Changing the Pre-Cat O2's on a M52TUB20 with Bosch Universal O2 Sensors

[NZ00Z3]

Decided that the pre-cat O2 sensors need to be replaced. Car has done 120,000 miles and the recommended life of the sensor is 100,000 to 120,000 miles. Have LTFT's in the -5 to -6 range. That's a rich condition. It's not the MAF. Since the pre-cat sensors have used their time up, they are going to be replaced. The molding date on their connectors say 1999, so they are the original sensors.

Just jacked the car up to give the sensors a spray with penetration oil and let them sit over night. The M52TUB20 is not like the M54's with easy access to the pre-cat sensors. The sensors are in a really fun place to get to. They are right on the end of the headers, just before the flange where the main exhaust system bolts on. No room for a socket and ratchet/breaker bar. It's going to be a spanner job. So, first job in the morning is removing the exhaust system. Flange nuts sprayed with penetration oil to make it easier.

I wanted to use Bosch sensors, since they have a really good reputation. The only ones I could get in New Zealand are Bosch universal sensors. I need to do a cut and splice in the leads to reuse the original plug. The kit comes with some splice connectors. The Youtube video and instructions are straight forward. Don't solder and heat shrink the connections. The sensor uses air movement in the wires for it atmospheric reference and a solder/heat shrink joint prevents this from happening.

Will update the thread with a couple of photos and let you now how it went sometime tomorrow.

 

[NZ00Z3]

Its been an interesting day.

Got the exhaust off easily. The old pre-cat O2's came out very easily. Fitted a draw wire to the old connectors and pulled them down behind the engine to remove the original O2 Sensors.

Here is the Bank 1 O2 sensor

Here is the Bank 2 O2 sensor

Here are the pre-cat O2 sensor connections. The front one is for Bank 2 and the rear one is for Bank1. If you have an M54 Engine, then these are the post-cat sensor connections. For the M52TUBXX they are the pre-cat sensors as the cats are well back in the exhaust system and not in the headers (M54). The M52TUBXX post-cat connections are well under the car, next to the rear of the cats.

As I was using Universal Bosch O2 sensors, they do not come with a connector on the DME end. You need to use the supplied connector kit and cut the connector off your old sensors. Here is what it looks like when you follow the instructions.

There was just too much unprotected wiring for my liking. Off to the shops for some suitable heat shrink.

Here is the finished connection.

The O2 sensors fitted easily. The draw wire made it sot of easy to get the connector back to the top of the engine. The connector boxes were a pain to work up behind the engine. They are just too bulky. I was concerned when doing the connections, where they would end up sitting. It turned out that they sat on top of the engine under the inlet beauty cover. Here is a photo of their final resting place.

You can just see them poking up under the cable housing by the wind screen/shield (far right).

Would I use Bosch Universal O2 sensors again? No I would not. It took more than twice as long with the universal O2's due to the connectors. Plus the connector boxes are bulky and hard to push up behind the engine. Normal O2 sensors that do not need the connector box would have been far easier.

Did they fix my problem? Well, sort of. My long term fuel trims are still in the -5% to -6% rich areas. The total fuel trims for each bank are spending more time in the good +/-4% range, but there are still some major rich deviations. The total fuel trims are now following each other a lot better than before, so the old Pre-Cat O2 sensors were on the way out.

Next on my list, it to investigate the Cat's to see if they are partially blocked and causing the rich conditions.

 

[amne]

I have an m52tub20 engine and my o2 pre-cat sensors are in the same place as on the m54: on top of the headers right next to the valve cover. Also my flanges have 2 bolts per bank instead of 3 like your pictures are showing. What am I missing?

 

[NZ00Z3]

Hi Amne

I see that this is your first post in the forum. Welcome.

My 2.0L Z3 is built to the UK spec. You may be in another country and the Z3 was built to a different spec.

There are also a couple of possibilities that your car has been modified. They are after all over 20 years old now.

• The Cat's and exhaust may have been changed for a set from a M54 engine due to the Cat's being blocked?
• The engine may have been changed with a M54B22 or similar engine and the later model Cat's where shifted over too?

 

[amne]

Thank for the welcome.

RealOEM.com - Online BMW Parts Catalog

BMW parts Z3 E36 Z3 2.0 Lambda Probe Fixings

www.realoem.com

RealOEM matches exactly what's on my car. I had in the past an e46 325i (m54b25) and the head part is almost identical: same CCV setup, DISA (maybe smaller on the b20), exhaust headers with cats integrated (I have two cats, yes).

Initially I thought maybe you have the normal m52b20 but that wasn't on the Z and also that engine has only 1 o2 pre-cat sensor. That's why I was a bit more curious why yours is different.

RealOEM does show a variant of the headers that have the catalysts outside the headers. I assume that is what you have. I just never knew it existed. That's all.

RealOEM.com - Online BMW Parts Catalog

BMW parts Z3 E36 Z3 2.0 Lambda Probe Fixings

www.realoem.com

Wire leads are also much longer on your setup to reach the plugs next to the fuel rail while for my setup the wires are short because the plugs are on the side of the valve cover.