Misfire is usually ignition-related (as you plainly know since your first step was to change the plugs and leads). How sure are you that the fault is on cylinders 3 and 4 specifically? It’s curious, since 3 shares an ignition coil with 2, and 4 shares with 1.
It’s hard to think of anything that’s common to 3 and 4 but not to 1 and 2.
How severe is the misfire, and does it happen all the time or under particular conditions (temperature, revs, load)?
Absolutely clutching at straws now, but if you had poor coolant circulation, cylinders toward the back of the engine would get the hottest. (Though that’s hardly a problem which would show itself in this weather. [:)])
The misfire is very bad when cold and persists to a slightly less noticeable degree when warm and reving it over about 2500 revs. (although it’s still there)
Both plugs from cylinders 3 & 4 are black with carbon so they do seem to be firing and burning rich.
Is it possible that the timing could be slightly out and not effecting cylinders 1 & 2 due to the double spark set up?
Also I had read somewhere that there was a problem with some models wearing the valves and resulting in them not seating properly. But there was no apparent problem during the compression test.
the only thing that seems to be common now is the ECU unit
This may sound obvious and i’m sure you’ve checked, but, could the plug leads have got mixed up when you replaced them? Just a thought, hope you get it sorted!
Surely if all ignition has been checked correcly and you would assume it has. And the engine has no compression problems then the only bit left must be fuel related. (Dam I can’t start new paragraphs again ) this could easily be an air leak, or something similar causing the problem . My knowledge of the MX5 specifically is not that great yet as I have only had mine 3 months ( and touch wood has not gone wrong yet ) but . Misfires are usually fairly straight forward to cure . i.e. ignition , compression or fuel. Over the years I have seen many caused by air leaks so may well be worth checkng out.
Yeah, I realise how easy it can be to transpose the leads but I triple checked that and it’s been into a very competent local garage that i use from time to time so i’m sure he would have picked up on anything so simple. After checking everything over, he could only advise that I went to a garage with Mazda diagnostis.
My train of thought was exactly the same as yours Mick D, thinking that maybe a simple air leak could be making the mixture rich but I’ve just heard from the garage that claim that they are “Mazda Diagnostic specialists”. They tell me that they have spent a full day on it and checked things including all sensors but for some reason their diagnostics have a “coms” problem with my car. They want to bring in an auto electrician (at my cost) on top of their days investigations. I would take it to Mazda at this point and bite the bullet on the cost but I can’t locate one nearby who say they do any repairs other than bodywork. The only other Mazda garages around here (South Manchester) only seem to want to sell cars. At this point I have no faith that a mobile auto electrician will solve what two garages up to now can’t.
I’m no technician but to me it can only be fuel/air mixture problem or a badly timed spark. But why only 2 cylinders (3 & 4, the 2 farthest back from the front of the car)?
If it is an air leak you can check for it yourself , you just need to be carefull. We use to use ether to find air air leaks , just spray it in around areas like inlet manifold ,servo pipes e.t.c, and if there is a leak the engine draws in the ether and the engin revs will rise. the only thing is you need to take care as ether is quite volatile ( easy start is ether).-----------------------------------(sorry no line break again)--------------------------------------------------------------
Regarding the leads why not just swap them anyway to be sure , the worst that will happen is it will misfire ( no change there then ).
I changed the Leads right at the beginning of the problem along with the plugs. If the garage gives up, which I think they will, when i get the car back I will try and get some easy start and test the air leak the way you suggest but I cant imagine that the professionals haven’t already tested for leaks. I’m starting to feel like I’m going to be stuck with a car that’s never going to run again. If garage professionals with all the best equipment can’t solve the problem, what chance do I have?
This guy is in Congleton and is pretty familiar with '5s. I know one or two people have had less than satisfactory experiences with him, but I believe that they are very isolated incidents
I was rather assuming it wouldn’t be the ECU which was your problem, but when you mentioned the garage’s diagnostics equipment had “comm problems” talking to your car, it made me think of this thread on miata.net : http://forum.miata.net/vb/showthread.php?t=300474
The guy had plainly had several inces of water in his footwell at some stage. If you lift the carpet in the passenger footwell, any similar problem should have left obvious traces.
The first garage gave up on the car in the second week of december - he did a load of investigative work on the car but didn’t charge me a penny.
Then i took it to a garage that advertised themselves as diagnostic experts. They had the car for two weeks to no avail. They called in an auto electrician at my cost and he drew a blank so I found my nearest Mazda dealer and put it in there on the 23rd december. They drew a blank again on the diagnostics so i had to authorise further investigative work at £85 an hour. Although I asked them to check the ECU first, because everything else had been checked and double checked, they insisted on starting from the begining to see for themselves. It became apparent to them fairly quickly that this was no run of the mill problem and they told me that they would not charge me further until they have found the cause of the problem because they were as baffled as everybody else. They were fairly sure that it was the ECU but they couldn’t confirm this until a model of the same year was brought in and they could temporarily exchange the ECU and immobiliser. So, after 8 weeks in mazda they tried my units in another car and the other car behaved in exactly the same way mine had. It was confirmed that it was a faulty ECU. Then I got the quote to change the ECU, Immobiliser and the Ignition lock and barrel - £2100 plus VAT!!! The car is probably only worth about £3500.
I paid them what I owed them and limped the car back home. After 3 weeks searching for the required parts from scrap yards I found one I thought I could trust not to send me a pile of junk. I ordered the parts for £205 - fitted them last Monday and have driven about 250 miles since then and it hasn’t missed a beat.
I have gone from my lowest moment cursing Mazda and vowing never to buy another car off them to now loving every minute I’m in the car.
Total costs=£450 and 3 months off the road - joy of having my little black baby back=priceless
Sounds like a long and difficult path but at least you are now enjoying the fruits of perserverence…enjoy it while its running well, you kept it going and hopefully it will give you some good service back…my recipe to a bad experience with the car is usually to drop the hood, give it a blast and decide all is well after blowing the cobwebs out.
