Firstfives.org

Hi,

One of the final steps ( ) of my carb-to-motronic 1.3 conversion would be the addition of the oxygen sensor to get closed-loop mixture control.
My car of course has no predestinated mounting position for anything like that on the exhaust. What would be a good location for the sensor? Close to the head? Or close to the first muffler? Or even on the exhaust header (easy access)?

I found this picture, seems a rather nice location:
http://blog.bavauto.com/wp-content/uplo ... 11/112.jpg

Normally you want it close to the engine, so it heats up quick and the exhaust gas doesn't cool much. Heated sensors diminish that need, but my E36 has it right in the manifold itself IIRC.

Since you're doing Motronic 1.3, you would have the wiring for the heated four wire oxygen sensor. On E28 535s with the heated sensors, the sensor is on the left side of the catalyst, which puts it just about right behind the downpipe.

wkohler wrote:Since you're doing Motronic 1.3, you would have the wiring for the heated four wire oxygen sensor.

I do, so at it would be easy to add to the engine management.

wkohler wrote: On E28 535s with the heated sensors, the sensor is on the left side of the catalyst, which puts it just about right behind the downpipe.

On my (gone) E32 it was just before the catalyst but you could only access it after you dropped the exhaust.
I would prefer to have easy access to it, in the future I want to experiment with MegaSquirt and wide-band lambda, so I probably have to change back-and-forth a couple of times as I will both be testing & daily driving the car.

Closer to the manifold would satisfy that need, but I was wondering if the temperature would not be too high, or if flames might damage the sensor.

One of the ideas behind putting the sensor in a "cooler" part of the exhaust is to increase the longevity. I don't think you want a wideband sensor in the manifold. Also, since Motronic 1.3 is always closed loop, it would be a good idea to add a second bung for the wideband.

Thanks for the info. I will try to find a place somewhere further away from the manifold.

So I put my hands on 2 possible studs for the 02 sensor:



The idea is to mount one of them on a downpipe. I saw in pics that Blaise also uses this set-up.

But, which one to pick? The long one or the short one? The long one might starve the sensor of fresh exhaust gasses (?), the short one will allow the 02 sensor to penetrate / block the downpipe more than I like, taking in to consideration that the pipe is from a '76 528 with stock exhaust (one exit at the rear) and the engine is a 3.5..

I guess you saw where it is on my car. Basically at the end of the downpipes, as they start to travel under the car. The O2 sensor comes with a loooong wire.

I will likely add another bung at a later date, for a proper fuel mixture meter and some fine tuning of my car.

I would go with the short bung to let the O2 sensor get into the exhaust path. I think most are that way.

Good luck.

helow its really nice. i really like it

I'm with Mike W. on placement - closer to the manifold is better. You want to sample as many cylinders as possible, but also keep the sensor very warm.

I've got experience converting from 530i to 528i FI. Some of this is described in the FAQ. I had the sensor at the end of some headers sampling 3 cylinders. I quickly moved to a heated 4-wire sensor to help keep the sensor warm. I still had driveability problems in the rain. I determined cold water splashing on the sensor killed its operation until it stopped getting flooded with cold water. Once hit with water, it could take 10 to 20 min or longer to stop idling like crap. I have an air/fuel meter, so it's pretty clear the sensor was the issue. Maybe one could build a baffle around it, but I don't think placement far back is worth it.

I've moved to Euro manifolds (3 into 1) and modified Euro downpipes. My O2 sensor is now less than a foot from one of the manifolds and I haven't seen the rain susceptibility yet in 4.5 years.