Intermittent Misfire when warm after 30 - 120 minute rest.

[pearsos]

symptoms : After driving and then parking so when warm or hot.
Park from 30 minutes - 1 1/2 hours, when you attempt to start the car it sometimes missfires for about 5 miles. Smells of unburned fuel.
Have replaced plugs and leads - what next - Coil pack ???

[tuttle]

Could be plugs & leads.
Unburnt fuel smell? fuel line fault? Could be green temp sensor or O2 sensor starting to crap out. Have you checked with a diagnostic fault reader?

Instructions for fault codes here

Edited by tuttle on Friday 12th March 16:00

[pearsos]

Thanks for the codes - I dont have one of those diags widgets - where can find one ? Have changed leads and plugs - no improvement. what typically happens is the
- car drives fine all day long until parked.
- I leave it parked up for a half hour or so.
- When starting the vehicle it will sometimes but not always miss like crazy.
- The car will drive but very poorly.
- After about 6 miles the missfire goes away and the car drives normally again.
- alterntively if you leave it for a few hours, it starts and drives normally.

Also I think I may have fried the cat as when the missfire first happened, I drove it and car smelled of unburned fuel, then there was a strange carckling noise from the cat !!

[bluetone]

pearsos said:

Thanks for the codes - I dont have one of those diags widgets - where can find one ?

There are instructions on the above link on how to make one, if you can be arsed, alternatively .

[neil_bolton]

Green Temp Sensor; under and behind the coil pack, on the back of the engine by the bulkhead.

£30 from MX5Parts, get it done, with new plugs, and the car will be as smooth as a nut.

When doing it, the third (hidden) bolt is a pain in the arse, leave it off when remounting the coilpack - makes any future jobs pain free.

Edited by neil_bolton on Monday 15th March 14:07

[youngsyr]

I'm guessing Pearso is long gone, but I have the exact same problem on my 1999 10 AE and was wondering if he ever got his fixed?

So far, I've replaced:

- coil pack
- plugs
- HT leads

Still no improvement.

I'll try the "green temp sensor" and report back, but to be clear, is this the engine coolant temperature sensor? Mine seems to work perfectly well?

[MX-5 Lazza]

There are 2 engine temp sensors, one used by the ecu (green one on back of block) and another for the gauge. At least that was the way on Mk1...

[ukianj]

Had the same problem for 2 years, asked all the same questions, changed all the same parts. In the end it was fixed last month - one of the plastic fuel injector caps was broken so there was an imperfect connection some of the time - when hot or at other impossible to predict moments. Check your fuel injector caps and the connections underneath.

[youngsyr]

MX-5 Lazza said:

There are 2 engine temp sensors, one used by the ecu (green one on back of block) and another for the gauge. At least that was the way on Mk1...

Interesting, thanks. MX5 parts only lists one temp sensor for the mk 2, and twoe for the Mk1, the rear one does have a green shroud.

The one for the Mk2/2.5 is blue.

Edited by youngsyr on Saturday 20th April 00:19

[youngsyr]

ukianj said:

Had the same problem for 2 years, asked all the same questions, changed all the same parts. In the end it was fixed last month - one of the plastic fuel injector caps was broken so there was an imperfect connection some of the time - when hot or at other impossible to predict moments. Check your fuel injector caps and the connections underneath.

Thanks, but I lose two cylinders at the same time, both on the same coil pack - unlikely to be a fueling issue I'd have thought?

[MX-5 Lazza]

That sounds like a simple coil pack issue. I know you've already changed it but do you have access to another know-good one you can try?

[youngsyr]

MX-5 Lazza said:

That sounds like a simple coil pack issue. I know you've already changed it but do you have access to another know-good one you can try?

I've tried swapping coil packs and then replacing the one with the dropped cylinders.

Neither has cured it.

I also found a way to "reset" the ECU yesterday online that someone had recommended to cure the issue (disconnect battery, foot on brake for 10 seconds, leave disconnected for 20 mins, reconnect). That didn't work either.

I understand that the coil pack that feeds cylinders 2 and 3 gets its power from a "bridged" connection from the coil pack that feeds 1 and 4, i.e. the 1 and 4 coil pack gets the power first, then tranfers it to 2 and 3.

So, I had the loom replaced and still no joy.

It's a very odd problem and no-one has been able to pin point the cause.

I hear there's an "igniter" somewhere in the system too, so this may be the cause, but I know nothing about what it does or how it does it.

I hope something simple like the temp sensor change works, but I can't see how - the temp gauge works fine, so unless the ECU is ultra sensitive (or has a dodgy connection between the sensor and the ECU, I can't see it being that simple.

I've also ordered a diagnostic indicator kit, so maybe the OBDI system can give me a clue.

It's a real pain because I want to supercharge the car, have had the kit for ages, but shouldn't really do it until I get this problem sorted.

[MX-5 Lazza]

I'd still try another coil pack & set of leads. If one coil is firing correctly that suggests all the sensors are working fine or at least enough for that one coil to be firing.

[youngsyr]

MX-5 Lazza said:

I'd still try another coil pack & set of leads. If one coil is firing correctly that suggests all the sensors are working fine or at least enough for that one coil to be firing.

Good point.

It all suggests to me that there's a temperature sensitive connection somewhere that's playing up.

It's weird, because sometimes you can fix the problem by giving the plug that joins the engine loom to the coil pack a sharp push on the tab at the back of it (as if you were going to release it).

It doesn't always work, but sometimes it does.

I've had that plug replaced though, so that should rule out the problem being in the plug itself.

[MX-5 Lazza]

So it could be a connection within the coil pack itself...

[youngsyr]

MX-5 Lazza said:

So it could be a connection within the coil pack itself...

Yes, I think that's most likely, but I had the end of the engine loom that connects to the coil pack replaced, so that should have sorted it.

Also, it's only one coil pack that has the problem, the other is fine.