Questions and Answers

Ford F-100 4.9 Liter Low Oil Pressure Q. I have a 1967 Ford F-100 that I have recently installed a 1987 4.9 liter into. The engine has had machine work done to it, consisting of .020 overbore with new pistons and rings, new cam and bearings, new main bearings and rod bearings, new oil pump, new lifters and push rods, milled .010 off block and .030 off the head.

The head work is complete with new guides and new springs. The crank mic'ed out standard so I chose not to cut it down, which I did not Plasti-gage. Basically a new engine all the way around.

My problem is that I have what I consider very low oil pressure. When I built the engine I put 5/20 Castrol in for break-in oil and ran for about 200 miles. I noticed that the oil pressure was lower than expected, I thought it was just the thin oil. I changed the oil and filter to Rotella-T 15/40, yes I know that is diesel oil but it's good oil.

When I first cranked the engine, the top end was oiling very well with a standard oil pump, so I thought there was good oil pressure. The gauge is a stock electric manual gauge which showed above mid range reading. While driving it for the first time, I noticed that the oil pressure dropped to almost nothing and stayed there even after the RPM came back up.

This worried me to the point that I installed a manual gauge to verify a faulty sensor or dash gauge. To my horror the manual gauge told a worse story. With the 15/40 oil and a cool morning, 55°F) I cranked the motor and had only 35 psi. Once warmed up, the oil pressure got worse, 25 psi operating pressure and 5 psi idle pressure. I contacted the machine shop who had done the work and they didn't have any idea beyond excessive clearance on the crank. So I opted to go with a high volume oil pump thinking that would improve matters, it did to a certain extent but still not what I would call sufficient. Still 35 psi cold and 12-15 psi warmed idle.

Do you have any suggestions other than pulling the engine out and working the crank?

A. The oil pressure specification for a 1987 Ford 3.9 liter engine is:

Engine Lubrication
Engine Oil Pressure (Hot)	40 to 60 psi at 2,000 rpm

Idle oil pressure is not as critical as the oil pressure at higher rpms. If your oil pressure meets this specification, then you're good. If not, and assuming everything else is right, then you will need to Plasti0gage the main and rod bearings and rework them as required.