Q–I own an ’85 Ford F-150 pickup with a 351W engine. About eight months ago, it developed an oil-pressure problem. Once it was at full warm temperature, it would gradually lose all oil pressure at idle. Granted, I’m dealing with an idiot gauge to tell me these things, but the truck never sounds like it is starved for oil.
My mechanic looked at it, thinking it might be the fuel pump, but said he had seen this before on a Ford motor and said it was my bearings going. Which bearings did he mean? The motor still runs great (except for the carburetor, but that’s another story) and doesn’t show any signs of quitting. Can I expect it to keep going for a while .W., Chicago
A–You can expect the oil pressure to be low at idle on a fully warmed engine, but it should never drop to near zero. If that happens, oil is not being pumped through the engine–especially to the upper half, where the valves and rocker arms need lubrication.
We have a hunch your mechanic suspected a problem with the oil, not fuel, pump. If there’s too much internal clearance, it won’t produce the needed pressure. There is also a pressure-relief valve on the oil pump designed to open if, for some reason, the oil pressure gets too high. Usually this valve doesn’t open, but if it sticks open, oil pressure will fall.
If the pump is OK but the bearings are worn, oil pressure won’t be maintained in the engine. Oil passages are drilled through the crankshaft to lubricate the main bearings (supporting the crank) and rod bearings (for the piston rods). There are also passages in the camshaft for the cam bearings (that support the camshaft). These passages are part of a labyrinth of oil passages.
The bearings are not ball bearings–the kind that come to most people’s minds. Engine bearings are curved pieces of metal that more resemble a potato peal than the bearings on your bicycle wheels.
If the clearance between the bearings and the shaft is too great, pressurized oil leaks beyond the sides of the bearings, allowing the pressure to drop in those drilled passages. This is not unlike the drop in pressure when you are taking a shower and your spouse flushes the toilet.
Typically worn rod bearings, not main bearings or cam bearings, result in low oil pressure at idle. If the other bearings are worn, the pressure would be low no matter what the engine speed.
Though we have heard of idiot lights, we have not heard of an idiot gauge. But your allusion to its possible inaccuracy is legitimate. Most instrument-panel oil gauges do not display the oil pressure in pounds per square inch (psi) or Newton-meters (Nm). They simply give a range that is safe and unsafe.
The gauge does not commonly go bad, but the oil-pressure sender, mounted on the engine, does. All your truck may need is a new one. But we would not install it until we checked the oil pressure with a test gauge. If the pressure is OK, then the sender, which costs only a few bucks, might be all you need.
There could be another reason for your lack of oil pressure that is unrelated to the condition of your engine. You mention a problem with the carburetor. If it is allowing too much gasoline to drizzle into the engine, the gas can dilute the oil in the crankcase, causing a drop in oil pressure.
You can diagnose this yourself. Pull the dipstick. If it reads over the full mark, gasoline is probably getting into the crankcase. Ditto if the oil appears to be extremely thin when you rub a sample between your fingers.
Sniff it. If it smells like gas, change the oil and filter right away. If the oil pressure returns to normal after the oil change, have your carburetor repaired.
All of this is a long way of saying that you had better let your mechanic do some exploratory surgery on your engine. Only then will you get an answer to your question to whether you can keep your engine going for a while.
