124: Tips on oxygen sensor replacement?
 

All -

I've been chasing down the driveability gremlins on my 1993 300E. I now need to replace the O2 sensor - car still has the original sensor after 13 years/110K miles.

Any tips on how to get that puppy out of there? I soaked the threads with penetrating oil a few days ago. Access to the O2S isn't exactly great. Is it worth dropping the exhaust system to make more room? What kind of tool is the best to use?

To recap, I'm chasing the usual driveability issues on an HFM-SFI 104 engine. So far I've replaced the plugs/wires/coil connectors to remedy a misfire problem. Pulled codes and determined I had a dead MAF - the computer was in "guess the mixture" mode. Replaced MAF and I'm now free of error codes and car drives fine. Still have a bit of an idle oscillation - it'll go up and down perhaps 100 RPM when rolling to a stop. After ~5 seconds it settles down to a smooth idle.

Many of the usual suspects were replaced under Starmark in the last few years: engine wiring harness, fuel pressure regulator, OVP relay. Car still has the original coils and EA/CC throttle actuator.

I can't find any vacuum leaks; will look again tonight.

The EGR valve is dead, won't hold vacuum. I've got the line to it capped for now. So long as the EGR valve isn't stuck open, I shouldn't have any idle issues from it, correct?

Current plan is to replace the O2 sensor, run some fuel injection cleaner through it, clean the throttle plate (I've read they stick due to accumulated oil vapors from the crankcase ventilation), and check further for vacuum leaks. If that doesn't fix it I may try an exorcism. :silly:

- JimY

Jack up on drivers side front jack piont... used an oxygen sensor socket, from any parts store (has a slit down one side to let the wire pass through), and a Craftsman 3/8ths rachet with a pivoting head (to get up around the exaust tunnel) and a piece of pipe as a cheater bar.
Old one out and new one in and routed in less than 15 mins.

I thought the O2S was accessible from the passenger side compartment under the floor mat? I'll have to do a search and check again, but I thought that was the case on the W124.

For underneath accessibility, I'd use ramps. Much safer than a jack/jack stand combination. PB Blaster has helped me with a lot of situations like this..ie..old exh. nuts/bolts. Spray on, let it soak over night.

This is a shot is the dark, but regarding the occasional up-n-down of the idle.
My 103 motor has been doing this on and off. Lately it's taken to stalling once underway for 15-20 mins. and in hot weather only -> fuel pump - nearly 16 yrs. old. When they start to go, the pressure can vary and you can begin to get a little bit of idle variation.

Hope to swap out the old Peirburg pump this weekend.

Good luck with the 104.

It's the wiring that is accessible under the floor mat. I have yet to buy a socket for O2 sensors. I usually just thread the wiring through the box end of a wrench, which gets the wrench onto the O2 sensor for removal.

I couldn't get it out except by allowing the car to run, thus naturally heating the exhaust...then standard ratchet it out, wiring harness under the mat.

And the winner is... combination wrench! I ran out and purchased a 22mm wrench on my lunch hour - my collection of tools stopped at 19mm.

Super easy job; took longer to put the car up than to change the oxygen sensor. I soaked it in PB Blaster a few days ago, it broke free with one easy pull on the wrench.

The lead from the sensor actually comes in the side of the transmission tunnel rather than the floor. Minor quibble, but it's not even necessary to pull the carpets on the passenger side if you know this. Just peel up the rearmost edge to access the blue cable that is the O2S.

I'm happy to report the m104 now runs absolutely, spectacularly well. It never surges at idle, is always smooth, and responds instantly to the throttle. Idling in park it's so smooth I cannot even tell the engine is running. Schweeet. :huepfenic

Now if I could just figure out how to adjust the steering gearbox without removing the intake manifold...

- JimY

I'm not sure but I think a 7/8" (22.2mm) wrench would have fit also.

Possibly, but why would I pass up the opportunity to purchase more tools?

- JimY

Very cool! I wonder if replacing the O2 on my M103 will clear up any "quirks" as well since it's 15 years old...