Hey all,
I've posted here quite a few times trying to diagnose the oil pressure problem on my newly purchased 89 f150 with a 351 with 120,000 miles. Long story short, after it warms up the pressure drops to zero. It seems to run just fine and doesn't knock or anything but even after an after market gauge, it still shows zero. From what I've gathered from advice and knowledge shared on this forum I have to assume my main bearings are bad. I'm fairly mechanical (been working on cars my whole life) but have never rebuilt an engine. Here's my question: Do I have to pull the engine to replace the bearings? Can I just jack up the engine and do it from underneath? It that just a BAD idea to just replace the bearings? I don't need this truck to be perfect, I just want it for camping and hauling and really don't want to spend a small fortune on it.
Any advice would be greatly appreciated!

 

To rule out defective oil gauge & sender take out sender and install a mechanical gauge to get a true reading and go from there.

 

Truth is the after market gauge will do nothing for you if the oil pressure sensor is messed up. U say that when the oil pressure drops off it still doesn't make any noises or seem to run any different. That makes me think u have a sensor(aka sender) issue. Like stated above, before u do any thing else u need to hook up a mechanical gauge directly to a pressurized oil passage and see what really is happening. A gauge at harbor freight should be less than $30 and will be suitable for your use. An overhaul will cost you upwards of $600. Don't forget to check that u have enough oil in the engine. Low oil will cause this problem as well.

 

When I hooked up the aftermarket electric gauge it came with a new sending unit. Do you think I'd possibly get a different reading with a mechanical? The gauge I installed reads about 50 psi on start up, then drops to about 25psi when it warms up in my shop. Then I go out and drive it and within about 15 minutes if I come to a stop sign it will drop to zero. I've been told that's my bearings. It seems odd to me as well that it doesn't make any noise or lose any power. The thing seems to run great. I suppose I should get a mechanical gauge before I tear into the thing. Oh, and I've flushed the oil several times and then added fresh 10-40. After running it the oil still looks absolutely clean. You mentioned a pressurized oil passage... Where would I find that?
Thanks so much for you help!!!

 

What I'm referring to by a pressurized oil passage is one of the channels cast into the block where oil under pressure from the pump is flowing. If ur new gauge came with is own sending unit and your using it u might be able to just forget about the mechanical gauge. you said your running 10w 40. I would never recommend moving away from the factory determined oil weight. The various valves inside the oil system (I.e. the pressure relieve valve) will be affected by improper oil weight. The pressure relief valve may be the only thing wrong, if it is sticking open then oil pressure will plummet, although flow will still exist, a result u have no oil pressure and no gut wrenching noises at the same time. By using the proper oil weight the problem may resolve itself. For now don't assume that ur bearings are shot. As metal heats up it expands, which would tighten bearing clearances and basically oil pressure would increase. That theory is while using single weight oil. Oil pressure only decreases because the dual weight oil thins out. Try changing your oil and see what happens.

 

Not my thread but still very interesting info!
Thanks!
Scott

 

Thanks for the info Lowrider! I'd absolutely love it if it wasn't bearings. I thought the truck was supposed to use 10-40 oil? What oil is it supposed to take? I'll do some research but where do I find the pressure relief valve?
Thanks again! Great info!