I had a similar problem about 2 months ago, the car never seemed right since I had it, and a few months ago got progressively (but quite quickly) worse. Really hesitating in 1/2/3rd...ok once you'd got some speed up... I changed the Splitfire Ht leads for some 8.5mm Magnecor leads... The problem dissapeared completely, and the car felt smoother and had better pickup than it had since I bought the car over 12 months ago. The magnecors are excellently made, one piece, the right lead size for each plug, with excellent packaging and instructions, I cant praise them highly enough.

I would say change your leads to either the magnecors (www.magnecor.co.uk) costs @62 quid 2 day delivery, or others on the list recommend NGK leads available from Moss motors about 40 quid. I think you'll find this is the problem, it certainly was in my case and a lot of my friends 5's and in any case is a good point to start at. (1/00)

I drove from Reading to Brentwood in Essex (with the top down on the M25!). Went a fairly spirited pace but when I came off and stopped at the first set of lights, the car stalled and wouldn't start. Eventually started but seemed to be mis-firing considerably Got my dad to come and tow me to his house and I called out the RAC.

Very helpful guy came round but, as is the way, the car behaved fine. He checked the plugs and leads and pronounced them all ok. His opinion was that one of the sensors to the ECU was not working meaning that when the car has been running hot and reasonably hard for a while and then changes to a a slower pace, one of the sensors tells the engine ........something! He then recomended taking the car out again to get it warm so I spun it down a few junctions and it was fine.

Part 2 - went to go home and got all the way around the M25. Was doing about 90 most of the way until I came to join the M4. Slowed down a bit and the car started doing exactly the same i.e. very lumpy, stop starting, felt like it would stall if the revs dropped at all. I pulled to the hard shoulder so as to leave it for a while and see if things improved again but after 15 mins when I started again, exactly the same problem after a few miles. Repeated the exercise and the result was again the same.

Called out the RAC again. He fiddled about for ages without doing anything before telling me that he thought that it could be anything. This time the car just wouldn't start and when it eventually would, it sounded like a tractor from the exhaust and the exhaust fumes were stinking and dirty.

Tried to pin him down to something more specific and his thoughts were:
Leads and plugs (checked and ok)
ECU Turbo problems (no more specific - can't think of what he could mean)
Running too rich

This last one seems reasonable as I'm only getting about 180 miles to a tank. Is this about right?

Is my best bet to get it to a Mazda garage and put it on diagnostic equipment? Does anyone know whether the '92 MK1 ECU has a fault recording system so that they can see what's happened? RAC 1 said no, RAC 2 said yes! (2/00)

Bear in mind I'm also a newcomer to this and the brief problem I had was not quite as you describe but similar (ie fine at higher revs, v.rough mid range) - and noting that we're looking for an intermittent or temperature related fault - there could be other causes but I'd go down the following path:

1. Have you ever put new plug leads on? From what I've read it is always worth doing and you won't be wasting money to spend #50 on good leads. Ditto new plugs.
2. Check all the intake hoses for loose/leaky connnections.
3. Disconnect the oxygen sensor (copper probe that goes into ex manifold) - trace the wire back to the plug at top rear of engine - disconnect the plug. Then try it - if consistently better disconnected you probably need a new sensor - see www.realbig.com/miata or www.miata.net for more info.
4. If still no better I'd try (I've not done this but only read it) disconnecting the battery to avoid damage and simply open up the plate in the passenger footwell then unplug and replug the connectors in case there is a poor connection. The advice is DO NOT wipe the connectors but you can spray switch cleaner or WD40 on them before reconnecting.
5. Go to dealer. BTW - if the ECU or a sensor is playing up consistently you should get an engine warning light on the dashboard - then the diagnostics are invaluable. (2/00)

If you have some plug leads handy then give these a go. Also check your plugs. It really does sound like a spark shortage! Check for a fault code using an LED+resistor or get a Mazda garage to do this. You open the diagnostic connector, connect TEN to GND and then monitor the voltage between +B and FEN with the ignition turned on but the engine not started. The ECU flashes code (hence an LED is needed) using long and short flashes.

These two really are the obvious first steps. There are lots of other things to test if these two don't fix/diagnose the problem.

The turbo does complicate matters, as I don't know much about it, and I doubt many others do, but Mazda did OK the conversion so should be able to fix the car! (2/00)

y far the most likely thing is the plug leads. "Checking them" doesn't work - they will appear OK to multimeters, etc. The problem is that they get damp, the insulation breaks down (but only under high voltage) and they conduct straight into the head.

