Dead O2 Sensor

[madboy]

OK, been looking to trying to find some things have have been bugging me with the 5, I read how to check an o2 sensor in the Haynes manual, and it describes getting a certain V+ under certain conditions and the only thing it says about not getting the right V+ is that the sensor is dud, get a new one. I tried to test mine today, i get 0V+ at any rpm, i am guessing this means its stuffed?

I was was suspect that i didn't have a good connection or a good earth, so i carefully cut around the shielding on the wire, exposed a little and made sure i had a bare metal contact, i also used the GND connector in the diag box plus various other grounds just to check. Also i was testing at the just after the plug that is at the CAS, I couldn't get at a length of wire long enough that didn't have heat shield on closer the sensor.

Cheers
MB

[Matty]

just pull the plug in the sensor and probe it while you rev the engine by pulling on the throttle cable (engine must be fairly hot to get the O2 sensor working)

Assuming it's dead, buy a generic sensor and crimp it on to the old plug.

[grump]

M.B, I got a new 02 sensor from Repco for about $60, takes about 5 min to install, made absolutely no difference to the car what so ever.

[madboy]

hmm well, i am trying to track down why my cat is over heating and making funny noises, why i get little backfires at gear change when i gun it from 1st to 4th and why i am only getting 300kms a tank on 98RON fuel.... from what i have read so far, a dud o2 sensor causing the ECU to go open loop can cause all three of these problems

[Hellmun]

why i get little backfires at gear change when i gun it from 1st to 4th

When you are WOT the ECU uses maps rather than trimming fuel according to the O2 sensor I thought. What modifications have been done to the car? Additionally if the CAT is getting extremely hot have you checked for a blockage at it?

[Steampunk]

1) The ECU does not use the O2 sensor at WOT, in short, the O2 sensor is more for emissions and economy at cruise. So a dud O2 sensor is not the cause of overrun popping/backfire at WOT.

2) although 300km is on the bad side of the fuel-economy range, it all depends on how you drive. If you drive 90% of the time "gently" then it's a crap figure, but if you drive 50% or so at WOT then it's fairly acceptable. Try filling up completely next time, and drive the whole tank gently and see what you get.

3) I'm assuming that you have a temperature sensor in your catalytic convertor and thus you have a Eunos Roadster instead of an MX-5? If so, then send a PM to JCS86, as his car runs ridiculously rich, but has no problems with economy or anything.

4) backfires can be caused by both a too rich and a too lean situation. In your case, it's probably too rich. jcs86's car backfires at WOT too.

IF your O2 sensor is dud, I wouldn't worry about it as you don't really need one anyways.

[madboy]

why i get little backfires at gear change when i gun it from 1st to 4th

When you are WOT the ECU uses maps rather than trimming fuel according to the O2 sensor I thought. What modifications have been done to the car? Additionally if the CAT is getting extremely hot have you checked for a blockage at it?

The car has an HKS Ram pod filter and 2.25 Jasma headers through the stock cat into a 2.5 cat-back, as far as i can tell they are the only real engine mods apart from iridium plugs and 8mm magnecore leads. From what i have read these mods usually make the car lean out more then run rich. I haven't had a chance to unbolt the cat to have a look for blockages, its going up on the hoist saturday, will try and get a look then.

2) although 300km is on the bad side of the fuel-economy range, it all depends on how you drive. If you drive 90% of the time "gently" then it's a crap figure, but if you drive 50% or so at WOT then it's fairly acceptable. Try filling up completely next time, and drive the whole tank gently and see what you get.

I do get a bit happy on the gas sometimes, but i'd say %75 miss daisy %25 hoon

3) I'm assuming that you have a temperature sensor in your catalytic convertor and thus you have a Eunos Roadster instead of an MX-5?

Correct

IF your O2 sensor is dud, I wouldn't worry about it as you don't really need one anyways.

so eveyone keeps saying. whats the point of having one if they are no use? I was under the impression that when the ecu is not getting feedback from the o2 sensor then it just dumps fuel in to prevent a lean AFR...hence the sh*te fuel economy even when driving not at WOT. ah well maybe i am %50 miss daisy / hoon and just don't realise it yet

Cheers
MB

[Fatty]

the o2 sensor is used when the car is cruising (closed loop) so i reckon it WILL provide better fuel economy than just running from the ecu fuel maps (open loop), coz i'm guessing the fuel maps would have a safety margin built in to account for poor quality fuel etc. hence you will be dumping in more fuel than you really need.

so i'd disagree with the comments that the o2 sensor is not needed. whether it is the source of your problems tho, i can't answer that.

[Tezzax5]

What is the timing set at....?

[madboy]

What is the timing set at....?

I am unsure yet, but i think its stock, 10deg, i don't own a timing light and haven't had a chance to use the one at work, tomorrow for that too, i need to replace the o-ring on the CAS as well and didn't want to do that till i had a timing light to use.

[Hellmun]

When I changed from standard headers to Racing Beat 4-1's, with a new exhaust and just a pod filter on an elbow my car ran really rich. Was back-firing and similar economy. I actually then hooked mine up to an aftermarket ECU. My car is an NB8B though. Some people run those mods fine however....