Hi folks,

I have had an recurring issue with my idle hunting when the car is warm.

Tonight I un plugged the rear o2 sensor and the revs stabilised somewhat, so I think this is my issue.... The car is a 1999 wrx rb5..and the o2 sensor appears to be a 3 wire jobbie, from what I have looked at so for I can only see the 3 wire being a front sensor.. is it possible that I have an incorrect sensor fitted? or would it not plug in?

The car does not have a front cat... would it be possible that the rear o2 sensor is plugged into where the front one would go? or again is it not possible for them to fit?

so in basic terms..how many wires should my rear o2sensor have?

Thanks

Dan

 

ok with a front and rear lambda sensor, a code reader will tell you if the "O2 sensor" is faulty. So recommend you get the codes checked before replacing it, and you will likely need to replace if its faulty because you can't clean them these days. Also, if the O2 sensor is faulty it will through up the EML (MIL) on the dashboard. So, has this come on? If not, then again recommend checking the codes because a lambda rarely fails unless VERY old like with 150,000 miles or more.

With lambda sensor replacement, the code will not automatically disappear. You need to run the engine and stop/start at least 2-3 times before the ECU recognizes its been replaced. This is typical for a lambda sensor replacement. The EML will disappear when its ok and found to be working normally.

If it remains on, the new lambda is either faulty or not recognised. Also, never try and clean a lambda sensor because these days a cleaner will destroy it, e.g. Petrol, Oil, WD40 etc will contaminate and ruin it.

With normal running, the lambda sensor should be a light brown colour with the holes visible to allow the exhaust gases to be analysed by an electrical potential difference in the lambda so the ECU can read this info and make the engine adjustments accordingly.

If you find the live wire (red) and the earth wire black (usually two or four wires or only two wires) then you can connect a potentiometer or volt meter and see if there is voltage in the wiring to the sensor - with the ignition on it should read above zero Volts - if not, first check the fuse in the fuse box and then if not this then you need the last resort of following the wiring back to see if there are any breaks and earth issues anywhere.

 

hey dude, thanks

No code but it has flashed up once when I was in the course of replacing the iacv and messing with the maf.

I only have either 2 white wires and a black or 2 black and one white...the only thing that makes me think its the o2 sensor causing the issue is that the car idles better when it is disconnected while running.

 

Is it true a pre 2005 Rear 02 sensor can't alter fueling?

 

I believe its just a warning light (cat on Fire) but makes no changes to the running of the car.

 

I'm happy to bow down to superior knowledge, I am....but the car idles around 500 to 900rpm....then I disconnect the o2 sensor and it sis steady around the 900rpm mark?

 

Im not sure about on a classic, is it the o2 sensor in the downpipe your referring to ??

 

correct bud, as far as I can see there is no up pipe o2 sensor

 

The other one is in the headers, accessed from the drivers wheel arch and its the one that has the main control on fueling. On a blobeye i was told the rear O2 sensor is just a warning light and usually appears after fitting a decatted down pipe and the code is just deleted by the mapper so wont throw a cel. I can confirm on my previous wrx it made no difference to the running of the car when lit and didnt alter fueling or cause a lose in performance. It may well be different on an earlier ecu.

 

Disconnecting a rear O2 sensor won't make a lot of difference because it only adjusts emissions minimally from re-cycling the exhaust gases. However it will through up a MIL EML CEL light on the dashboard. If you connect a voltmeter to live and neutral then you should see voltage. If not, then the wiring is likely suspect. If there is Voltage then it's possible the O2 sensor is faulty...


But from what you say, I suspect the O2 sensors aren't faulty. The idle speed control valve sets Idle speed, and the MAF sensor will feedback to alter mixture. If either is faulty, then this will cause a rough idle. If it's these, then you'll get a code come up. The only other thing that can cause rough idle is the variable valve timer sensor or solenoid in it which could have failed but you won't see a code thrown for this.

 

And where would the vvt sensor be ?

 

You may have a slight air/boost leak, when the cars cold there is added fuel in the mixture so it would idle happier than it would once warmed up.

 

That's a good call. An air leak will cause variable running on idle. Check pipes for cracks or splits through from air filter in and out. You mentioned you'd looked at the MAF so maybe around that area?

 

Did you get that mangled/melted MAF plug sorted?

 

Not yet, i have just had a word with richard at fb tuning.. im going to take the recirc dump valve off and block the hole and make sure that isnt the culprit for a leak.. other than that im sticking a new maf on asap and richard will stick a new plug on for 25ish quid.

You still having this polo ? :-D