mx5cartalk.com

Hi all,

I'm a regular reader but don't really post. I'm hoping someone can point me in the right direction. My nb8b hadn't been dríven in 3 months and the battery was very flat. I jump started it and took it for a blast. Got home, turned it off and restarted it. All good. Another 2 weeks later and it was flat again (all it could manage was dull dash lights). Attached the charger and left it for a few days but the 8 year old battery wouldn't come back to life. Bought a new battery, fitted it and it cranks fine but randomly will fire and then dies after about 3-4 seconds. It's just like the key has been switched off. Tried both keys and the same result.

I didn't disconnect the battery while it was charging. Possibly a stupid decision.

Googled it and found a few cases and lots of suggestions but as usual the op doesn't post after they find a solution.

Any suggestions before I spend $150 getting it towed to a workshop?

Cheers,

Warwick

Probably fouled plugs from trying to start it a heap of times with a flat battery.... Maybe not, not a lot else that makes sense in my head, but nothing i touch has an OEM ecu typically.

Dann

Could be lots of things.
Firstly, are the terminals clean? Then are they nice and tight?

NitroDann wrote:Probably fouled plugs from trying to start it a heap of times with a flat battery.... Maybe not, not a lot else that makes sense in my head, but nothing i touch has an OEM ecu typically.

Dann

Thanks Dann. Not fouled plugs. Didn't try and wind it over heaps of times. It wouldn't crank at all with the old battery.
When it does fire it revs cleanly and you can hold the revs up but after a couple of secs it dies like the keys been turned off.
I used to fit aftermarket immobilisers 20 years ago and they some were similar if you didn't disarm them. Fire but die after a few seconds.

Should have said that it's completely stock.

bruce wrote:Could be lots of things.
Firstly, are the terminals clean? Then are they nice and tight?

X2


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bruce wrote:Could be lots of things.
Firstly, are the terminals clean? Then are they nice and tight?

Thanks Bruce, but as I said in the original post, it cranks fine. Faster than it ever did with the old battery.
It won't fire, or if it does it runs great for a few seconds. You can rev it and hold the revs up but it dies after a few seconds.

Fuel, spark and compression and it should start. Sounds like you have compression so first off - check for spark. Pull a plug - ground it by resting on the cross brace - turn the engine over and see if there is spark. Fuel is a bit harder to test, but turn on key and listen for the fuel pump kicking in. Try a good shot of start-you-bastard down the vacuum hose inlet on the inlet manifold – if it runs for a second or so you have a fuel problem. No fuel-no run. Could be the fuel relay above the gas pedal, could by the injector relay in the engine bay, could be the fuel pump. I had a fuel pump die not long ago in my NB8B, they don't last forever. Same starting problem, sometimes start, then no start. Not too hard to change over - plenty of info on the web. Buy the best pump you can - no cheap e-bay jobs. Good luck.

1. Blocked fuel filter. There's enough fuel getting through it to build initial pressure at the injectors when you turn the ignition on but not enough after that to sustain combustion in a running engine.

2. Faulty MAF or MAF connection. What you're describing is what commonly happens when a MAF dies. Borrow someone else's MAF and try it.

Sounds a bit like no fuel pressure.

Does better than mine in this cold weather.
Cranks for a bit then backfires like a shotgun while blowing flames out the exhaust.
Sometimes it'll do it out the intake too. (which I really hope hasnt killed my MAP sensor since i'm using the internal one on the ECU... worst case, I have to buy an external and wire it in)

But, I haven't had my winter cold starts tuned yet.



AS for your car,
I'd think possibly fuel, as others have mentioned.

#1: remove the plugs and have a look, black/brown = normal (for MX5s usually more black)
VERY black and dry = too much fuel
black and wet = flooding
Unless you're using expensive iridium plugs or something dont bother putting the old ones back, just replace them.

Then just move down the list from cheap to expensive (plugs-fuel filter-fuel pump-etc)

Does now sound fuel related.
Is your tank really low? If so, it could be a combination of sediment/stale fuel causing the stalling.
Perhaps try putting in a jerry can of 98 RON (for once I'm recommending a higher octane!).

Or the immobilizer is shorting out the fuel pump?

Thanks for all the advice guys. I'll get stuck in this weekend and see what transpires. I'll post the result.

Also the number 1 biggest random asshole problem we ever get here is 15 year old immobilisers.

Dann

To be honest, i've never actually heard of a non factory immobiliser being any good.

They all tend to just be a constant headache to own, and as far as preventing theft, most take all of 5mins to google and get a wiring diagram to remove/bypass them.