When the car first starts up oil pressure is good, it is always good when car is cold pegging 60 without much rpm as expected on thick oil. When its hot though I sometimes see it drop very low at idle below 1000rpm, Less than 10psi. Other times it holds 15psi with no issue. Is this the symptom of a bad relief valve?

 

Might be the relief valve or your oil pump may be worn out. In either case, you're pulling the engine to repair.

But, before you do that, it is worth connecting another gauge. The OEM gauge sometimes reads incorrectly.

On cold start, you should see it go much higher than 60. 60 is a good pressure for normal cruising after reaching temperature.

 

It also makes a difference how hot the oil is. When it's 115 out and I've had the A/C running and get off the freeway (having done 70 for ~30 mins), until the oil cools a bit driving on surface streets mine will hover in the ~10psi range at idle until it gets a chance to cool some. Under "normal" operating conditions mine sits in the 15-18 psi range at idle (850 rpm).

I've thought of getting an oil cooler to help fix this condition, but meh...

 

Does this cause scary rattling noises? Because I get noises after what you described, and I've always assumed it was heat and oil related. I'm reinstalling a legit OPG this weekend.
Hah! I was thinking the same, but I know its not going to fix a dubious oil pump.

 

There are indeed scary rattling noises when I let it go low. I have seen it come very close yo reading zero and other times when I know the oil is hot it won't go below 15psi

 

BTW, what oil are you running? Diesel formulations are kind to our cars. Rotella T-6 seems to be the MT standard and is fairly inexpensive at Wally World. At temperature, T-6 acts like 40 weight and helps keep pressure up.

 

I don't know about OP, but I am running T6, that's what freaks me out

 

1995 and later cars have an oil pressure gauge that says "good" above 7 psi and "bad" below 7 psi. Apparently the people who designed your engine feel that 7 psi of oil pressure is safe.

When it's hot, I'm generally satisified with 10 psi per 1000 rpm. If you don't like the low pressure, then buy a thicker oil. Safety of your engine is not determined by oil pressure, it is determined by oil flow. When your oil is at 60 psi and flowing through the relief valve, your flow is reduced, and engine wear is accelerated. When the engine warms up, viscosity is reduced, pressure drops, flow increases, and your engine is happy. When you're idling at 800 rpm and no throttle, why do you feel you need 30 psi of oil pressure? No load on the engine, no speed on the spinny bits...just a thought.

People that complain about low oil pressure when hot idling (such as yourself) is the EXACT reason why Mazda replaced the oil pressure gauge with an idiot gauge in 1995. We have idiots to thank for the fact that we have to retrograde our oil pressure gauges and sensors. I feel like the most cost effective solution for you is to trade senders and gauges with a smart person who has a 95 or later gauge and sender.

Your car will still be happy, and you'll still be just as....uninformed.

 

Yes I'm familiar with how oil works with temperature But I think OP and I are in the same boat here: at what point does this go from being low pressure due to hot, thin oil to low pressure due to relief valve issue?

Rattling from the engine is generally does not indicate a happy engine. I'm tempted to believe its actually a bad thing. Does your **** rattle at idle after a long highway run or a 20 minute session on a roadcoarse?

spforces: Sorry to threadjack, hopfuly this is relevant. What brand/weight of oil are you running?

 

Nope, no engine rattles. Running solid lifters. Typically see 8-10 psi at hot idle. Ocassionally down to 6psi if the fans kick on and the idle drops to 400 rpm for a quarter of a second. I would expect it to be a relief valve issue if the pressure falls relative to RPMs as RPMs increase. If my RPMs increased from 800 to 5000 and my oil pressure went from 8psi to 20psi...well, that would make me sad.

 

Exactly

Sounds like OP and I have rattles with the same conditions, but I need to get some real OPG up in herr to confirm teh dataz.

FML. Maybe I should call Andrew and find out about a shortblock. I've been whomping on this thing for a while, I know its not going to last forever.

 

Wow... I think I know how to use an oil gauge for intelligent people...such as yourself... When the gauge says its at its minimum and the lifters can't stay pumped up due to lack of pressure and are clanking against the cam I feel pretty sure I have an issue. My automotive service background and mechanical engineering degree don't mean I gave all the answers about my car but they are enough to go yo against someone who is just out to troll. Is my LOW oil pressure issue problem consistent with what those of you that have had relief valve problems have seen? That's all

 

I put in 10-40 in an effort to combat the issue so I don't think thin oil is my problem. No problem at all hope we can both learn something here from someone genuinely interested in helping us out!

 

spforces: Do you have another gauge installed yet?

I can't find my stupid BSP adapter so I haven't installed mine yet

 

I see the same thing, 8-10psi hot idle. I've never paid attention during near-stall speeds but I wouldn't be surprised if it drops like yours does. Running Rotella T6 and VEI gauge in with the sender in stock location. I may switch back to Mobil 1 0w-40, it was consistently 1-2psi higher than with T6.

Oddly enough, with the stock sender completely removed the stock gauge still reads as if there was sufficient oil pressure.

All this time I assumed the low pressure was caused by FMs -8an cooler lines, gauge and cooler were installed at the same time.

 

Given that variability, is there any chance it could just be some glitching in the pressure sending unit?