harrisk@inet2000.com

Hi.My 1990 F150 5.8 oil pressure goes crazy .Up down ,nothing and so on.I changed the pressure switch and same thing!I just bought it and the guy said it has a knock and no oil pressure.The knock seems to be fuel related(starving for fuel)and runs rough and then smoothes right out???I will check fuel pressure with a gauge ,but it almost seems electrical ???It runs better on rear tank than front.I haul wood with it and it runs good ,but somes times starves for fuel???Any help would be appreciated.Thx.

Just call me Sean

iTrader: (1)

First off, check the computer for codes. This will tell you how:
https://www.f150forum.com/f10/how-retrieve-trouble-codes-obdi-10907/
Then post them here.

I assume the truck has oil in it, so the oil pickup screen may be clogged. If you think the knock is fuel related you should get some codes pertaining to the problem, most likely timing issues.

andyman7931

you also may want to get a mechanical oil pressure gauge to verify that there's really pressure, it could be a faulty factory gauge or loose wire.

harrisk@inet2000.com

Thanks for the reply guys.I hooked up a mechanical oil pressure gauge and on fast idle it went to 30 lbs. As it warmed up it went down to 25lbs. then I revved it to 2000rpm and it stayed at 25lbs!!As it slowed to an idle .(800rpm )it slowly went down to about 3 lbs!!Rev it to 2000..back to 25lbs..idle 3lbs and starts to run rough so ..Boooooooooooo How hard of a job is it to pull the oil pan off ??The truck sat for 6yrs and I only paid 300 for it for body parts ,,but it runs and looks great except for the low oil pressure.I drove it for 400kms like this with Prolong in the oil so I guess it helps.It does not use a drop of oil or ever heat up.A/c is even cold after 6yrs.Really nice truck but is it worth the fix?Before I drove it I did notice that the oil had some sand in it so I changed it and looked in the bottom of the container and it had a few small round ***** of sand.When I touched them they fell apart instantly and looked like maybe casting sand?Oil is very clean now.Thanks for the help

aliens8mycow

I would consider it a fairly big job... I think you need to raise the engine and prop it with wood blocks, drain the oil and coolant, etc. Some of these guys think it's no big deal - the rest of us probably feel like it's a bit more than a Sunday afternoon in the driveway.

The chilton manual will walk you through step-by-step. Some of the pros on this forum can fill in any blanks along the way and offer some guidance.

andyman7931

you're better off than me. My truck had 0 lbs when idling warm and between 3-5 lbs when running at 2000 RPM. Since I use my truck to pull a camper long distances I went ahead and put in a reman-ed engine, but if you're looking for a cheap around town beater you could get away with using thicker weight oil and possibly a high flow oil pump.

Just call me Sean

iTrader: (1)

Why exactly do you want to drop the pan? Are you planning to change the oil pump? You can try that and see if it fixes your problem, but I don't think it will. More than likely the bearings are worn, especially if you found sand in the oil. Worn main bearings will cause a drop in oil pressure, and worn connecting rod bearings will cause the knock. If you change the bearings before you spin a bearing then you should be able to get by with just bearings instead of having to buy a crankshaft and other stuff that will be ruined. You will need to measure the main bearing and connecting rod journals to see how much they're worn and see if they're out of round. Then allow for oil clearance and buy the proper sized bearings.

You can always buy a new engine or a junkyard one, or find a rough truck for cheap and take its engine since you should be able to hear that one run.