Idle speed issue NA 1.6

[Purfler]

I’d be grateful for advice before I do anything invasive.

The car: NA 1.6 LE. Unmodified engine 235KM

Problem:

Intermittent failure to idle when coming to a stop. On one occasion, in the garage, it would crank but not start. (I have been unable to replicate this).

What I’ve done:

Testing of the idle speed control valve (ISC) as set out here: https://www.miata.net/garage/isc.html, but not yet removed either the ISC or the air valve (AV). Using a jumper between TEN and GND causes the idle to drop dramatically, but not quite stall. I have used a little Subaru upper engine cleaner into the ISC, but no great improvement.

Questions:

1. Should I remove and blank off the AV as suggested by Miata.net to eliminate that as a possibility?
2. Should I put more cleaner into the ISC, or
3. Remove and clean the ISC, or
4. Cut to the chase and replace the ISC?

Many thanks for any advice, thoughts or experiences.

Tim

[bruce]

Spark plug leads.

[Purfler]

Thanks Bruce. I just tested them with a meter, all in spec.

Regards,

Tim

[bruce]

It's a common issue, especially if the original leads (or a lot of age on them). They would test in spec even if faulty as you say it is an intermittent issue.

[Purfler]

Thanks again. They are only a couple of years old, but I'll try putting the old ones back (they were working fine, the change was preventative - which might turn out to be a bit ironic)

Regards,

Tim

[93_Clubman]

Tim, have look at the following:
http://mx5cartalk.com/forum/viewtopic.php?t=60434
viewtopic.php?f=18&t=73272
Btw, Gladiator on here had a very persistent problem with his old NA6, which took a while to fix after trying many, many things. Think it was idle related & ultimately tracked to something intake manifold & immediate surrounds related.

[slomo]

If you have idle droop, the engine is warm and runs well but the idle drops (near stall or occasionally stalls) when coming to a stop say at traffic lights then check the clutch activation switch at the clutch pedal. There is a bluish coloured plug that the white pin of the switch comes in contact with.
This plug is brittle and can break and fall out. The clutch switch is there to send a signal to the ecu that the clutch is being disengaged and for the engine to run in idle mode. Common problem, had it with both my cars.

[Purfler]

Sorry for the delay in replying and thanking. Have OS house guests and have had very little time to get to the car.

Haven't replaced the leads yet, new set on its way. In the meantime slomo's description fits my problem, many thanks. This is what it looks like:

Inserted link isn't working in preview, this is the link: https://photos.app.goo.gl/ze5PPzcRuuMYN2yS8


I will put in a plug and see how it goes.

Not that it matters, but any thoughts on why it is designed this way?

Regards and thanks again to all.

Tim

[93_Clubman]

Tim, below are pics just taken of the clutch switch in my 1993 NA8. From your pic, yours appears to be missing the white seat base or collar:

clutch pedal not depressed.jpg

clutch pedal depressed.jpg

More detail here:
http://mx5cartalk.com/forum/viewtopic.php?t=71780
&
viewtopic.php?f=18&t=73272

[project.r.racing]

If you are jumping under the dash to replace the rubber stopper on the clutch. I'd suggest doing the same for the brake pedal.

[Purfler]

Many thanks again for the replies. I have not taken "after" photo but I inserted a plastic plug - like one used to secure trim. It fills the hole but does not have the thickness of yours. Is that an original part?

I just test drove the car and the idle problem has not recurred. I have now done three things that might have solved the issue, singly or together:

1. inserted plug in clutch switch
2. new plug leads
3. used small amount of cleaner in the idle control valve.

The clutch switch was definitely faulty, but who knows for how long. The old leads tested OK, but if it was an intermittent fault I'd probably never isolate it until they completely failed.

Hopefully the issue is resolved now, some days of driving will confirm.

Thanks again for the advice.

Regards,

Tim

PS- Now my back has recovered I'll have a look at the brake pedal.

[Purfler]

PPS - just took the driver's side floormat out to give it a clean and found the crumbled remains of the original plug. I see how thick the striker plate was and mine is much thinner. Does not seem to have effected anything even though it must change the timing of the signal change to the ecu.

I cannot help but wonder why the plastic striker plate. Would have been easier and cheaper to have just used the folded steel that it fits in.

Tim

[93_Clubman]

It fills the hole but does not have the thickness of yours. Is that an original part?

Yeah, it's the original part. A white plastic replacement tap washer looks like it would also do the job, be a bit thicker, & appear almost original.

[project.r.racing]

When the one on my brake pedal failed. I found a piece of ply in the same width and glue it on. Lasted 4 years. 50 laps of lakeside. Numerous motorkhanas.

[slomo]

The clutch switch is on/off for signal to ecu.
Not using the bluish plug may alter the timing of the signal change slightly.
If using trim plug instead of OEM, you may have to adjust the switch position, but only if the problem of idle droop reappears.
Also glueing a small disc of say hard rubber, the same thickness as the OEM plug onto the trim plug will do.