I wonder if anyone can assist? I have just started my MX5 2.5 (2004) up after being parked up for the winter.
I have a misfire which hopefully won’t be much. But I went to plug in my bluetooth OBD reader & initially all the lights on the reader flash but there is no bluetooth connection with my iPad. I have tried another reader this one is a wired connection and that does not power up either.
I have checked the voltage at pins 16,4 & 5 of the port and I am getting 12.6v.
I have also tried disconnecting the battery and leaving it a few minutes and then reconnecting.
Still no joy.
Thanks in advance
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The diagnostics operate through the ecu.
It does seem that the OBD has power, so I would think the issue is ecu related.
Is the car parked outside with softtop?
Worth just removing with n/s sill protector, lifting the carpet, removing the ecu Cover plate, easier said than done, if its a virgin…and check for signs of water ingress.
With the cover plate removed and clear access to the ecu, any water ingress will be obvious.
There is a good chance that the misfire may be an ecu side issue too.
Thanks rhino666. The car has a hardtop and has had a cover on it. It has not suffered with moisture inside everything feels dry.
Is the ECU a pain to get too?
I started the car up and moved it so I could replace the brake fluid. It had no issues then, no engine management light or miss fire. After changing the fluid I took it for a spin up the road and after a few minutes I could feel the mis fire followed by the engine management light coming on.
They light was permanently on but is now flashing.
The cover plate is a pain to remove on these for the first time because of two shear security fixings. Basically they are hex nuts at the factory that snap off at an adequate tightened torque and leave a dome of metal, either side of the bottom of the plate, as their legacy.
The last ones I removed were smoking with SDS chisel application before they finally let go.
If there was an engine management light showing, there should definitely be an associated error code. Not at all sure what would cause a flashing engine management light on a MK2.5. it is definitely the engine management flashing and not ABS, handbrake or security light?
Flashing handbrake light would fit in well with low or marginally low brake fluid.
A flashing engine light means that catalyst damage may occur if the vehicle continues to be driven. Basically unburned fuel could be going through the exhaust.
There should be 4 wires going to the OBD plug (I’m not familiar with the pin numbers that you’ve quoted) but there shout be 12 volts on the Blue/Red wire protected by fuse BTN3 and ROOM. A Black and a Black/Blue wire going to ground and a Green/White wire that is the communication to the PCM.
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It’s definitely the engine management light.
I will have look tomorrow at the ECU fingers crossed.
Thanks I will let you know what I find.
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The ECU cover plate had never been off (maybe a good thing)
There was some rust under the polystyrene wedge and on the bottom of the cover plate. I could not see any water damage on the ECU connectors or the circuit board.
After putting the ECU back I thought I would look at the plugs/leads and found the number 4 very wet.
Cleaned all the plugs and dried the leads.
Started on the key with no misfire or engine management light.
Happy days I thought!
I tried me code reader and it made a connection to the car but can’t read the ECU, may be disturbing the plug connectors got the connection to the car?
I have ordered some plugs and leads and will replace them before giving it a run up the road.
If I have to replace the ECU, its the immobiliser and keys as well?
Is the immobiliser just above theECU and towards the centre of the car?
Thanks in advance
Ecu looks clean from that position. If the other side is similar and connector pins all present and shiny silver and gold you should be OK with the ECU. Only other upset would be a short where there would likely be an acrid smell and obvious flash marks on the case and damage to pcb components/tracks.
You will probably find that water has got to the very fine, exposed copper wire strands where they leave the plastic coated wire and hit the metal connectors in the three plugs. Usually when this happens the ecu pins gets discoloured. Anyway you’ve bought plugs so solder and heat shrink and the issue should be resolved. I sold a set of 1.6 connector plugs, two wires added to work on a 1.8 a couple of days ago, so not you.
That’s a 1.6 ecu. Worse comes to the worst I can clone to another similar ecu for you but generally these work with a bespoke immobiliser, under dash just to right of steering column and transponder chips in your keys.
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Thanks rhino666,
Sorry some confusion the plugs and leads were ignition as the number 4 was wet and the cap was looking very second hand.
The pins on the connector to the ECU looked ok too.
Is that the alarm unit that’s just above the ECU and to the centre?
I will give it a run with the new spark plus and leads and that’s OK run it over to a friend with a decent code reader.
Ha, ha, well confused; there are three ecu connector plugs but they are labelled 2, 3 and 4. Strangely enough, the big one, No.4 is usually worst effected by water ingress.
The MK2/MK2.5 1.6 coilpack is the weakest but given what you say, the misfire could well be caused by the damp in the high tension ports. Probably corroded too; if so I would replace the leads, if corroded and clean out the ports to shiny metal. Lemon juice left in to soak and rolled up newspaper will clear light corrosion.
That ECU looks really clean but it was worth just getting that plate off to check and I think you said there was small water ingress to legs/plate. You can use a thin disk to cut slots in the security fixings to re-use, if they were not too mangled on removal. Worth just having a glance at the ecu every once in a while.
