Hi,

I have have a problem with Oil pressure . It's been doing that for a while. I can figure out why it's doing that?

Description on the tuck:

- 1994 Ford F-150 4X4
- 300 cu I6 4.9L
- 5 speed overdrive manual transmission
- 3.08 axle ratio

Problem:

I've been running the truck for the past 4 years. No problems at all. but since last year I started to notice that my Oil pressure started to act strange once in a while. If I'm only running into town. well no problem. but I've noticed that the Oil pressure is not as high as it used to be but still in the normal range. But if I go on highway after an hour or so that i'm driving the pressure is going down. still in the normal range but pretty close to very low and then after a while, the pressure goes up again. What the hell is going on? I always have enouph oil in the engine. the Oil has never been under the ADD 1 Quart on the dip stick. Please help.

Work that has been done with no success:

- the Oil sending unit as been changed. but stil same problem
- Oil change has been made. this time I"m using Castrol Syntech.
Can't go wrong with that. I'm using the ford Filter , the one that
is recommended for my truck. I thought he coul've been the filter
but i've changed the filter at most 3 times since I got the
problem. So can't be the Oil filter.


So what could it be? Is it possible that it could be the Oil pump? Can it be a a crack on the crank shaft?

What puzzle's me is that if it's the OIL pump. why is it only doing that once in a while?

I really need your help on this one.

thanks
Daniel

 

it could simply be the oil thining out on the highway (when it heats up it will do this) and then once it cools a bit its thinkening again. I might start to worry once the presure goes below the normal marks, then it may be an oil pump. But just let some of the more knowledgeable guys chime in.

 

Your truck doesn't have an oil pressure sender - it has a switch. If the pressure at the switch is over ~6psi, the switch closes, sending current thru the gauge in the dash. There are resistors (20 ohm) built into the instrument cluster that are supposed to make the gauge read in the normal range, and the needle isn't supposed to move regardless of what the oil pressure actually is.

It sounds like your instrument cluster is developing a poor connection, which is causing some fluctuation of the needle, OR the gauge is aging and going bad. I'd pull the cluster (read how here) and clean the terminals of all 3 connectors. (The middle connector is for the speedometer "PSOM".) Do the terminals on the cluster AND the ones in the connectors. A clean piece of paper is good for polishing them, if there's no visible buildup of corrosion. It could also be a poor connection at the switch on the engine, so clean it and crimp it slightly so it gets a good bite - it has only one wire, which is white with a red stripe (W/R). The R/W is the temperature.

If you want a real sender, get one from a parts store for an older truck (like early 80s - the sender is a BIG stamped steel cylinder) and modify the cluster wiring so the resistors are eliminated.

 

i didnt know that abou the switch. i have a 1970 model and when im idling the oil pressure drops down to the middle between low and high. when i and running it, it moves back up almost right at the high mark

 

steve83 great reading. I don't know much about the newer rigs (never thought i'd get outta the 70's) but one thing you can do if you know anything about plumbing or a friend that might is go to the local hardware store and get small threaded pipe (close nipple) and a "T" fitting and remove your sending unit (used to be called idiot lights), install the nipple with the "T" (position the "T" 90 degrees from the way you see the letter here) in the top of the T install your stock sender, in the bottom of the T install the nipple to install into the location you removed your sending unit from, in the side of the t install a copper line as you can get with any good "real" gauge and mount it somewhere convenient for you in the dash or below the dash. What happens here is you don't effect the computer and you get actual real oil pressure. I never have liked nor ever will trust sending units. Take caution while routing the thin copper line as not to kink it, you can use the plastic lines I just prefer the copper, and you'll need to use brass T and close nipple fittings of the right thread size. Ford was very common for oil pressure drop in early models, i've run my 390's and 302's where the dash gauge (idiot light) was on or below the low indicator. Never had a problem. But then my thoughts are if it ain't broke I can't fix it.
Does your rig require more than a quart of oil per month, whats the mileage, what type of pulling or hauling have you done with this rig, do you have any leaks, you may not be leaking but just may be normal burning. Do a compression check. Worst case scenerio is your crank or cam bearings are wearing and not allowing pressure to flow properly, if you pull the engine you can plasty gauge the crank, any body else know how to check the cam journals, I dunt, good luck. I'm gettin long winded hear let me know how it goes E-mail broncky@cs.com if you want. I'm always open to sugestions, comments statements or ideas.

 

The gauge/light circuit is totally independent of the computer, so there's no need to keep the OE sender/switch if you're not interested in the OE gauge. You can put the mechanical line directly into the sender/switch hole.

 

steve83
really no kidding.
I would never have thought that you could change the idiot light for a real gauge and not effect the computer, thanks for the input, wish i would have know that before i through all that typing into the forum this one fingered thing is nuts, but what about the pressure problem the boy has someting going wrong what da thin

 

Yep. Check the Haynes wiring diagrams. They're worth their weight in gold.

Since we can't be sure WHAT the pressure is, we still don't know IF there's a pressure problem.