dherr (Forum Supporter)
dherr (Forum Supporter) GRM+ Memberand Dork
9/27/22 2:50 p.m.

So my Spitfire powered by the turbo NB Miata drive train has developed a warm shutdown issue. It starts cold, no problem, primes the pump, all is good. But once it is hot 5-15 minutes or more, it will shut down and stop priming the pump. Letting it cool down, starts back up. The fuel pump relay is pretty hot to touch after the car has been running. So simply a case of a bad relay, or is the Megasquirt tune to blame? I have a new pump relay coming tomorrow, and my pump is brand new, so 99% sure it is just a bad relay but what do you all think?

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) GRM+ Memberand MegaDork
9/27/22 2:56 p.m.

Check the grounds!

edit: check all the connections, including the tightness of the terminals that the blades of the relay push into.

dherr (Forum Supporter)
dherr (Forum Supporter) GRM+ Memberand Dork
9/28/22 10:01 a.m.

So apparently a know issue can be that the cam position sensor can heat soak and turn off the injectors leading to this. Cheap part, so I replaced it but still having the issue. Car will run and idle for about 20 + minutes and then shut down, won't restart until it cools off. Fuel pump relay is firing when I cycle the key when it won't start, so guessing it is not relay related, but replacing that as well just to be sure. Thoughts?

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
9/28/22 10:47 a.m.

You've got a MS, see if you can catch a log of this happening. That will tell you if it's the pump shutting down or the CAS signal going wonky and causing the MS to stop firing injectors.

Definitely check the connectors on that relay, especially if you're using a stock Miata relay/wiring harness. They don't usually shut down while running unless you're leading the Targa Newfoundland 4 days in, usually the symptom is "car was running fine until I parked and turned it off, now it won't fire". But a hot relay is not a happy relay.

GameboyRMH GRM+ Memberand MegaDork
9/28/22 10:47 a.m.

The hot relay is very suspicious, either something is wrong with it or its connections, of the megasquirt is driving it in a bad way (like PWM mode rather than old-school on/off).

dherr (Forum Supporter)
dherr (Forum Supporter) GRM+ Memberand Dork
9/28/22 11:08 a.m.

In reply to Keith Tanner :

Yes, should be able to do that as I can get it to simulate the problem just by letting it idle from cold for about 15-20 minutes. The harness is a stock Miata harness (pared down to only the circuits I needed). Since this was not happening until recently, looking at what changes I have made to the car, primary change being the relocation of the battery from the front to the trunk area, so will check grounds carefully.  

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
9/28/22 2:19 p.m.

If it is actuating the relay but the fuel pump is not running.... the relay is bad.

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