rattlecan New Reader
7/21/22 3:01 p.m.

Hey Miata people

I’m about to fire up my rebuilt engine for the first time and I have questions about building up oil pressure before firing the engine. There seem to be 2 schools of thought on this.

  1. Pack the oil pump and then let ‘er rip
  2. Crank the engine with XYZ unplugged to build pressure first

My oil pump is packed and everything else has a nice coat of assembly lube. But I would be sad if I spun a bearing or something because I was being lazy

Which brings me to my question. There seems to be a thousand ways to prime the pump. Disconnect the CAS and the crank the engine. Pull the plugs and crank the engine. Disconnect the injectors and crank the engine, etc. I’m leery of cranking with only the CAS unplugged as I once blew a coil pack doing this during a compression test.

Any wisdom you can share with me?

Also, bonus question: The build took twice as long as anticipated due to supply chain issues and the gas in the tank has been there since last October. I’m gonna guess I should drain it, right?



Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
7/21/22 3:15 p.m.


If the oil pump is packed and you put assembly lube on the bearings, let 'er rip and watch that pressure gauge.

If you want to unplug things, I'd recommend CAS and coils. The injectors will try for a prime squirt but won't fire during cranking.

Mr_Asa GRM+ Memberand UltimaDork
7/21/22 3:59 p.m.

I know that most modern vehicles cut fuel via the ECM when you have the pedal at WOT while cranking.  Does that year Miata do this?

Be much easier to just hold the throttle down and crank till you see pressure than disconnect stuff.  Hell, I do that in my truck when I do an oil change just because my filter is mounted sideways and I can't guarantee I fill it all the way.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
7/21/22 4:09 p.m.

I'm honestly not sure with stock ECUs.

I don't think I've ever heard of someone toasting bearings in a Miata engine on first start. You're really not loading them much with just idle. Failing to seat the rings, now that is a lot more common.

NickD MegaDork
7/21/22 4:43 p.m.

With my engine we took the plugs out and unplugged the coil and cranked it until we saw pressure. We also built a pressure pre-luber that worked pretty well. A long chunk of PVC pipe with caps on both ends, then we drilled the ends and put a pressure gauge and a Schraeder valve in one end and a hose nipple and a ball valve in the other. Took an old oil filter and drilled the top of it, and brazed a hose nipple into that as well. You threaded the filter on, hooked the hose up between the pipe and filter, unscrewed the cap and poured a quart of oil in, then charged the thing up with an air compressor and opened the ball valve. With the key on, we would see about 50psi of oil pressure. Put a filter back on it, cranked it up and it had instant pressure.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
7/21/22 4:52 p.m.
Mr_Asa said:

It would be easy enough to check with someone that has a stock computer of this year and an actual pressure gauge.  

Fuel pressure? If you're seeing that drop when the injectors are open, you have other problems :)

Sure would be interesting to know what year, but I'm assuming 1990-93 because they're the least sophisticated. And page F-124 of the factory manual says that WOT does indeed disable the injectors if the engine is cold.

The manual very specifically says "while cranking" and doesn't mention the priming squirt, so there's the chance it will fire on a wide open throttle if you try this.

Mr_Asa GRM+ Memberand UltimaDork
7/21/22 4:57 p.m.

In reply to Keith Tanner :

I meant oil pressure.  Get stock Miata, mat the gas, crank, see if pressure rises.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
7/21/22 5:06 p.m.

That's going to happen no matter where you have the throttle pedal :)

Note that the "idiot light" oil pressure gauge in the 1995-05 cars registers at 7 PSI IIRC. The ones on the later cars are complete fiction.

Streetwiseguy MegaDork
7/21/22 6:02 p.m.

I'm pretty sure you are far more likely to damage stuff that is not pressure lubricated by unplugging and cranking the engine until the starter melts.

Make sure stuff is right, fire it up and pretend you have a flat tappet cam and need to break it in.

APEowner GRM+ Memberand UltraDork
7/21/22 6:36 p.m.

Stock engine?  If it's not easy to pre-lube I just pack the pump and fire it up.  I'm not convinced that the crank it till it get's oil pressure approach gains you anything and I didn't notice any change in engine life when I switched to the just fire it up approach sometime around 1990.

Performance engines I'll pre-lube using an Accusump that I keep around just for that purpose or by spinning the pump if that's an option.


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