Annoying misfire

[drew74]

Hi All, sorry for the length of this post, but I figure the more you know the better you may be able to help here!

I've had an annoying misfire in my 91 NA6 ever since the waterpump and cambelt were replaced back in January.

The car was overheating on a drive back from Akl airport to West Akl and we discovered the waterpump had blown. I had the waterpump and the cambelt done by Autoway Services in Glendene, but when I picked it up there was still a lot of water pouring out of it. They also said it had a mis-fire and they thought it could be the leads. Being 5pm I took it home (5 min trip) and left it for the morning.

Looking under the hood in the morning, I found the leads (factory) had all been melted due to the overheating and after checking the possibilities for the water, we found the thermostat had gotten stuck in the closed position. (amazing the mechanic never found this!)

So after a new thermostat, plugs and leads the car is running a lot better, but the misfire still occurs about once or twice a day, usually when its under load (or perhaps when I should change down sooner!).

I've put a bottle of injector cleaner through it, and changed to 95 or 98 octane fuel, which has shown a huge improvement (particularly the 98 octane from Mobil, the misfire is gone!)

I'm about to change the air filter and I've read that the air flow meter may need cleaning - how do you clean it?? I figure it's a good idea to do this at the same time as the air filter?

After that, I'm thinking fuel filter?? And checking the plug gaps? Am I right that the standard gap is 1mm?

So if anyone can advise on cleaning the AFM I would be very appreciative, also any other suggestions on how to get rid of the misfire!

[Trevor]

It would appear that things became really hot and you may find that you have done in the head gasket and even possibly warped the head. This happened to me when the heater hose at the back of head, blew half way over the harbour bridge.

Is there any evidence of oil in the cooling fluid, water in the engine oil, or bubbles of air visible with the radiator cap removed? Even if not, as I found this is not conclusive. First thing is to do a compression test.

Don't like the attention you received from the Glendene outfit.

[SLYDIT]

have you had your timing checked??
Chances are that they may have one of the cams out a tooth. probably the one with the cam sensor on it. either that or the crank wheel crooked or something.

[drew74]

Thanks guys,

It's due for an oil & filter change very shortly, so I will get the mechanic to do a compression test and check the timing for me. Thankfully, no sign of oil in the coolant, water in the engine or air bubbles in the radiator. As you say, it's not conclusive, but I'm taking it as a positive sign all the same!

Wont be going back to the guy in Glendene for this service though, I'll be heading to my regular mechanic this time! He's been great with every car I've taken to him, so I'm happy to make the journey over the bridge to see him.

Just going back to the AFM though, is this a task I could do myself anyway? I've seen a few comments on here about it and I figure it makes sense to do it at the same time. I've read its just a case of spraying some sort of cleaner into it, which I'm more than capable of, but I've also read stories of people dis-assembling them for cleaning, putting them back together and then the car wont start!

[Stevo]

yep, gotta agree with Slydit on this one. Definately sounds like timing....
Cheers Stevo

[WideOpenThrottle]

I,ve cleaned our NB AFM with CRC Mass air flow cleaner from Repco.
With the unit removed you can access the sensor to spray & leave to evaporate.I also did the same to the pod filter as it was greasy from using a filter oil on the pod which apparantly is a no! no! as the oil particles get blown across the AFM sensor & causes incorrect oxygen readings.
I also cleaned the throttle body valve as it was gummed up as well.
All the above cured our fast erratic idle problem.We did not have any misfire like yours but it may help to check,clean & eliminate
You may have water in the plug housings or a lead thats on its way out or perhaps a spark plug?

[sprsta]

i dont think the afm will be the issue
id say its all in the timing
i have a perfectly good afm sitting in the shed at home if you want it
but i doubt itll make any difference

however if you do take your afm apart (i would'nt sugest) do not take the top screws out and remove the black plastic square this is why the engine will not start

[drew74]

Thanks for all the info guys!

I have the car booked in for a regular service and they're going to check the timing and compression while it's in.

I've only had the car since January and I don't know how well it was maintained prior to us taking it over, so I'm progressively working through all the tips I've read on here such as the air filter, fuel filter, etc as well.

The leads were 2nd hand from Mazspare in Rosebank (post-Xmas cashflow issue!), so as someone mentioned, it could be one of those is worn too.

Will update on progress as time and funds allow!

[SLYDIT]

oh and one thing to check also are the grounding points from the block to the chassis on the passengers side and also the one under the wiper motor that bolts to the chassis with a whole pile of wires on it. bad grounds can have you scratching your head some times.

[drew74]

Just a bit of an update on this one. My retired engineer father was nosing under the bonnet the other day (he's damned handy when you're a novice like me!) and found oil in the plug holes and around the seam of the rocker cover... suggesting the gasket was perished and in need of replacement.

We did some research on here and on miata.net and found the instructions to do this, and when we took the cover off to see what state the gasket was in, we found it was brand new, but the guy who did the waterpump and cambelt had done an absolute shocker of a job, not cleaning off the old gasket remains, etc and putting the rocker cover back on, there was old sealant sticking to everything!

So about and hour and a new tube of hi-temp sealant stuff later, we had everything cleaned up and the rocker cover back on.

The best part of this... On the morning drive from Glendene to the CDB, not a single misfire!!! I'm tentatively optimistic and we'll see how it goes for a week or two.

[Growler]

Hey Drew,

I know a good mechanic in Glendene who can help you out.
I'm in the same area an have a set of leads you can try if that helps.
Send me a pm if you want his details.

[drew74]

Thanks for that Growler.

I put a brand new set of leads in not so long ago and was still getting the misfire until yesterday when we took the rocker cover off and re-did the mechanic's work on the gasket. Only one run since then, so I'm keeping my fingers crossed for now.

Always good to get referrals for a good mechanic in the area though, if you're happy to share,

[Growler]

Thanks for that Growler.

I put a brand new set of leads in not so long ago and was still getting the misfire until yesterday when we took the rocker cover off and re-did the mechanic's work on the gasket. Only one run since then, so I'm keeping my fingers crossed for now.

Always good to get referrals for a good mechanic in the area though, if you're happy to share,

I do all the maintenance/modifying etc myself.
The company is McLeod Road Motors situated at 190 McLeod Road, Te Atatu.
His name is Graeme and works by himself. I take all my cars there when I don't have time (Except the MX5 - I'm too fussy to let anybody else touch her!)

[Stevo]

Its always good to get feedback after the problem has been fixed so we can find out what the issue really is. cheers stevo