mx5cartalk.com

spark plug leads would be a good place to start maybe

When you say "...put your foot down and it's back to 3 cylinders again....", do you mean it is misfiring and gutless or just gutless?

Ignition timing is out - perhaps not enough advance at idle.......cam could be out by one tooth too - sounds like timing for sure. Not fuel otherwise it would do it all the time.....

Make sure to bridge the TEN and GND terminals in the diagnostic box before checking/adjusting timing or idle speed.

You really need a timing light with a clip on the sensor wire that just clips around the plug lead rather than having to plug it in the line to the spark plug.

You must do the GND-TEN thing or the ECU will be trying to control the timing/idle speed etc. while you're trying to adjust it. Bridging the GND-TEN terminals takes the ECU out of the picture.

MX5s are very simple. Once you can confirm that you have 10deg at idle and that the CAS is not either moving or failing, your coils show the correct impedance and that your plugs are correct and correctly gapped and your leads are in good nick and not cross firing - then start looking at things like fuel filters, injectors and pumps. The timing can be confirmed in about 20 minutes then you know it is OK and it MUST be something else. In my experience, most poor running as you describe is ignition, not fuel related.

JBT wrote:You really need a timing light with a clip on the sensor wire that just clips around the plug lead rather than having to plug it in the line to the spark plug.

You must do the GND-TEN thing or the ECU will be trying to control the timing/idle speed etc. while you're trying to adjust it. Bridging the GND-TEN terminals takes the ECU out of the picture.

Also, if you are looking for a 12v power source for a timing light, such as the one mentioned by JBT. There is a small blue plug just behind the passenger side headlight. Remove the cover and connect the + lead to that.

J

Supercheap, Autobarn etc. should have them.

1) Any good auto shop will sell the "inductive" timing light

2) in addition to what the others have stated, before doing anything more tricky, I would simply take out the spark plugs and check their condition. It may be just fouled plugs.
Just replace them even if you don't see fouling. Cheap and simple.

3) check the Cam Angle Sensor at the back of the engine. It's the palm-sized round thing beside the coil pack. Sometimes the retaining bolt can come loose and cause it to move around.

Glad to read that you have fixed the issue. Most of us here have experienced the dodgy plugs or leads issue at one time or another. You didn't really need to go to the extra expense of the NGK-Iridium plugs, but they will last a long time (100k Km?). The leads will probably need replacing before the plugs.

J