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Greeting humans!

I've got a '90 NA6 with another motor in it (big nose na6) as put in by the previous owner.

I started chasing an idle dip - the idle will drop off to 400ish RPM and recover a second later when doing things like turning the parkers off, or even revving it up just above idle, then dropping your foot off the accellerator.

I've checked the ECU/non-ECU idle, no difference in idle with or without the jumper (while hot, no fans, etc) - tested a couple of times because I wasn't sure I did it right the first time with the GND -> 10 bridge wire.

The person that sold me the car advised the extractors were the factory ones (They didn't look right, but at the time I'd had about an hour worth of MX5 experience) - but there is no heat shield, and the (i assume) O2 sensor is near #4.

(See attached photo)

I've seen posts stating that idle dips can be cause by the O2 Sensor, and that there's an NGK one you can lop the plug off and crimp to the OEM plug, and "bob's yer uncle"

What confused me is the fact that the current O2 Sensor appears to only have one wire... Curiosity got the better of me, and I decided to follow the lonely O2 wire, and see if OM (Other Mechanic) had done some splicing on the loom to perform some majik or ... I don't know. But at the end of the lead is a plug, which it zip-tied to the back of the metal tubing for the AC gas. Standing back and looking makes it look like it goes somewhere. Investigating reveals the truth: it's not plugged anywhere.

So, I get decent fuel economy (8.5L/100k) mostly hooning around town - but I've only had the car for 4000km - the worst economy I got was 9.1 with the airbox on the car (it came with, and has again, a pod filter)

So, the actual question is this: if there is no O2 sensor on the car, would it still run OK? I have the idle dip mentioned above, and it will hesitate a bit (drop RPM slightly) and then pick up when stabbing the go pedal.

... I think that covers everything for the moment, sorry for rambling - I've got no internet of my own at the moment, so I've loaded up with everything that may or maynot be relevant

Welcome to the forum!

Fuel economy and daily driving should be better when you've got a good O2 sensor and it's plugged in. That won't necessarily fix the idle dip problem though. It's a common problem on NAs ,with lots of different causes. Google for "Idle dip" on miata.net or look here on viewtopic.php?f=76&t=60434

Thanks Magna!

I have done some google-fu which is how I got to the O2 sensor. I'll look in the manual and see where it's supposed to plug in, and if there's something on the loom end that's plugged in unexpectedly....

What should I be seeing for "better" fuel figures?

I think the o2 sensor plug (car side) is up under/near the coil packs on the NA6.
I;m going off my GF's memory here, she has an NA6.


On the NB it's front and centre, big black plug between the cam bumps on the rocker cover.


if it;s unplugged there's a chance it may be not working, often people will unplug them when they are acting up (the car then goes to a default map)

I don;t think the NA6 ones are particularly expensive, so just budget to replace it (plug it in though and see, they may have just forgotten to plug it back)

I replaced the one in my NB (4 wire) with a Bosch one. No issues at all, I just needed to splice in the wires (box included a weatherproof thingy to splice the wires with, and instructions showing wire colours of most models)

NA6 only have 1 wire O2 sensors, and they really dont do that much haha.
The oem plug for it should sit up behind the coil packs.

They are not factory extractors, and are only reading one cylinder's gas, whereas the stock reads all 4 (once they merge together).
Shouldnt really be a problem, but I would still get a new O2 sensor from an autoshop like Repco/Bursons/Supercheap auto etc.
You can save money by buying the 'universal' Bosch unit, which only means you have to solder your plug onto its wire.

Idle dip sucks, seems to happen to so many NAs now that they are getting older.
Taking off the throttle body and iscv and cleaning them can help a lot in minimising it.

Cheers hks! I've just put my workshop manual down, it helpfully said "disconnect and remove ..." then "installation is the reverse ..." I'll have a poke around tomorrow and see if I can find it


And cheers for the 1-wire-is-normal confirmation emily!

Yes, NA6s only have the old single wire sensors. They're only meant to last 60,000kms according to Bosch but at least they're cheap. I'd go looking for a bung hole further down the headers from there.

I had a whole short story typed up, then my phone ate it.

Abridged version: dad noticed the connector wasn't unplugged, but that the wire had come out of the back of the plug.

Some spade connectors and a little bit of crimping later, and we're at least reporting *something* to the Ecu...

I put the O2 on the multimeter and it came back reading 0.88V at most revs, so I've got a new one on the way ($50 at beeps)

Well, I got my new O2 sensor, put it on, plugged it in, and the car runs about the same as it did with the previous one unplugged - the previous one (when plugged in) was making the car loose power (like the AC cutting in) probably by reporting jibberish to the ECU.

There was no other bung for the O2 sadly. Is it the kind of thing someone that can weld can move? (Fair warning: I'm an IT geek by nature, so everything can be done, just "install linux and RTFM" - I'm learning the whole "physical objects" thing)

You really cannot solder the O2 wire - I had assistance from an uncle that told me crimping was no good, and soldering was the way to go(*) - I can solder fairly well - not on those bad boys though - ended up giving up and crimping the cable as per the advice of everyone else.

End of story so far. Idle dip stil present, but O2 sensor now functioning and plugged in! Solved a problem I didn't even know I had. Go Team!

(*): "What you need to do is..." is a running joke amongst my cousins - it's something all of our fathers say. It's not always followed with good ideas. But it's easier to let them run their course, then do it your way then it is trying to argue the point.

Can you confirm the idle stays the same when GND and TEN is shortened?

For reference, with the wire you can solder or crimp, both will work well if done well.

Solder joints can fatigue and break with vibration, crimps won't.

Personally I would crimp it, quicker, easier, less likely to fatigue.

gslender; I'll test it again and see if the new O2 made any difference, before doing the GND->TEN link made no difference

It’s been years since I’ve owned a NA but I think you should look at the throttle position sensor.

My NA would have the revs drop to 400rpm when I pulled up at a set of lights, sometimes it stalled.

The solution was to clean the throttle sensor, from memory located towards the front half of the throttle body. As my memory has faded on NA’s, there must be someone here who can post a picture of where it is if it hasn’t been done in past posts.

Do a search, I’m sure there is a lot of information about this.

Yep, having the TP switch fail wouldn't help, but just make sure the idle before and after GND-TEN is exactly the same. Most common reason for idle dip is the computer only comes in when the TP is closed, and the idle cushion mechanically stops this from happening immediately, so the idle screw needs to ensure the base idle is high enough. Make sure you are doing the GND-TEN test when the engine is really hot, like after a good drive and fully warm. Don't do it warm... do it hot! Fans on is the best way to know you're at the right temp.

G

Thanks gentlemen I'll investigate the TPS, to the googles! *adventure music*