Another start but won’t run issue

Hi there,

I hate that my first post on here is a problem post, and I’m looking forward to sharing both my Miatas with you at some point.
I have a 1993 mk1 Miata 1.6 Eunos Import, which will start and fire perfectly first time, and then die Almost straight away. I’ve read loads of the same problems and lots of people have had lots of different issues that cause this, and I’ve also read and diagnosed it. I’ve Bridged the FP to the GND ports in the Diag box, and the car runs perfectly then like there’s no problem, which led me to believe the AFM and it’s Run signal was at fault. I found a replacement AFM and swapped over the whole lot, and it’s still doing the same, only staying running when I ground the fuel pump.

Could there be a problem with the actual plug or any of the 7 pins inside it? The car has had an immobiliser in the past but this has been bypassed.

Any suggestions or help will be greatly appreciated.

Thank you.

I’ve read problems like this could be caused by a bad earth…thinking that when you say “stays running when I ground the fuel pump”
Suggest you check all earthing points including the battery one, they do corrode or break given the age of the Mk1 now.

Thank you for the reply. I shall check all the earthing points and see if they are any good. This is a project car, and I should point out that I made an idiotic mistake and a rare one for me to do, by that I didn’t disconnect the battery when I was welding the passenger side inner arch. So I’m thinking that I may have blew something as the car ran fine when I parked it up to weld it, then would run afterwards. Just started up first turn of the key perfectly then cut out. When the Fuel Pump is grounded by the terminals in the diag box the engine runs faultlessly. So it has to be a fuel system problem. Thanks again

If it helps, there’s a pdf of the wiring diagram here:

The relay which powers the fuel pump is the “Circuit Opening relay” on page 2, zone C4.

You’ll see that there are two wires which can operate the relay. One is violet and comes from the “ST SIG” fuse (hot while cranking) and the other is light green and comes from the airflow meter. It looks like the violet wire supplies 12V to power the fuel pump while cranking and the light green switches to ground to keep it running after that.

I don’t know which of those wires is activated if you test the fuel pump at the Diagnostic port, but I suspect there’s a fault with one of them and the smart money must be on light green wire (or the black wire which brings its ground to the AFM).