1.8 Miata oil pressure

Bill Meyer

Bill Meyer

Well-known member
I have a 1.8 Miata (DP AUTOX, basically stock engine) with a concerning oil pressure issue. Cold (idle) about 60 psi, hot (180) goes to zero. Gage (mechanical) checked: ok. Have no idea what oil was in it. Experimented with straight 50w. 75 psi at idle but drops rapidly as engine warms up.
Pulled the engine, plastigaged all the bearings (they all look great): rods are all 0.001+(#3 max @ 0.0015). Mains are all 0.0015. No smoking gun here.
I’m baffled. Loose clearance oil pump gears? Cam towers? PRV?
 
Bill,

Check obstructions in suction side of oil pump; crimped tube, plugged screen, etc.

Had similar experience after start-up on a very cold morning when still using STP. Oil just too thick to be pushed into oil pump.

Ron Sorensen
 
Have you checked with another gauge as a reference?

the electrical sender/ gauge in my old STU car never read above about 40psi cold and 30psi at temp/on track, but plugging into same port with a different gauge read much better news. it was closer to 100 cold and 50 hot.. with the pressures as the gauge indicated, no way that car would still be alive let alone still running well after 12 years and hundreds of hours of racing.
 
Checked dash gage (piped, not electric) with another: it’s ok (dammit). Put straight 50w oil in (since I had no idea what was in it) just for grins: 75psi cold dropped rapidly as engine warmed up (dammit again). Pans off (obviously…): no smoking gun yet. Oil pump & restrictor inspection next.
And….prv is OK.
 
Is this a newly built engine? Or an older engine that just started do this?

Any chance oil is building up in the head? Perhaps a head gasket without the right drain-back holes or something. Wait a few minutes then fire it up again when warm, does the oil pressure start out OK? If so it's a running time problem, not a warm oil problem.

Is there enough oil? What is the oil level right after the pressure drops?
 
Not a new engine: I bought this car in California 3 or so years ago. Seller was flipping it so I got no insight about technical details. So….I didn’t know what oil was in it. Or anything else. Cold starts always had reasonable pressure. Then as water temp went up, pressure went down ( to essentially zero at idle. Up a bit with rpm). No improvement/change on hot restart.
Pan off & partial inspection above noted so far. Stock rods. The engine has been apart: it’s got coolant re-route out of back of head. More news to follow when I pull the head.
 
no oil in the water or leaks anywhere?
compression/ leakdown?
how do the spark plugs look? any signs of oil?

With that much drop, I'd be concerned about a crack or leak somewhere that's opening up with temp.

there should be a smoking gun somewhere.
 
I believe those engines have an oil restrictor in the block to limit flow to the head. If that was accidentally left out, that would explain your problem. You should be able to see if it's there when you pop the head off.
 
185 compression all 4. No leaks, oil in water, bad plugs, etc.: ie no smoking gun. I was hoping for loose clearance main bearings but that thought died rather quickly. Missing restrictor is #1 suspect now.
 
Ok, let me chime in here.
There's no removeable restrictor, it's pressed into the block. Even if it was gone, that won't cause this. What causes this is an old oil pump pressure relief spring. Seen it a million times. Change out the oil pump and your problems will be gone. One more thing to check is that all 4 of the piston oilers are still in place in the block.
 
My intuition/subconscious seems to be working: I ordered a pump last month. Checking mailbox every day.
 
Well, after an extended period of sloth & procrastination (this sounded better than the real, more serious reason), I installed a Boundary 60 psi pump. And…put the car back together in the last 3 days. It seems to run great, 25psi @ hot (200 degrees) 800 rpm idle. 60 @ 1500 up. Autocross @ Ozark Mountain/Neosho tomorrow will prove the cure was successful. High confidence level!! Thanks to all the responses: particularly Jesse.
 
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