In need of help diagnosing a mis-fire

[howard.i.know]

Hi,

Firstly I apologise for what will probably be an extremely long post but I need some advice please about my mx5 so I`ll try to give as much info as possible.

It has a misfire above 4000rpm, starting it is fine and anything below 4000rpm is fine, runs like a dream actually. But once its warmed up and you put your foot down a misfire appears around 6000rpm and if you keep on doing high rev pulls, the misfire gets lower and lower in the rev range but never below 4000. The rev counter drops when the misfire occurs and it doesn`t seem to do it when cold but I don`t like revving it that hard whilst cold so I`ve only tried it twice.

Its a mk2 (NB) 1.8 , I've had it about 3 years, brought as an MOT failure with only a couple of days left on the ticket so never really drove it before I started to restore it (in hind sight a big mistake). Not quite sure if this was an existing problem with the car or something I may have caused.

Here's a list of the things I`d done to it before I noticed the misfire:

New timing belt and idler/tensioner pulleys.
New camshaft and crankshaft seals plus cam cover gasket.
New water pump inc flushing the system.
Coolant reroute which included removing the EGR valve because I couldn't reroute the pipe from the exhaust manifold to the EGR.
New coolant pipes & new 50mm rad.
Spark plugs & HT leads.
Full service (all oils and filters).
Drained the fuel tank completely and replaced with fresh fuel.

It was at this point I noticed the misfire, so since then I have:

Rechecked the timing on the belt.
Replaced the fuel filter again (so I could cut open the previous new one to see if any muck had made its way out off the tank, All clear)
Reconditioned the injectors (same reason as above)
Pressure test on the fuel system (32psi rising to 40psi)
New Coil packs.
Replaced all the vacuum hoses.
New crank position sensor.
New cam angle sensor.
New mass air flow sensor.
New intake air temp sensor
Removed and cleaned all earth points including replacing the earth strap between the engine and chassis as it was non-existent.

The only fault codes that come up on the diagnostic are EGR valve related.

So today I finally got to use my compression tester, here are the results:

Cyl 1 - Cyl 2 - Cyl 3 - Cyl 4

50 ----- 35 ----- 35 ----- 50 First revolution
165 --- 150 ---- 160 ---- 185 Max psi
225 --- 185 ---- 210 ---- 235 Wet

So, Finally getting to the point, is it feasible that this misfire could be caused by what seems like a loss of pressure in cylinder no.2? My thoughts were that maybe the head gasket is leaking pressure once the engine heats up between cylinders 1-2 and 2-3 hence why 1 & 3 are lower then 4 and 2 is lower still as its got 2 means of escape even on the wet test.

Any advice would be greatly received and thank you for reading till the end.

Ben.

[Mazda Mender]

Everything i would of told you to check and fault find with you have done and covered...it is looking like lower compression on 2 and 3... from your report.
M-m

[blackyb]

I am no expert but I don't think the lower compression on 2 and 3 would cause a misfire as you describe, scrathing my head at the moment.

As Mm said, all that you can do to eliminate things you have done, I have heard of dodgy plugs causing a misfire at the top end!!

[pbarwell1]

Hi Buddy I would follow the wiring back from each coil pack and check for visual breaks in the wiring, should be three wires to each coil pack. I have had the same issue turned out to be a kinked wire which had gone brittle, split and corroded causing an intermittent 02 sensor circuit dtc.

Or check your earthing straps.

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[Guest]

personally I don't think your compression readings are that low , more than likely its an ignition fault misfire

also check for resistance readings on the 02 sensors or any cracks or holes in the exhaust pre sensor as that would cause allsorts of running issues and random cylinder misire

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[howard.i.know]

Thanks for the reply's, definitely worth trying other options first before taking the head off. I'll check the above mentioned items this week.
Thanks again.

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