mx5cartalk.com

Howdy,
Well it seems I either have bad luck or a bad MX5 (2005 build NC). Latest problem is that an intermittent misfire has been traced back to the output from the ECU. To avoid paying the roughly $3k for a genuine replacement, does anyone know of any used ones for sale, either a wrecker or someone who has upgraded to an aftermarket unit? Alternatively are there aftermarket units available at reasonable prices and an easy substitute?
Cheers,
Stephen

Are you SURE its the ecu?

Im sure a wrecker would have one.

Dann

Give Danny at MX5 Plus a call, his details are at the bottom of this page.
Greg.

It seems a bit odd that the ECU would be the suspect for an intermittent misfire KK. Intermittent faults are generally caused by something breaking down under certain conditions i.e. leads, plugs, coils, connections, wiring etc. If the ECU has an internal fault, it would normally be permanent. Have the connections to the ECU been checked?

The ECU is a bit vulnerable to damage being in the engine bay under the air filter box. I guess there is always a remote possibility that it has some intermittent problem. MX-5 Plus is where I'd start with enquiries.

Have you checked the wiring at the coils? Very common on early NC's for the wires to break inside. Easiest way to check is gently wriggle the wiring next to the coil plugs and see if it drops a cyl. 3 & 4 are the most common but I have seen 2 go as well. If this is it then the wire will be broken inside the coloured cover. Won't be the black wire as it's a heavier gauge.

If it is the ecu they are not plug and play but coded to the car so will need to be set up by someone with the appropriate computer.

My brother is doing the work as I wouldnt know an ECU from the spare tyre. The misfire is on number 4. They swapped the coil between 2 and 4 and the fault stayed with 4. Will get him to check wiring as not sur eexactly what was swapped with coil.

Any error codes showing? Engine check light on?

At times engine light would flash, sometimes stay on. Error code had logged.

Apparenlty there is a 5v signal that comes from ECU to coil, traced back that it isnt exiting ECU.

Kuntry Kuzzin wrote:At times engine light would flash, sometimes stay on. Error code had logged.

Apparenlty there is a 5v signal that comes from ECU to coil, traced back that it isnt exiting ECU.

Is that an intermittent or continuous problem? Have all the wiring and connections on that line been checked and confirmed OK?

Seems odd that the ECU would fail one coil/plug output.

Lack of signal traced back through wiring to ECU and confirmed there is no signal coming from ECU. The fault was intermittent but is now permanent.

That's bad news KK. MX-5 Plus should have used NC ECUs but then there is the issue of "matching" it to the car. I wish you good luck and let us know what happens.

Well, lucky I went for a second and third oponion. All the historical evidence was pointing towards the harness.

I had a play around a couple of days ago but with my limited knowledge I didnt prove anything.

This evening I had a look with my smarter friend Cambo. We did lots of testing, swapping, etc but still couldnt prove anything wrong with the harness. We were beginning to convince ourselves it must be the ECU.

I suggested one last test of the connector and then bingo, as Cambo was plugging it in the wire broke clean off the back of it. So it was just as everyone was saying, it just took ages to find as when tested for continuity it was fine but wouldnt work when the engine was running.

Cambo pulled the connector apart, soldered in a few inches of wire and now it's running sweetly again. If the repair doesnt hold up or I have issues on other cylinders at least we have now confirmed to order a new harness and not an ECU (I would have punted on a new harness before fitting an ECU anyway).

Thanks everyone for comments and advice.

Now on to the next problem I noticed when working on it tonight, but thats another thread.

Great result KK!