Kit car - help needed before it gets scrapped!!

[elephantpoo]

Bit of a long shot, but worth a try.

DH is building a kit car, based on an MX5. I say building, but actually it was all done and ready to tax tomorrow (1st April) until yesterday.
He went to start it yesterday and it ran for a few seconds and then cut out.
This is all it does. He's had a couple of mates look at it, and they're all of the opinion that it's the immobiliser, which would mean a lot more work for DH. Apparrantly the air flow meter (another possible problem) is working.
Surely if it's the immobilser it wouldn't start in the first place?
As you can imagine DH is very frustrated, and in danger of trashing the car before scrapping it.
Any suggestions greatly recieved.
TIA.

It's very difficult to diagnose at a distance but I'll chip in a few ideas....

It's likely to be either fuelling or electrics so look at fuelling first (because car electrics are scary!)

Does it have fuel injection or a carb? If it's a carb then there may be a problem with fuelling so it gets fuel at the start but can't sustain the flow. If it always runs for about the same amount of time and can't be restarted immediately then it's worth checking the carb.

It could also be that the fuel pump is not maintaining the flow of fuel so check that.

Then you're onto electrics and you're stretching my memory as it's 15 years since I built a car. If he starts the car, releases the key and it runs for a few seconds then it's likely to be a problem with the immobiliser. If it stops as soon as he releases the key from the "start" position then it's likely to be an issue with the distributor. I think there's a capacitor that is bypassed when the starter motor is running (to provide extra energy for the spark). If there is a fault with the capacitor then the engine will stop when the key is released. Or it could be a problem with the starter motor relay.

That's as much as I can dredge up from my brain without standing over the car with a spanner in hand, sucking my teeth in a concerned manner

nocake - thanks for your reply.
You're right electrics are scary! If it's electrical, which is looking most likely, he's going to get rid
It's not the carb (it doesn't have one)
Apparantly you can hear the fuel pump working.
From what you've said, it's the immobiliser. It starts, runs for a couple of seconds and then cuts out. Surely the immobilser wouldn't allow it to start?
All of your suggestions have been useful.
What would be the next step if it's the immobiliser?
Apparantly it can't be bypassed. Could we take the ECU from another and test it that way? Would that help diagnose the problem?
This is so frustrating for DH. He was all ready 2 days ago to tax it and hit the road
Thanks again.

It sounds like your donor car is far newer than mine was. I'd have run some wires direct to the ignition circuit to try and bypass whatever might be causing the problem and work out if it's electrical or fuel. With an ECU you can't do that so your options are to try and remove/bypass the immobiliser or get a new ECU.

He could also ask a Mazda garage to plug it into their diagnostic computer.

Yes, donor car is 1997 (it's what we had already) With hindsight, would've gone for older (less technical!)
Anyway.......IT"S WORKING!!!
DH started at the beginning, checking EVERYTHING!
Called a chap out to check the key was still chipped. Then started checking wiring. He found a couple of stripped wires (they've been stripped all along, even when it was previously running) They got covered and now it runs-woohoo!!
I don't have to move out now!! (DH was very stressed about it)
Thanks for your suggestions x

I'm very pleased it's working . I remember the agonising frustration of a non-working kit car. I hope you both enjoy it.