NA 1.6 Idle Issues & Diagnostics Box Shorted Out

Technical Area Engine, Transmission, Exhaust etc
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  1. My model of MX-5 is: 1993 NA 1.6
  2. I’m based near: North West
  3. I’m looking for technical help or recommendations on: Electrical diagnosis & troubleshooting

Hello All,

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 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 and then some smoke came out of the diagnositcs box, with the engine cutting 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 !

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I think that the first thing to establish is are you sure that you had the jumper wire/LED in the correct terminals in the DIAGNOSTIC connector, and if not, which terminals did you accidentally plug into?

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Hi Robbie,

I had the shorting cable in GND & TEN. Im convinced I had the LED positive in B+ and the negative in FEN, but maybe I made a mistake and put into MEN or had it the other way around.

Initially I thought that maybe the mistake I made was trying to insert the LED into the diagnostics box with the car running, and maybe I should have inserted it before starting the engine ?

I had a look inside the fuse box last night and could instantly see that the Head 30A fuse was blown, and I couldn’t get any continuity on the Fuel Inj and Cooling Fan relays, so I’m going to get those replaced and see if that will get me back to square one at least.

I’m curious though, speaking from experience have you ever had any issues with running diagnostics on these cars, similarly to what I managed to do which caused a short circuit ?

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Symptoms suggest you shorted B+ to GND somehow - that can blow some fuses and stop your car starting. Check and replace any fuses, don’t forget the 30A fuse (may or may not be labelled “air bag”)and/or main relay.

Personally, I think the autolink diagnostic cables are poorly colour coded. GND to TEN is a red cable? GND on red is asking for trouble. B+ is red which makes sense but the only black cable is the FEN connection? Why?

Seems unlikely to me that a few seconds of shorting would brick the ecu, but I’m basing that off general electronics knowledge rather than mx5 or car specific elec. knowledge.

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I replaced the blown 30A fuse and the main fuel relay today with spares I managed to source during the past week, and the car started up again so it looks like all is fine.

I was going to go ahead and replace the ISC to see if that would resolve the idle issues, but as I inspected the throttle body after removing it I realised that the adjustement screw wasn’t actually moving the butteryfly valve so I’ve ordered a replacement throttle body too, and will replace both that and the ISC next weekend.

2 Likes
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@Roadster_Robbie , just wondered if I could pick your brains a little ?

I replaced the throttle body, ISC & air valve last weekend. Started the car up and all seemed well, with it now idling at around 1500/1600 RPM from cold and gradually coming down to around 800/900 when warm. I took it for a short drive and all seemed well.

Today I took it for a slightly longer “test” drive to see how it would run, and initially it was fine but after about 45 minutes of urban driving (Covering around 9 miles) and getting up to proper operating temperature (I could feel heat coming off the exhaust tunnel which is expected when everything is warmed up and there’s limited airflow) I suddently started experiencing the same idle issues as a few weeks ago.

The car would get a very rough idle bouncing around between 600-800 RPM and completely stall when coming to a stop in neutral. I’m starting to run out of ideas as to what is causing all this.

Having had replaced all of the parts I mentioned earlier, all I can think of is maybe the AFM or fuel pump ? I tested the battery a few weeks ago and it read 12.2V which is still within spec but on the lower side, so maybe that’s one potential fault.

Have you got any ideas or suggestions as to where to look or what to do ? Thank you.

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Has the engine coolant temperature sensor been changed?

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No it hasn’t, so I’ll make that the next priority. I swapped out all the hoses and the thermostat with genuine OEM parts, but never considered the sensors at the time,

Would you recommend replacing both the outlet temperature sensor (Located at the back of the engine) and the sensor sat in the thermostat housing at the front of the engine at the same time ?

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Really hot? Look at the back of the engine, where one finds coil-pack and CAS.

Both can suffer from high temperature issues, my guess because of thermal expansion eventually cracking a wire inside which reconnects when cold.

Good luck, and well done for your persistence.

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I know you have changed the ISCV, but go back to basics, have you reset the base idle yet?
If it needed it before you blew the fuse, it will certainly need it after changing the Idle valve

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I wouldn’t say it was “really” hot, more just up to temperature coupled up with a hot day and less airflow from being sat in traffic. I don’t think it’s overheating, but I’ve ordered a replacement coolant temperature sensor (One that sits behind the block) and will get it replaced next week.

I’m pretty hopeful this will sort it out, as all the symptoms point to it.

I also set the base idle last week after replacing the throttle body and I’m sure it’s within spec.

1 Like
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Most of the car electrical problems I’ve come across have resulted from lead-free solder crystallising and failing, sometimes aggravated by poor mechanical assembly. This is one reason of several why so many modern connectors use crimp or IDC instead of solder.

Here is an illustration of it found in my favourite tiny palm-sized mouse. It developed an intermittent left button, the one most used. I fixed it less than an hour ago.

20250411_205854_c
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This pic shows the grey crystallised solder on one pin of the mouse button switch, probably because it was not sitting completely snug to the board when made. After a decade or more it gave up on that pin’s solder. It still worked with a harder press.
After re-soldering with old fashioned tin-lead it’s nice and shiny and has the light touch again. It will out-live me.

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Have you done the switch test with the diagnostic box? Clutch and neutral switches assuming its a manual.

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I hate the lead free stuff, never looks “right”
I have a big reel of the proper stuff.

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So I fitted the new coolant temperature sensors tooday, start it up to go for a test drive and all seemed fine initially.

The car was idling at around 1,800 RPM which is higher than usual, but I expected it to come down as it warmed up. After about 1-2 minutes of just sitting idling, I gave the throttle a few blips to see how the idle would behave, and all of a sudden it started “sputtering” (Best way to describe it) again.

I’ve recorded some videos and uploaded them to Google drive as they can’t be uploaded here.

The first one shows how it stalls after raising the revs: 1.mov - Google Drive

The second one shows how it “splutters” and struggles to maintain an idle unless I hold it on the throttle: 2.MOV - Google Drive

I also noticed this on the wall behind it afterwards:

IMG_1375
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A huge sooty splatter. Making me think it might be running rich, which I thought was the issue but had hoped swapping the sensor over would solve.

Have you got any ideas as to what else I could try @Roadster_Robbie ? I’m not sure where to go next really. AFM maybe ? I’m thinking it’s definitely something with the fuel/air mixture. Would @Mazda_mender know maybe ?

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I wonder whether the O2 sensor might be the next possible culprit maybe ?

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In an earlier post in this thread you mention changing the throttle body because the idle screw doesn’t move the throttle butterfly, it doesn’t work like that, it varies the airflow through a bleed into the Venturi.
The garage wall mess points to a rich mixture, could you borrow another ECU just in case something was damaged during the diagnostic box shorting? It will eliminate the ECU if the problem persists.

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Thanks @russbif, I wasn’t aware of that. I still think the throttle body was suspect as the butterfly valve didn’t seem to seal too well in the closed position.

I’ll see if I can follow that lead and investigate further.

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Which engine coolant sensor did you replace? You needed to change the one at the back of the engine.

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If there’s anyone here in and around the Greater Manchester area with a MK1, I would greatly appreciate it if I could borrow a spare ECU and/or AFM to run a test on my car :folded_hands: