325 Fault code

[Marc T]

Hi, to anyone reading this. I am new to this forum but have spent the morning searching though and found some faults close to my problem but they didn't have the solution. I've now hit a brick wall and I'm hoping somone will have the answer.

The car is a 91/92 BMW 325 E36 manual. In September it had an O2 fault so a new sensor was fitted. All seemed well until 3 weeks ago when the exact same fault happened (Idle hunting and almost stalling) I had it plugged in to a computer (My UK car doesn't have a check engine light) and it was confirmed the sensor had faulted again. A friend also lent me a 'scangenie' to allow me to check the codes myself. The codes were 'O2 sensor short or break' and O2 sensor fail, possible air/fuel leak'

Upon fitting the new sensor all the codes were cleared and the car seemed OK but upon checking the codes again I found the 'O2 sensor short or break code' and if I kept using it, I sometimes got the other code (but very rarely)

I have checked the wiring to the ECU which all checked out good and also connected a multimeter at the ECU on the sensor wire and monitored the voltage. It tends to be between 0.1v and 0.8v but if I keep my foot down it does read 0.0v, then on backing off I sometimes get a -0.2v (Somepeople say this is bad, others say normal?) I also tried cutting the BMW wires and putting my own in, incase of a short but no difference. I have also swapped the ECU, but there was no difference. (Sometimes after a drive it has a fault stored, other times it doesn't)

One thing I have noticed is that if you put your foot down the O2 heater relay is turned off by the ECU at about 3750/4000rpm but if you slowly speed up it stays on. - Is this normal?

I have heard of faults with airflow meter causing O2 sensor faults so I have one coming and a new heater relay (although I believe it is the ECU turning the relay off so I don't know if this is normal or not)

Sorry for the size of this post but I am really hoping someone has the answer as I don't really want to have to get rid of the car as it drives great, but I don't want to keep burning sensors out every few months and risk killing the cat before my MOT.

[Jeron]

Are you sure you only have one O2 sensor?

You might want to read the thread on surging, they seem to be having Cat problems.

[Marc T]

Hi, thanks for the quick reply. Yes mine only has one O2 sensor.

I'll do a search on the cat fault you mentioned and check that out.

[Marc T]

Jeron, thanks again for the info. When I looked though I only really stopped at O2 items.

After reading though, I think it must be cat/O2 sensor related (Although I still want to get the fuel pressure checked as the car takes a couple of seconds cranking some times to start (as if it hasn't got fuel ready))

Anyway, back to the main fault. I tried bypassing the O2 heater relay but that gave me a O2 heater fault on the 'scangenie'

I also fitted the new airflow meter and took it for a run but it was no better. What I did notice was that if you have your foot down the O2 heater relay was turned off so if you drove hard for a couple of miles the heater would not be on all that time. (I have an LED connected to the O2 heater circuit so I can tell at a glance if it is on or off)

I guess that since the sensor is quite far down the exhaust that if the heater stayed off to long, the sensor would cool off and not work. When I got home, the O2 wire break or short was back.

As a bit of a test, I have placed a wire in the the relay power feed and wire out to the O2 heater and connected these together with a fuse. I took the car back out on the same road and tried to drive as close as possible to the previous journey, when I got home there was no code present in the system.

I assume the relay is meant to turn the heater on and off as both the ECUs I've tried do this and on another post someone has bought this up so I don't know if maybe the cat could be breaking down and maybe not keeping the heat in and the sensor going 'open loop'

I am going to keep using the car with this wire in for a while as I can't believe after all I've tried it really can be this easy and I will post up the results afer a bit more use

I am also going to find out if the local tire/exhaust place can test the cat as if this is at fault I would rather get the car right.

[LiquidBMW76]

I know this might be simple but go through and check all of your vaccum lines I finally solved my O2 sensory fault by replacing a line that had a really small leak. Also as far as I can figure out the O2 heater is not your problem. If the post I have read are correct the only reason it comes on is to make sure that it is heated to get a proper A/F ratio reading. Once the gasses are hot enough it should kick off. Hence the reason it would not come on during full throttle acceleration, but would if you barely accelerate due to it's location in the exhaust system.

[Marc T]

Thanks for that. I have had the code once yesterday. The heater been on all the time does seem to have helped, but not cured it. I am thinking of taking the inlet manifold off and checking everything underneath, after I check the fuel pressure.

Thanks again, all these tips help to try to stop losing hope as when its right its just so nice to drive.