Misfire on Mazda MX5 Mk 2 1800 Urgent help / advice needed!

[Hopalong64]

Hi
I picked up my 'new' Mk2 1.8 a few days ago, and am delighted with it, but have one problem with it that has 'taken the shine' off it a little.
The engine starts easily, ticks over smoothly, and pulls cleanly up until EXACTLY 4000 rpm.
At 4000rpm the car misfires badly and then pulls smoothly again from about 4200rpm.
From 4200rpm to about 55000rpm the car once again pulls smoothly. Above about 5500rpm, the car does not pull cleanly?

I have just come back from a main Mazda dealer, and they are unable to resolve the problem / dont know what to do next!

They plugged their computer in, but the car has registered zero fault codes!

Discovered that the misfire does not occur when just revving the engine on the ramp with no load.

They say they have never come across this problem....
I have changed the HT / plug leads , but I dont want to get into spiralling cost by changing, coil pack and possibly engine management computer or other components until I know what definately needs doing!

Please, please ..... somebody must have come across this before. ....

All ideas gratefully received - urgently!

Thanks

[juzzyp]

I was just about to suggest plug leads but they've been done, are they OEM or aftermarket, some aftermarket leads like to pop off the connections which causes horrendous problems.

Unfortunately the MK2 coil packs do tend to breakdown very quickly and cause real misfiring issues, have you done the spark plugs aswell.

The only other thing I can think of is the O2 sensor which (if it is bust) means your car is running pig-rich and that'll cause some running problems.

Have a search round:

www.mx5ocforum.co.uk

or

www.miata.net and look for the Garage Section or Forum

Hope you solve your problem, I'm a member of the MX5 Nutz Forum (www.mx5nutz.com) pop along and register and our resident gurus should be able to help you.

Edited by juzzyp on Wednesday 27th September 12:24

[andy g bmth]

Is it the sport version?

i bought the missus a mk2.5 1.8 sport which hesitates at around 4k similar as you described but not a misfire. i always thought it was to do with the induction as it feels strangled momentarily.

Incidently Mazda said there was nothing wrong at all with ours and prehaps i was just too used to the free revving Elise i drive.

If you find anything out please post

thanks

Andy

[cptsideways]

Above or at 4k they go into open loop/rich mode, sounds like a fuelling/ignition issue. Though it may be as simple as water in the plugholes.

[vixpy1]

If its misfireing as soon as it moves from the probes to the AFM, i'd check the voltages your afm is giving out, could well be fooked

[Hopalong64]

Just an update and to thank those that responded to my panic / need for advice.
The problem was traced to a faulty crankshaft angle sensor and for £159 including labour my car is now revving cleanly all the way to the red line. Let the fun in the bends commence!

Thanks to all that posted suggestions to assist!