Misfire when cruising

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: __NBFL 1.8 turbocharged
  2. I’m based near: __ Chelmsford
  3. I’m looking for technical help or recommendations on: __ misfire problem

Further to a previous post Ive made Im still having problems so thought I would create a more specific and detailed post in the hope that someone has any ideas.

I have a 1.8 VVTi with 35k on the clock running a turbo with an emerald ECU professionally tuned. Has run well for years.

It has had an occasional misfire that would be felt during steady cruising, but never under acceleration and always performed strongly. I couldn’t identify the source of the misfire and presumed ultimately it would evolve to the point that it might be easier to trace or Id flush it out on a rolling road…

This winter I upgraded my exhaust and manifold to a 3 inch set up up from a 2.5 inch and also renewed my cambelt and water pump and fitted a fluidamper uprated harmonic damper.

After this the car started, but takes a few goes, before it doesn’t stall. It holds a steady idle and sounds perfect. Out on the road it accelerates strongly and smoothly, no issues apparent. But when cruising the misfire seems more persistent.

I downloaded the Emerald software and plugged my laptop into the ECU, more as an experiment in case I had to take it anywhere as Im not competent to actually tune it myself, however it allowed me to view the AFR and I noticed that at a constant throttle say 2500rpm it will drop to 2000rpm every other second or so and the AFR shoots up to 20. It doesn’t do this at start up, only after a few minutes running. This is a pain because Ive given the engine bay lots of looking over, found a loose earth, and a plug that wasn’t fully home and thought that I had cured the problem only for the surging to start again. after a minute of running again.

Ive had my spark plugs (NGK BKR7s) out and found them all looking quite black and smelling of fuel. Normally the plugs look the perfect colour when Ive had them out or changed them. To my mind, the fact that all the plugs look the same rules out a dodgy plug, spark lead or Coil on plug.

Im sure that the cambelt is in the right place as I turned the engine a number of times to double check all the marks lined up and like I say it sounds sweet at idle and accelerates great.

I am wondering if I could have introduced a problem with the replacement harmonic damper. I run a 36-1 trigger wheel which I presumed because of the keyway could only go on on one way, but if this was wrong, would it cause these symptoms? Really Im hoping that this is a development of the earlier issue

If not does anyone have any other ideas?

Ive got a new crank position sensor on order as the existing one looks old but otherwise I m at a loss now.

Cheers

Ben

PS Emerald, my goto tuner are booked up for months and I was hoping to get this fixed before then.

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Any error codes?

Could be any one of the non standard parts.

Get back on the tuning dyno - anything else is a waste of £s

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Unfortunately I’ve no error codes to go on as it has an aftermarket ECU.

A dyno would be optimal, but not available for months and Id rather fix sooner than that.

I was hoping that given the countless MX5s in use, someone would have encountered similar symptoms and could point me in a certain direction.

Cheers

4

Your post has many inconsistencies in it…

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Allow me to expand on that then. I bought the car already turbo’d 7 years ago. It has been on rolling roads 3 times since Ive bought it in the course of upgrading aspects of its build. The last time it was on a rolling road was 21/2 years ago.

I think I first noticed the misfire 18 months ago whilst touring in switzerland.

For 5 years it would pass its MOT no problem. The last 2 were problematic with emissions - I think the cat has failed.

Ive changed the plugs 4 times according to my records and they have always previously been a good colour.

So it has seemingly run well for years, but I would say a problem has been developing which has now come to a head.

Hopefully this clears up the inconsistencies.

Cheers

6

Lambda Sensor??

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I had a wideband lambda put in when it was last on the rolling road. It’s probably the youngest electrical component on the whole car. That doesn’t mean its not faulty of course although Ive no idea how Id check it.

Cheers

8

You could try disconnecting it and see if it makes any difference. If not, it’s probably not working.
Also, it might be worth giving the connector a clean and wiggle.
My wife had a Peugeot that had a new lambda sensor fitted but the improvement didn’t last long.
On a recommendation, I gave the connector a clean and wiggle and hey presto…

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The lamba would have been disturbed during the exhaust swap I undertook recently,so maybe it took a knock. I believe they’re pretty fragile for something that lives just downstream of a turbo.

cheers

10

Disconnecting the lambda does stop the misfire so is likely the culprit and will be replaced.

Anyone else chasing similar issues be aware than the lambda has to operate at a high temperature and incorporates a heating element, running it for any more than a few minutes disconnected would probably kill the lambda.

Cheers

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