This is a '69 mustang with a 302. It runs great, except after it ‘warms up’, the oil pressure will start dropping at low rpms. It never drops below the “safe” level, but often sits right on the line. No fluids are leaking. When this happens I can sometimes hear a ‘chatter’ while going up hills and stuff like that, but only while my foot is on the gas. This may be unrelated as sometimes I hear it even when the oil pressure is normal. There seems to be performance issues, and the temperature rises slightly, so this leads me to believe that there actually is an oil pressure problem and not just the sending unit.
Oil pressure will drop as it warms up, because the oil gets thinner.
Stock gauges are notoriously inaccurate. Buy, borrow, or steal a mechanical gauge and double check. As a general rule, if the pressure goes up with engine speed (rule of thumb: 10 psi per 1000 rpm), then you’re probably safe.
What weight oil do you use? If it’s too light, that could be a possible cause. Check your owner’s manual, Haynes, Chilton’s and make sure you’re using the right stuff.
If it’s still low, it might be loose (tolerance-wise) bearings. How many miles has it been since it was rebuilt, if at all?
If you’re hearing a chatter when the engine is under increased load, it may be pre-detonation (pinging) from running too lean or not enough ignition advance. Leaning out is more likely, so first get one of the service manuals Gunslinger mentioned and check the mixture. The fuel filter and the grade of gas you’re using could also be to blame. Older engines generally like higher octane. Changing brands sometimes helps, too. If it goes away if you open the throttle further, leaning out is the most likely cause.
If it is more related to engine speed and load than throttle setting, it could be mechanical, like main bearings, but rod bearings could also be to blame. If your engine has never been overhauled, it would be a whole lot cheaper to do it now and give the bottom end a good inspection and replace all the bearings (you’ll probably have to have a shop do this). It’s not cheap, but it will extend the life of the engine considerably.
As far as the oil pressure problem itself, it could be normal, you could be using the wrong grade of oil, or the oil pump could be failing. If, as I said before, the engine is original and hasn’t been overhauled, and if the pump needs replacing, it wouldn’t be a bad idea to go ahead and do an engine overhaul. A 32 year old car, even if well maintained, is probably due for one anyway.
It’s 10w-30. Pressure does go up nominally with engine speed, but not a lot. I haven’t been able to find Chilton’s here for years, and Haynes sucks so bad, it’s worthless.
I always use the good gas, maybe I’ll try some octane booster and see if it goes away. If so I’ll give it a tune up and see what happens.
btw, it’s got about 55k on a rebuild, and the heads were done about 15k miles ago.
If you are in neutral and hit the accelerator sharply, you should see a spike in oil pressure. (maybe not huge) if you don’t try a thicker oil (though 10-W30 sounds good). As your engine wears you’ll drop in oil pressure a bit. If thicker oil (10-W40) doesn’t help, you have one of three problems.
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oil loss- leaks or burning in cylinders. Since you said you have relatively low mileage on the engine, I doubt that this is a likely cause.
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sender unit/guage problems - yes, they can go bad. Plus, a sender unit is not that expensive and it sure couldn’t hurt to put a new one in any way. However, since you are getting a small spike in pressure and it starts out OK until it warms up, I’d guess that it isn’t the sender. Gunslinger is dead on about the stock guages (which he usually is on all things engine related).
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Oil pump is worn. More expensive to replace so you want to check all else first. I would imagine that it was replaced new when the engine was rebuilt, but one never knows.
I guess it could also be sludge in the oil pan. If it’s not too inconvenient (I don’t know what the cross membering of a 'stang is like) see if you can drop the oil pan. If there’s sludge and gunk in the bottom, or anything else that can clog the pump filter clean it out. Don’t forget to replace the gasket!! This seems possible if one thinks that as the oil heats up it softens the gunk allowing it to flow near the pump and clog the filter. However, I must go on the record as saying that I seriously doubt that this is the problem. One, there are so few miles on the engine one wouldn’t expect that much gunk in the oil pan unless you use crappy oil or never replace the filter or stuff like that. Also, I’m not sure if that would produce the symptoms that you describe.
Like I said, I kind of doubt its the sending unit because of the increase in temp., but I think I’ll replace that and flush the little tube that leads to it. I’m sort of leaning towards oil pump at this point. I’ve never had one go bad, so maybe this is what happens.
the symptoms sure sound like oil pump, but the timing seems off (new engine) unless some old parts were used.
That was done before I owned the car, so anything is possible.