Suspect O2 sensor - but no CEL when removed

[ultreos]

Hi all,

First post! I've recently just bought a 93 Clubman that I was hoping to fix and learn a little bit about cars along the way - mistakes will be made I'm sure.

I've been driving it for a while now, and I have been concerned about the lack of power below 4000 RPM. It seems really quite sluggish and hesitant, and the moment it reaches around the 3000-4000 RPM mark, the power seems to come back. I've done some research on these forums and on Google, and it seems that the ECU uses the inbuilt map for fuel above the 4K RPM mark, and relies on O2 and other sensors when below - so I pinned my hopes on a new O2 sensor.

Picked one up from Repco (4 pin which I think I got shafted on), and installed it, and it seems to run even worse! When cold, it wouldn't breach 2.5K RPM in first gear, and when it finally warms up, it was having trouble revving to 4K RPM.

On a whim, I decided to unplug the O2 sensor completely, and rely on the stock maps from the ECU, and it runs beautifully. No hesitation, and power delivery is smooth right up to red line. However, at no point during any of this did the CEL turn on or flicker - even with the O2 sensor disconnected.

I've run through a number of scenario's in my head as to why the CEL isn't turning on, which may be indirectly related to the engine running like a pig when the O2 sensor is connected, but my knowledge is far too limited. Can anyone shed any light on this?

Thanks!
Ult

[hks_kansei]

Australian cars don't have a Check Engine light.
So that explains why it's not coming on.


As for the o2 sensor, being a new one and running like crap (ie: not even revving above 2500) I'd suggest that it's either stuffed, or the wiring is incorrect.

Double check the wiring (since I assume it's a wire in, not a plugin) and make sure all the wires are correct. The colours on the non-mazda sensors don't match the colours on the car's wiring. (my Bosch sensor came with a little booklet that listed the differences in colour)

a 4 wire sensor has one signal wire (to the ECU) a pair of 12v wires (pos and neg - for the heating element) and a 4th wire which I can't remember (probably signal ground)

The stock wiring should also have 4 wires leading to the o2 sensor plug, just a matter of matching the sensor colours to the car (ie: Sensor black may go to car blue, etc)

[ultreos]

Australian cars don't have a Check Engine light.
So that explains why it's not coming on.

You're kidding me. I'm feeling pretty red right now.

The O2 sensor I bought was a direct plugin. Unless the connector plugs are the same and the wire's are somehow mismatched, then I would be guessing that both O2 sensors would be stuffed? However unlikely it is for a new O2 sensor to be screwed, it's not entirely implausible.

I'll keep poking around, but I'm not sure how successful I'll be at attempting a return on the sensor.

[hks_kansei]

If it's a plugin that should rule out wiring issues (unless it was wired incorrectly from the factory, but I doubt it)

All I can suggest is that you double check the model number of the sensor and make sure it's the correct one for the car.


As for the CEL, yes, Aussie MX5s didn't get it (maybe late NB and NC had it, but anything prior didn't)

You can however check error codes using the diagnostics box in the engine bay, the process is basically just to solder up an LED and a resistor (of certain resistance, but I can't remember exact specs, works out to under $5 of stuff at jaycar)
You then take a paperclip and short 2 pins while attaching the LED to another two.
The car then goes into diagnostic mode and the LED blinks to tell you the codes (kind of like morse code, long-long-short-short etc)

There's a few guides on how to do it on the web, mostly for NA6 (not sure if NA8/NB8A use the same codes/process)




edit:

http://www.miata.net/garage/Engine_Diagnostics.html

http://www.miataforumz.com/how-40/diagn ... nouts-349/

As above, I don't know if the NA6 codes match the later cars, or if the LED pins used are the same.

[ultreos]

Awesome, verbose information. Much appreciated

I was actually ready to check error codes, but given the absence of a CEL, I thought it would be useless.

The O2 sensor I picked up was the cheapest model they had at Repco (made by a company that is owned by Repco) and was recommended by them unsurprisingly. I'll take a stab at finding a Bosch unit online and see if it makes any difference. Thanks again!

[hks_kansei]

I'm not saying the Bosch is any better or worse, just that's what I have.

I bought mine from Repco, the options were "fuelmiser" or "Bosch"
I went Bosch since I know they've got decent quality control.


I do have my old OEM o2 sensor around somewhere, it works but reads incorrectly (makes the car too rich)
You're welcome to it if you want to try it out (and in Melb)


Only issue is it has no plug since I cut that off to use with my new sensor.



Also have you tried looking at the spark plugs? to make sure it's not running rich and fouling them?
Also maybe replace them.

[ultreos]

Yeah, I went with Fuelmiser as O2 sensors were dearer then I expected. I live in QLD, but I'll ask around for a spare O2 sensor that I can borrow to test with.

When I first bought the car (early Dec 13), I replaced the spark plugs and ignition leads thinking this might fix this hesitation. I recently cracked open the spark plugs again and had a look, and the heads of the plugs do seem fairly fouled, which lead me to think that it was running rich. Again, my experience is limited, so my definition of what fouled means may be different to someone else - they just looked too black for 3 month old spark plugs.

[Cort500]

This sounds very similar to my vehicle. I have checked the forum sponsors websites but couldn't find any O2 sensors. So it's just repco etc to get this item?
Sorry for the hi-jack.

[hks_kansei]

Black plugs are pretty normal for an MX5, they do run rich.
But, it should be DRY black, not damp/wet.

Fouled to me (and I think the normal definition) is that the plug is contaminated to the point it either won't spark at all, or doesn't spark every time.

[ultreos]

This sounds very similar to my vehicle. I have checked the forum sponsors websites but couldn't find any O2 sensors. So it's just repco etc to get this item?
Sorry for the hi-jack.

Indeed - that's where I picked up my O2 sensor. If it was a bad O2 sensor, then I'll report back and I obviously won't be recommending them.

As for the spark plugs.. well they are indeed black, but they definitely aren't damp. I'll give them a good clean after I find a working sensor.

[sailaholic]

Mx5 parts uk is good for parts, you get quality and often cheap enough to make up for the shipping.


Sent from my iPhone using Tapatalk

[JBT]

If another O2 sensor doesn't fix it, check the crankshaft pulley key way - yes, it can and has happened to BP engines.

[hks_kansei]

If another O2 sensor doesn't fix it, check the crankshaft pulley key way - yes, it can and has happened to BP engines.

But unplugging the o2 sensor fixes the issue.

Wouldn't a keyway issue show up at all times?

[93_Clubman]

Given your NA8 Clubman is a recent purchase you may not have had a chance to baseline service it & therefore have not checked the ignition timing like this NA8: viewtopic.php?f=29&t=60066&hilit=no+power+below&start=15

Also, take a look at manga blue's posts on page 1 & 2 of this:
viewtopic.php?f=29&t=61790&p=775895&hilit=open+closed+loop#p775895

Ref Bosch O2 sensor for NA8:
viewtopic.php?f=77&t=47164

viewtopic.php?f=29&t=25609&p=459686&hilit=oxygen+sensor#p459686

[Mr Morlock]

I agree with 93- it could be a number of things and its often quicker and cheaper to take it to a pro.