I have a '97 F-150 with the v6 and 5 speed. The truck has just over 200,000km on it.

I was merging on the highway today and noticed that the oil pressure dropped (according to the stock guage)... so I started playing around with it.

The oil pressure will read normal (just a tad above half way) when the truck is idleing or being driven normally. When you get on the gas the oil pressure will drop.

Ie, when I hit approx 45km/h in second gear and above (no tach in the truck but with 3.75 gears i'm guessing it's approx 3500rpm) the oil pressure guage will drop to about 1/3 pressure. As soon as I let off the gas (shift) it returns back to normal. I can gun the engine and you can see the oil pressure guage move slightly.

Now to really confuse me, to my understanding the oil pressure guage is an "idiot light".... how is it possible i'm seeing it "drop in pressure" instead of just going to zero?

Other possibly unrelated items.
1) We checked the oil first chance we got... it's clean and full.
2) oil was changed 2000km ago.
3) The truck has recently developed a hesitation at low rpms (get on the gas at low rpms and the truck will hesitate slightly)... we had the same problem last year and fixed it with a set of new plug wires. i'm just supprised it could the plug wires again.
4) the battery is new (6-8 month) and the alternator has been replaced (prob just over a year).

 

I had that happen on a '72 1/2 ton with a 302. Found out the wire was loose on the sending unit and the increased air flow caused a poor connection.

 

first of all i would find someone that has a manual pressure gauge with a long enough hose to where you could go drive it and see if the manual gauge acts like the dash gauge.if it dosnt drop like the dash gauge does then i would say you have a failing oil pressure sending unit. and if the manual gauge drops like the dash gauge then you have internal problems in the engine.