The diagnostics is a mystery. There should be a single 12 volt pin receiver on the car OBD socket. If the reader is powering up but not connecting, that would imply issue with the connecting wire to the ECU. These are colour coded, so whilst everything exposed, just do a continuity test. It would be very unusual for a generally functional ecu to drop diagnostics. A car failing to provide diagnostics would likely be a non starter ie. catastrophically damaged ecu.
Quick update. Spark plugs and ignition leads arrived today.
Fitted them and then took the car for a long run. That’s cured this misfire.
I tried the BT OBD reader again and it connected but still can’t read the ECU. I tried a very basic Autel wired reader and that did connect and said no fault codes. I am going to try a friends Snap On reader hopefully at the weekend. 
The 1.6 coil packs from 89 to 05 have always been the strongest against the 1.8 packs from 93 to 05 , the main issue you find with leads and packs on these roadsters that they suffer from is furring/oxidation in the packs wells and on the connection for the leads on them all giving a barrier for a good solid connection ,but the 1.8 packs are the weakest for failing.
Which Bluetooth OBD II reader are you using ? with them not being that expensive, there are good and bad ones on the market that don’t last and do as you describe, flash and light up and connect but will not read, we have tried a good few makes to find which are reliable and last if your interested.
M-m
I tried the friends Snap On Triton reader and it connected and read the ECU.
My OBD reader obviously does not work with the MX5. It connects to my VW, I might have to invest in a better one!
Thank you for the help and advice.
Not entirely correct buddy.
1.6 coilpack on MK1/Eunos 1989 - 1998 is the strongest.
The MK2/MK2.5 1.6 coilpack 1998 - 2005 is the weakest, as noted in my earlier post, hence the response; perhaps you did not read it?
I do agree that the earlier 4 wire MK1 1.8 coilpack 1993-5 is a liability because when a unit lets go it often spikes and damages the ecu. That aside, it is still stronger than the MK2 1.6 unit.
Later three wire MK1/Eunos 1.8 coilpacks and similar MK2 1.8 coilpacks are reasonably reliable, as are the VVT coil units. These also tend to die peacefully:-)
None of them like to have the ignition left on for any period of time without the engine running.
No it is from 30 odd years of repairing , problem solving,and fixing these, and I am not talking about couple of cars here.you get to see traits in them all , the early 1.6 packs are the strongest with the separate ignighter, the 4 pin 1.8 is the weakest and can cause major issues , the 3 pin 1.8 which is the same on the late MK1 and MK2 1.8 is a lot lot stronger than the 4 pin and don’t fail as much as the rest and with less damage to any other units ,the MK2/2.5 1.6 coil packs are pretty good and don’t fail that often, the MK2.5 VVT is not to bad but they do fail and in very wierd ways which destroy chips in the ECU’s, for from “peacefully” in my experiences…
I wonder how I have gotten to this view of judgement, maybe by hunting down thousands of faults and putting them right with many other different units on these roadsters from a lot of time and years and see the traits.
I have a list of everything in the flip files in my head and what fails the most ,on which mark and in which ways…
Always willing to help and share that knowledge to help everyone that needs it.
I really do not understand why you seem to feel the need to put people down and feel like you need to defend yourself every time someone with a good background of knowledge joins in…
M-m
I have sold hundreds of coilpacks and dealt with many related problems specifically. It is what I do and know.
You said in your post that the 1.6 coilpack was the strongest 1989 - 2005. This went directly against what I had posted earlier, so has to be put right.
There are a lot of areas of MX5 knowledge where I accept you know more than I do but this is most certainly not one of them.
Never knowingly come across an ecu damaged by coilpack or rather coil unit failure on the VVT 1.8 engine. Please tell me what you know about this? I’ve seen hundreds of water and otherwise damaged MK2.5 ecus but never one taken out by a failed VVT coil unit. The only spike damage I know of on the MK2.5 ecu is EGR stepper motor failure; we’ve been here before, haven’t we?
So i am lying then?
I have no experience and make it all up…
I am not playing tennis with you , you have my number for a chat .
M-m
You started it, as usual chap!
I don’t think you are lying I just don’t think what you have written down is correct and you stepped on to this thread and contradicted me; that is all.
On the coilpacks side, Autolink ebay sales of new units which more or less killed my sales of used units as it goes. These Chinese parts were rubbish for quite a long time but they appear to have got their act together MK1, 1.8 4 pin coilpacks compared with MK2/mk2.5 1.6 coilpacks Check out the sales figures and pricing.
VVT coil unit damage to ecu; just give me an example of this as I have never seen or heard of it and plenty of coil units and ecus supplied.
Now EGR stepper motor damage. I have at least 5 ecus here with the characteristic damage and many more encountered. This is the reason why I sincerely recommend buying a new EGR unit. Have to admit I actually have a used MK2.5 EGR valve for sale; should probably smash it with a hammer but whilst others are selling them, lets me off the hook:-)
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" you started it" my god listen to yourself man…
As I said , you have my number…
M-m
Yes, mate, I do have your number 
