MX5 Misfire

[Dakkon]

Recently aquired an '99 MX5 1.8 Sport, was running fine, however whilst follwing an M3 along the A41 yesterday and accelerating hard in forth it has suddenly developed an occasional misfire. Low revs in a high gear and the engine is very unhappy, keep the revs high and its ok, it also has an erratic idle now

Changed the plugs and have new HT leads on order, just wondered if there was anything else I should check?

Thanks

[thegreatsoprendo]

I'd say it's almost certainly your HT leads. This is a common issue as 5's are quite hard on leads.

I'd try not to drive it if you can avoid it before fitting the new ones, as unburnt fuel getting to your Cat could easily fry it!

[cptsideways]

Washed your engine bay recently?

Water in the plug wells, or as mentioned the plug leads.

[Dakkon]

I have read in many places that MX5's eat HT leads due to their placement above the head and the heat gets to them. I suspect they are still the origionals fitted to the car. Local Halfrauds and Mazda dealer have none in stock, but should have new ones tomorrow

Don't have to worry about driving it, car is only weekend fun, I enjoy the wonders of the unreliable, uncomfortable, overly hot & humid tube for work

Only wondered if I should be looking elsewhere as it was all fine until I was accelerating hard in forth up a long hill. I have put in a load of Optimax as did wonder if it might be poor fuel? b ut that has made no change

No water in plug wells either

>> Edited by Dakkon on Monday 26th July 14:14

[benjc]

I vote for HT leads too.

[KITT]

thegreatsoprendo said:

I'd try not to drive it if you can avoid it before fitting the new ones, as unburnt fuel getting to your Cat could easily fry it!

Defiantly! A mate's MX5 suffered this and then started firing on 3 cylinders. He phoned a local garage and asked if it was ok to drive the car to them ~10 miles to fix it (he's not clued up on engines). Well, this ruined the cat because of the unburnt fuel If only he'd asked us before driving it!

[thegreatsoprendo]

Dakkon said:

Only wondered if I should be looking elsewhere as it was all fine until I was accelerating hard in forth up a long hill. I have put in a load of Optimax as did wonder if it might be poor fuel? b ut that has made no change

I wouldn't worry anymore about it until you've changed the leads. I'm 95% sure that'll cure it, but if not, come back and we'll think of some more suggestions. If you start looking into other possible causes now it'll most likely just be wasted effort!

[Dakkon]

Alas, as I suspected new plugs and leads have made no difference, as soon as the engine warms up the idle drops a bit and in creeps a misfire....sigh

Any other idea's before it goes to Mazda?

Thanks,

[DanBoy]

Rotor arm and dizzy. Try that.

[KITT]

DanBoy said:

Rotor arm and dizzy. Try that.

Seconded. Defineatly sounds like an electrical problem to me.

[BenjC]

DanBoy said:

Rotor arm and dizzy. Try that.

More likely to be the coil pack Fuel filter?

>> Edited by BenjC on Tuesday 27th July 22:21

[Dakkon]

I would be the first the admit my lack of mechanical knowledge, but I don't think MX5's use rotor arms

Someone else has suggested the coil packs too

[PJLarge]

Not wanting so state the obvious, but have you checked the plugs?

No rotor arm on an MX-5 so don't spend too much time looking for it. It's electronic ignition.

Phil.

[Dakkon]

Changed plugs and leads, still not fixed it

[KITT]

PJLarge said:

No rotor arm on an MX-5 so don't spend too much time looking for it. It's electronic ignition.

Very true! Don't know much about coil packs but presumeably they send the sparks to the plugs in the correct order so could be a potental culprit of the fault. What happens when you rev the engine? Does it misfire then or only at idle? Could be the throttle pot sensor?

[Dakkon]

Misfire is at low revs, if you rev it hard the misfire goes

[PJLarge]

Low revs - are we talking up to 4000 rpm, which is low revs for an MX-5 or lower than that even?

Is it a stumble or a flat spot? What did you gap the plugs at? I run at 0.035", but I'm running 15 psi of boost. Fuel filter could well be the other culprit, they should be changed every 40k miles maximum. Fortunately they're only a £20 part.

Phil.

[DanBoy]

Dakkon said:

Misfire is at low revs, if you rev it hard the misfire goes

Still sounds electrical to me.

I had a similar thing once where if I ragged the nuts off the car, the misfire would disppear.

At low revs it would misfire and then stall seemingly whenever it felt like it.

Getting it home was a real battle sometimes!

Replaced various electrical btis and it was sorted.

>> Edited by DanBoy on Wednesday 28th July 13:15

[Dakkon]

Well compression tested it this afternoon, 10-8-8-10, so looks like there is a leak between cylinders 2 & 3....sigh...sob

[PJLarge]

How many miles on this engine? - Compression test or leakdown? What scale are your measurements in, I'm more used to PSI...

Phil.