MKii engine problem

[genericguy]

I have already posted on Nutz about this and had some useful advice but getting desperate and wondering if anyone here has experienced this.
My car is only running on 3 cyclinders, the coil pack seems fine and all plugs and HT leads have been replaced, if you disonnect the lead from the plug nearest the windscreen it makes no difference to the sound of the engine, so I presume it is doing nothing.
I have recently had a replacement cam cover gasket fitted, the old one was leaking, but I have seen some black/dark liquid on the plugs I changed. May have been left over from before?
I have also had the cat replaced recently.

The car was making this noise previously but seemed to improve when cam cover gasket was fitted, wondering if this could be leaking.
What else can I check?
Thanks in advance for any help.

[muckymotor]

A leaking cam cover gasket won't cause a misfire. Are you getting a spark at the plug that isn't firing? Is fuel getting to it? It should be wet and stink of unburnt fuel if fuel is getting through ok.

[genericguy]

muckymotor said:

A leaking cam cover gasket won't cause a misfire. Are you getting a spark at the plug that isn't firing? Is fuel getting to it? It should be wet and stink of unburnt fuel if fuel is getting through ok.

Definitely getting a spark.

Sadly my sense of smell is awful, can't tell if it's oil or fuel.

[MX-5 Lazza]

Put a match to it

[trackerjack]

Sorry but my diagnosis is that you have a burnt or chipped valve prob exhaust.

[MX-5 Lazza]

trackerjack said:

Sorry but my diagnosis is that you have a burnt or chipped valve prob exhaust.

A compression test would confirm that.

[snotrag]

As your getting a spark, that means its going be either fuel, or as above maybe you've got zero compression.

If you trust yourself to be careful, you can check your injectors by pulling the fuel rail and using something to catch the flow from each one, make sure theres no spark present or anything hot, and get someone to quickly turn it over - no squirt of fuel = your problem.

If there all working ok, then compression test time.

If you have a nice local garage to you I'd pop down and ask if they can try it with their compression tester - it doesn't take long and its an easy car to do it on as access is good.

[genericguy]

Swapped a spark plug today as I was right by somewhere that sold them and I thought I'd see if it made any difference (worth £2!)
The old plug looked like this:


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I *think* it is fuel from sniffing it, but can't be 100%, power has been down and changing the plug definitely improved this, although it still sounds awful.

The thing that really confuses me is that it was like this recently, then it was OK, now it is back, the noise and performance drop definitely went away, would this have happened if it was compression/valves?

Thanks for all your advice so far.

[genericguy]

I've been on holiday so update on this.
Took the car into Maztec in Slough today (very impressed with the service so far), and have eliminated coil pack, leads, plugs, and compression as the cause.

The issue is that one plug is only sparking weakly, it does spark and changing the lead/plug/coilpack doesn't improve it.
I'm taking the car back for further investigation Monday, if anybody has any ideas from the extra info I'd be very glad to hear them.

[MX-5 Lazza]

There are 2 coils in the coil pack and the plugs fire in pairs. 1+4 and 2+3 fire together. If one spark is weak but the other matching plug has a good spark then it can only be a problem between the coil & plug.

I guess it might be possible that the matching lead/plug is faulty and has particularly low resistance so is robbing the other plug of enough power to make a good spark - I don't know if that actually happens, just idle speculation.

[genericguy]

MX-5 Lazza said:

There are 2 coils in the coil pack and the plugs fire in pairs. 1+4 and 2+3 fire together. If one spark is weak but the other matching plug has a good spark then it can only be a problem between the coil & plug.

I guess it might be possible that the matching lead/plug is faulty and has particularly low resistance so is robbing the other plug of enough power to make a good spark - I don't know if that actually happens, just idle speculation.

The thing is he completely replaced one of the coils with a known working one, and it made no difference at all.
It really is a mystery.
he thought one of the four pins opn the crank pulley (?) was slightly shorterr than the others and may be the cause.

[MX-5 Lazza]

The crank pulley will have no bearing on the spark from a single plug. If there is a problem with the crank pulley then all cylinders will be having a problem.

[rdodger]

You may have checked this already, but I had a similar issue a while ago after doing some work on the car.

It turned out to be injector loom plug not being in properly. It is one of the plugs on the front of the engine clipped to the block just below the cam cover.

Worth a look.