New truck...oil pressure problems?

[Luminatorz34]

hey guys, i bought a 88 ford bronco fullsize, with a 351 in it...well my question is this...when i start the truck and drive it maybe 5 miles or less, the oil pressure is perfect. but if i drive for 5 miles or more, and come to a stop the oil pressure drops off to nothing when the motor is idling in gear at 750rpm or so. i put it in park, and it will still register no oil pressure, then will all the sudden pop back to normal pressure range. i dont hear any lifters or valves ticking, but its not cool to see a gauge drop to zero and back. i'd like input from anyone, it may be as simple as just the sensor, or maybe the pump is gonna take a shit on me...i do know the rear main seal is attempting to go out cuz of oil leaking between the motor and tranny, but this cant affect my oil pressure this much. i thank you guys in advance

[TbTalon94]

easiest thing to do is to replace the sensor...see if it still does it.

[Scott]

Its probably just an old sensor thats not very accurate or sensitive, you could try putting a old mechanical gauge on it to make sure, but low pressure at idle while warm isnt nessecarily that bad, its normal, shouldnt be zero but at least have 20psi or so. and no a leaky rear seal wont affect your oil pressure.. Im sure you have but make sure theres enough oil in it.

[Luminatorz34]

yeah, i changed the oil as soon as i got it, as it was about 2 quarts low (woman driver, and not knowing the rear main was out) obviously means just drive it and dont check anything, but it was 2 quarts low, i changed it, which was thin as water. and thought that would fix it. it didnt. im thinking its just a bad sensor, becuase of the fact that there is no motor noises or smoke or anything

[69gt4speed]

Most those newer fords have a pos oil pressure reading not much better than a light truly. Ol dad used to put senders and gauges in those broncos. Ya mech gauge is the way to check but if it isn't knocking and lifter tapping prob a ok. My 03 is a fake gauge reading itself. Means absolutely nothing almost. The old stangs with a real pressure sender was better IMO.

[69gt4speed]

88 351 btw I'm not sure if a one piece main seal or the old 2 piece. if it was 302 h.o. I'd say one piece seal, ya gotta pull the tranny to fix it. Course they don't suppose to leak as easy. Same answer don't know if that is a roller block or flat tappet. They did the roller a lot later for 351. And trucks were different than cars. Even cars 302's had several block changes along with head changes from 85 to 87.

[Luminatorz34]

thanks for that info man....im having a hard time figuring out what motor is actually in this thing...the person i bought it from says its a 351, but all the efi stickers and crap say 5.0. but the intake is a intake for a 351. and the title says it was a 5.0 truck to begin its life out. but i have no way to check if this is really a 302 or 351. and its pissing me off, cuz the 302 and 351 oil seals are different part numbers...and really cant afford to buy one, pull tranny/transfer case out and then find out i have the wrong seal, then the truck is out of commision

[Scott]

If the intake is for a 351 then thats what the motor is, a 351 intake wont fit a 302. If the engine is actually an 88 its probably a flat tappet lifter with a one piece rear main. its a pain the ass to do the rear main, I would just park it in the yard or on cardboard and add some oil to it every once in while. but Im lazy like that sometimes

[Luminatorz34]

LOL niiice. i like your way of fixing things scott. haha the seal is only 19 bucks, but yeah, a pain in the freaking ass to do. im pretty much rust proofing the frame right now

[69gt4speed]

Well get a tape measure and measure distance between the valve covers. 351's are wider as block is taller. If it looks bigger wider then 351w. Crankshaft diameters are larger too hence different seals, oil pan pattern is different. The early 289's 302's had the 28oz flywheel so once I put a 351w in a 67 w a c4. Only real issue was the exhaust. Any good ford guy can tell if 351 one reason the 347 came about along w smog laws in most states. Being you have probably a swapped engine hard to say what vintage it is. I put in a d2oe (72) in the 67 my old 77 van had a c9oe (69) in it. Ford part # look here
C5ZZ-2140-CR

C - Decade of Manufacture - 1960's in this case
5 - Decade Year - This is a '65
Z - Car line - Mustang
Z - Ford Service Part - This came from the Engineering Department
2140 - Basic part number - 2140 is a master cylinder
CR - Design Change - indicates manual disc brakes

http://www.mustangsandmore.com/ubb/FordTechData.html

[tylers88]

my 88 Sierra does it to so I just went and bought a $20 mechanical guage so i know it won't go out for almost ever on me if it even does.