Hello all, im a newbie to the forum but have had a few roadsters in the past. Have just bought this vehicle on the motorway returning home and after about 100 miles it developed a misfire. The vehicle drives no problem for about 5-6 miles (15-20 mins) then develops a severe misfire when it gets hot. The tacho needle bounces up and down from 2000 revs up to 5000 revs. It engine eventually stops running. Have changed the plugs and replaced the HT leads but still same problem. I am about to change the coil pack and was considering replacing the ECU temp sensor but does anyone have any other ideas. It is really frustrating as I bought this vehicle to cheer up my 8 year old son who is under the oncology department as he is roadster daft and you can imagine his disappointment at it being a non-runner. I really am desperate for any help or advice. Many thanks in advance.
Cheap fixes first:
Check plug gaps
Clean throttle body (use proper cleaner).
Then get yourself the diagnostics kit from MX5parts:
http://www.mx5parts.co.uk/product_info.php/products_id/35
You make up your own diagnostics reader by reading this:
http://www.miata.net/garage/Engine_Diagnostics.html
Or if you get the workshop book from MX5parts, there is enough info in their for someone to read the codes with a decent digital voltmeter.
That might point to an obvious problem.
Possibilities; bad water thermosensor (back of cylinder head with grean top, under £10 to replace), bad O2 sensor (£30), throttle position wiring (check earths), The coil pack is easy to swap, so if you can get a used one cheap, might be worth swapping, but they are expensive new. The diagnostics isnb’t perfect; a componant might be faulty but not throw a code. Similarly, the fault code may not be caused by the related sensor. On a 1.6, I had a fault code #9;water thermister. Easy I thought; replace the sensor. Code still remained. Eventually,the code was found to be caused by a faulty earth shared by the TPS and the thermister. Fixing the earth cleared the code and also fixed an odd 4000rpm stutter only apparent when warmed up (the TPS earth is behind the dashboard, rather than try and trace this earth, I just made a new earth from the TPS to the engine block).
Thank you very much for that quick reply Saz, the coil pack appears to be brand new on car but I do have a spare one. I’ve just found out that the The vehicle engine was jet washed just before I picked it up. Thats not going to help with water in the electrics!! I have ordered the diagnostic checker as you suggest and will check plug gaps too. Will replace temp sensor as a precaution as you say its a 10-20 quid fix. Fingers crossed, will keep you all posted.
Sounds like water in the plug wells, but you’ve already changed the plugs and leads. Try giving all the electrics a good squirt with WD-40, especially the coils and TPS.
Good Luck
Thanks Ralph, up to now have replaced coils plugs leads temp sensor o2 sensor , have cleaned TPS body checked earths removed plugs and cranked it. Good sparks at all the plugs too. ECU not showing any fault codes so im now stuck. Car runs perfect for about 15 mins from cold then it starts to misfire getting to the point where it stalls. Leave it a few hours and off it runs again!
Im at a loss now, could anyone recommend a good garage in the Newcastle area please. Many thanks
Hello again, well an update on this misfire. I checked the ECU today to see if it had water damaged, it was dry and clean. I unplugged the connectors to see if there was any damage and plugged it back in. Then ran the car for over 40 mins before the car started to misfire again. I checked the diagnostics and its thrown up a code 3 (G signal.) nothing else. Would the CAS cause the car to misfire when hot or am I looking at a defective ECU. Any help please.