I've got a 65.000 mile MX-5 I've owned for 5 years. Previously (say 10,000 miles ago) I had a intermittant failure of 1 of the 2 coils when it would run on 2 cylinders from time to time. Replacement of that single coil solved it, but I also changed the HT leads and plugs prior to the coil.
I've never had any other engine problem with this car until recently. Symptoms have varied a bit, but there is a sort of pattern. Generally it will fire and start, but it will either stop within a few revs, or idle and hesitate every few seconds, and then stop, or idle OK for longer, hesitating occasionally, and then stop, or it might run perfectly. If it decides to run, I can drive it and have driven it for 10-20 miles without a problem until I try to start it again. It does not seem to be any different when hot or cold, except that it seems less problematic starting if it had just been running OK. So it seems a bit different from the other threads here.
When it does stop this way, it makes an unusual sound rather like a rattling relay or mechanical valve which seems to be coming from the front of the inlet manifold area. When it stops by switching it off normally (by switching off with the ignition key), this sound does not occur.
I've asked lots of local garages for help and suggestions. Generally I was advised to look for electrical connector problems. I have removed, dried, re-connected and sealed every engine-related connector I can find in the engine bay apart from those at the bottom of the coils (lazy me, they are difficult to access) but I didn't believe they could be the problem as it either runs or doesn't run, never running on 2 cylinders. Sometimes after doing one I've tried it and it started OK and I thought I'd solved it. Sometimes I could stop it and restart it OK, but on leaving it a few minutes the problem has come back, so clearly it was a red herring. This has happened several times so I no longer think any of these was problem.
I have a fuel pressure guage fitted to the injector gallery and know that my fuel pressure is OK.
I have removed the ECU from the footwell and found it dry and not corroded in any way.
In desperation I took it (it started and drove on that occasion) to a local garage earlier this week, which has a range of engine diagnostic computers as they do rally tuning. They failed to find any fault (apart from seeing it perform its new tricks). They had the computers on its diagnostic port and diagnostic oscilloscope on various sensors and reckon they saw normal waveforms. They believed it is the ECU but were unable to test it. For 4 hours of failed diagnostic work I got charged £230. Ouch!
My local Mazda dealer offered little hope here, saying they could not test the ECU, and replacement for a test is difficult as the ECU is linked to the immobiliser and they had an MX-5 that had been in there for 7 weeks with an ECU problem that they couldn't solve! They suggest leaving it until the car fails to start at all, thus making it easier to find (they hope). So I've avoided taking it to them at least at the moment.
The previous owner fitted a Cobra Thatcham 1 immobiliser which might be the problem I suppose, although I doubt it. Circumstantial evidence here is that the passenger door central locking has not been working for about a year and it is a signal problem because I've tested the door solenoid and it works fine. However since it went to the garage I've noticed that the central locking causes this door to lock, but won't open it still. The driver door locking works fine. They claimed not to have touched the immobiliser, so maybe that's another red herring.
As a final piece of information, I fitted a Jackson Racing supercharger to it soon after I bought it which has been faultless (and extremely good fun!) and is believed not to be part of the current problem.
From other threads I see that replacing a "coil pack" might be a sensible move in view of comments in one thread about one coil affecting the other one. Given that I replaced one coil as mentioned above, what does this mean? Both of the coils situated right at the back of the engine next to the firewall, or something else?
Usually, when it is being troublesome which is maybe 95% of the times I try to start it, if I open the throttle with it idling, it will stop immediately, although sometimes it picks up a few 000 revs whilst coughing, barking and backfiring badly. When it stops on its own from idling, there is sometimes a slight backfire.
Before I go out to do some random chequebook engineering, can anyone please help with these symptoms?
Many thanks.
Edited by Pugugly on 27/11/2009 at 08:54
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I have experienced ECU issues before with MX5's that have had superchargers fitted. I trust you are using the specific plugs recommended when a charger is fitted. Have you noticed whether a coil gets hot if you leave the ign on, this is one indication the ECU is failing.
I suspect when a charger is fitted & the engine is under load the coils try to spike the ECU.
Fantastic fun with a charger on these except in the wet!
I would cast a serious eye on the Thatcham first. One of my customers flew to California to collect an ECU & it was about $100 he said it was so cheap in the states it was worth the fight.
The garages are quite correct that they really do need to have it in the non running state to trace the fault.
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Just a suggestion, but have you checked for fuel contamination, i.e. water specifically.
Also had it been stood for a while prior to theses issues, maybe a drain and refill with fresh fuel might help.
Edited by kith on 27/11/2009 at 11:46
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Thanks for your replies, I really appreciate them.
I didn't notice the coils getting hot, but will check in the morning when I try it again.
Fuel contamination isn't something I'd thought of. The car isn't used that much, but its previous journey before the problems started was a 100 mile return trip. Having said that I filled it up a few miles from home, but at a large Tesco where the turnover is high, so I think wet fuel is unlikely, but I'll sample it and see. Since I use 98+ octane, I guess they sell relatively little of that so it might be old fuel.
I don't know anything about how the add-on security works, other than to achieve Thatcham 1 it is patched into the car's electronics in several places to make it difficult to defeat. Do you think this might include eg the signal to the coils which if intermittant I think could give the sort of symptoms I'm experiencing. A problem then is that is the firm that fitted it is no longer trading, although I believe Cobra have a number of agents around the country who might help.
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"Fantastic fun with a charger on these except in the wet!"
Absolutely, it transformed the car. Up to last year I had Dunlop F1 tyres and despite the power they were fantastic, they gripped like mad and tamed the rear end completely. Previously with some unknown tyres on the tail was very lively and I had it off the road once - 180 degrees in less time than I noticed. Alas these tyres are no longer made and I had to change to something else (forgotten what) and I have lost a lot of confidence in the handling, although it's still reasonable.
I paid £8k for the car, £2k for the charger, and found I could loose 3 litre BMWs with ease on the track! That's value.
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Well, I think the problem is solved.
First I de-activated the immobiliser. Cobra gave me some phone help with this, and whilst this has de-activated the central locking as well, I can live with that for now. However the car didn't run properly after doing that, so I suspect it wasn't the problem.
I decided not to sample the fuel. I'm sure it wasn't contaminated fuel as you would expect that to be worse after standing as any water would settle to the low points. It is presently running OK - it started and ran OK this morning after standing in my drive for 2 days.
What did I do? Well I remembered that the garage had said that they found no signal from the throttle position sensor once, but then they did get a signal reliably so presumed it was OK and they had made a mistake. This was when it was running better than it had been afterwards when I posted the message here. So on Monday I removed it, but found it is sealed so I couldn't get inside it. So I very thoroughly and carefully cleaned the contacts and put it back. The result is it has started and run perfectly each time I've tried it since. I'll need a bit more experience to be 100% convinced it was the problem, but I've got fingers crossed.
Really frustrating problem! I have sympathy with people trouble shooting modern cars. Thanks for your help, all.
Stuart
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