So, has your engine been washed recently - by you or the dealer/valet? Probably got the plug leads wet - fatal. First thing is to get new plug leads - I strongly recommend Magnecor 8.5mm (the expensive red ones from Moss, or direct - see www.magnecor.com). Others swear by the NGK (less expensive blue ones from Moss). Don't buy cheapo ones, or pay for even more expensive trick ones. The magnecores are about £50.

It's worth changing the plugs - use NGK BKR5E-11 I think for a turbo.

It is most unlikely to be the ECU. It could be a sensor (but frankly unlikely) There is a diagnostic check system - you can interrogate it yourself, but it's too involved to go in to here. Full details are in my manual (Use and Interrogation of the Diagnostic Connector by John Cookson - £6 from Donutz - see www.donutz.com .

Can't imagine what turbo problems could do this. You are risking your cat. driving with unburnt fuel in the exhaust - due to plugs not firing properly, because your plug leads have broken down...... Just my 2d long range diagnosis. Hope this helps.

BTW - there is no engine warning light on UK spec and Roadsters - only on Miatas. There is a cat warning light on Roadsters, but it only indicates cat overheating, and by then it's too late (2/00)

The problem that I'm experiencing is missing at maximum boost (15psi on an FM2). Investigation has shown that there is no missing at 12 psi or when I close the spark plug gap from 1.1 mm to 0.7 mm. Apparently, with the new software that I'm now running with the ECU, I should be able to run stock plug gaps (e.g. 1.1 mm), but evidently can't. (Just as an aside the reason for the missing at high boost is that higher boost increases the resistance of the fuel/air mixture and makes the voltage required to jump the gap greater).

My problem is that I'd like to run 15 psi AND have a stock plug gap (bigger spark = better burn and more power), So after checking the grounds and contacts on the stock igniters and coils, which I've yet to do (anyone know where the igniter or its associated ground on a 1.6L is?)

I have a couple of choices as I see it,

1. Replace the stock igniters and coils and hope for an improvement in supplied voltage and hence spark.

2. Go to an after market kit. There are 2 that I know of: The HKS Twin Power Ignition Amplifier, see http://www.hksusa.com/html/ignitionsystems.htm this kit is £420, and the SplitFire Direct Ignition System see www.rspeed.net this kit is $559 before shipping, VAT and import duty.

So does anyone know anything on the two after market kits above before I spend hard earned cash? Or have ANY other suggestions on how I could go about increasing the supplied voltage to the spark plug.

FWIW I've just installed new Magnacore R-100 wires and NGK ZFR7F-11's to eliminate these from the equation. (5/00)

You cannot run 1.1mm gap .. the stock gap is 0.8 to 1.0mm gap .. and I misfired at 1.05mm at 14.5 psi. Running 0.7 to 0.8mm gap will not significantly reduce power .. you try and sense that lost power .. in extremes it will make the car harder to start .. but that is down at 0.35mm gap (wild guess) .. just be happy .. the driving instructor I was with the day from Knockhill races a Cossy .. with 420hp .. he needs different spark plugs to start the car .. and then changes over once the engine is warm!

How much data logging have you done on this.. when you get knock it pulls the timing and the engine goes very 'flat' ..another possible.

Check your spark plug voltage with a gap type tester .. cheap and nasty from Halfords and I think I was about 22,000 V .. compared to 18,000 V on my old Lancia with retro electronic ignition. This tool will tell you high much you can get out of your system .. but then remember the dwell goes up to 300 degrees after 6 psi of boost .. you could simulate this boost by driving the MAP sensor with a foot pump/tyre/compressor to 10 psi or so and cranking the engine to record the voltage.

Basically you need the volts to punch through the dense mixture .. and a reduced gap also helps.

Ignitor was an aluminium electric box on an inner wing on a 1.6 with the coils separate behind the engine (all from replies to you off miatapower..)

I have thought of aftermarket coils .. not sure how much the leads could take before the higher volts becomes a problem ... and as plenty of others are running more boost than you with the OE coils I would suspect something else.. but it is your money!! .. the interesting one to me was someone on about aftermarket motorbike coils .. these are normally wasted spark too so could be a cheaper route .. HKS is pricey but not always good. (5/00)