearing shell. It tells you if it's standard or under size. If the crank journal is not groved/damanged then just slide in a new set of bearings.

 

This is surely a bandaid...but it will buy some time.