1993 Eunos Roadster - Smoke from the Diagnostics box

[kacper-k]

Hi There,

I'm hoping someone more knowledgable and experienced than me might potentially be able to help out or suggest what to do next, as I'm at a bit of a cross roads with my 1993 Eunos at the moment.

For context, this has been a project car for the past year and a half (Thread detailing that on the MX5OC forum here: https://forum.mx5oc.co.uk/t/my-1993-eunos-me-a-tale-of-foolishness-perseverance-dedication/144252). I had the car laid up over winter after getting it back from the bodyshop, and I noticed when I took it out earlier in March that it had a strange idle issue that would cause it to stall if I didn't hold it manually with the throttle. After holding it for 20-30 seconds, it would eventually settle (Albeit at a lower than expected idle still, normally about 900 RPM even at cold).

When I took it out for a drive in March, after about 30 minutes and a short stretch doing 70 on a motorway, the fault developed even further to the point where the car would not maintain a base idle at all when stationery or changing into neutral whilst coming to a stop or slowing down.

To cut a long story short, I decided to cut my losses, get the car back and try to sort it out from home. My suspicions based on the symptoms were either the ISC Valve or air valve that sits on the side of the intake manifold. I did a resistance check on the ISC which came back as 11.3 which was just within spec, however when plugging/unplugging the ISC with power on I couldn't hear the valve open and close, so I came to the conclusion it must have seized up, and so I ordered a used unit off ebay with the intention of swapping it out one evening and seeing where that got me.

In the interim I also ordered the diagnostic LED from Autolink thinking I could use that to double check my suspicions and see what the ECU was outputting, and whether or not it would confirm that it was the ISC or air valve.

Well I had a go at that earlier today, with the intention of swapping out the ISC after, but unfortunately what had happened is that I started up the car as usual, got it to idle and then plugged in the diagnostic cable. First I plugged in the shorting cable into GND & TEN, then the positive terminal of the LED into B+ and as I tried to push the negative terminal into FEN I could see a few sparks in the diagnostics box and a load of smoke came out and the engine cut out.

I immediately unplugged the cable (It was quite warm, so no doubt I managed to short something out), and attempted to re-start the engine but unfortunately it now doesn't start up at all. It does crank and turn over, and I can tell the starter motor is definitely working, but I'm not getting anything else at all, so presumably no spark.

I just wondered if anyone here might be able to suggest what I can do next ? I don't know whether it might be something as simple as a fuse (Fingers crossed) or whether I've completely fried the ECU. Does anyone know how I could diagnose this further, or maybe what fuses I could try swapping out to see if I can get the engine to start at least ?

Any advice at all would be appreciated !

[Mazda Mender]

You could of been lucky and not burnt the ECU , have you checked your fuses?as for the dodgy idle , have you tried a newer ICV,,it sounds like it is that playing up, is the timing correct?.
M-m

[Mazda Mender]

?
M-m

[kacper-k]

Hi Wayne,

I ended up going down a bit of a rabbit hole with this car and completely losing track of things, apologies for not responding to your comment earlier.

I got lucky and it was just a burnt out fuse and fuel relay which I managed to replace with a few spares sourced online. In the end I replaced the throttle body, ISC and air valve, and also re-calibrated the TPS sensor whilst I was at it, just to ensure everything was in order.

Well long story short, the car ran fine initially and the base idle issues appear to have been sorted, but I've now encountered a separate problem with the car appearing to run rich. It's very intermittent, sometimes happening after a 20 minute drive, and last week after I depressed the throttle inside the garage and got the engine speed up to 3,500 RPM. Essentially what is happening is that the car loses it's base idle (Will stall unless I catch it with the throttle), and once it does manage to find an idle the revs bounce around an extra 100-200 RPM.

I replaced both the front and rear coolant temperature sensors (Thermosensors ?) last week thinking that was faulty and giving the ECU a bad reading, but unfortunately it made no difference whatsoever, so I've recorded a short video showcasing what it's doing. If you turn the volume up you'll be able to hear how rough the exhaust sounds.

https://drive.google.com/file/d/1QJwApi6HNitItG4Ey0oPrOp1w7fEEI-z/view

(For context, when the engine speed jumps up to around 1,500 that's me doing that with the throttle to show it at different speeds)

Below is an image of a big sooty splatter I noticed directly behind the exhaust after running it, which is what makes me thing it's runnig rich, as there was unburnt fuel there too.



At the moment I'm thinking the next culprit may be either the Oxygen sensor or the AFM, so I'm planning on running the car with those unplugged (separately) to see whether that affects it all.

If you have any thoughts or suggestions at all about where to look or what to test, please do let me know.

Thanks !

[Mazda Mender]

The running rich could be down to the o2 sensor,but you also mention that she is not running correct, check the coil pack pots for oxidation, on the HT leads and plugs, is the ICV ok? and the IPS
is the throttle body ok ,is it sticking?, is the air trunking ok, no splits?
M-m

[kacper-k]

I'll check the coil pack and HT leads and see whether there's any clues in there.

The ICV seems fine, I checked the resistance on it before fitting it and it gave a good reading within spec (11.8). I re-calibrated the IPS manually with a multimeter after replacing the throttle body and it all looks good to me - no sticking points or such.

The trunking/cross over pipe all looks in good condition too, but I can give it another look whilst checking the HT leads.

I'm purely speculating at the moment, but what I think is going on is that the ECU is increasing the amount of fuel going into the combusion chamber due to a bad input, but because there's no additional air coming in, unburnt fuel is being pushed out into the exhaust and the engine stalls (Unless I open the throttle manually).

[kacper-k]

I just thought I'd update the post to let anyone that is experiencing similar problems know that in the end I managd to resolve this by replacing the AFM.

Turns out that the flap inside my original AFM was getting stuck and not closing, therefore giving the ECU a bad reading (Thinking there is more air going in than there actually is) and causing the car to run rich.

All is as it should be with a replacement AFM and a normally functioning flap.