Unplugging O2 sensor

Ted_Richardson · 01-07-2003, 08:07 AM

Upon pulling away from a stop my 1984 911 has been hesitating. The dealer serviceman suggests disconnecting the O2 sensor. He said the drivability is greatly improved and the engine actually runs cleaner. He has tried replacing it but it runs the same. I asked if this was a common problem and he said some 911’s of this vintage have this problem. Have you heard of this before? It the O2 sensor he replace mine with also bad?

Thanks,
Ted

APKhaos · 01-07-2003, 08:43 AM

Well, not exactly.
This may have been true for CIS cars, but the 3.2 DME relies on the O2 sensor. If it fails, or is unplugged, the DME reverts to a full rich mode [open loop mode] which it also uses for cold start. Like running with full choke all the time. This is suboptimal.

Best bet is to replace your O2 sensor. Do a search - several good threads on replacing the [spendy] Porsche part with a generic O2 sensor.

Reading your post again, your service guy could be grasping. Better to have a definite diagnosis rather than replacing parts until it gets better. This can be a frustrating and expensive journey. Some dealerships love 'em.

Dan Owens · 01-07-2003, 09:09 AM

One of the often over looked issues with the O2 sensor going bad is that your air flow meter may be mis-adjusted. If your sensor was recently replaced and the car still hesitates, bucks at low RPM's or has "flat" spots during normal accelleration I would have a garage check and reset the air flow mixture.

I would also question the "serviceman" that says just disconnect the sensor? Sounds like fishy advice to me.

Dan

ChrisBennet · ChrisBennet's Garage

Disconnecting the O2 sensor make sense - in a way.
If the motor is running too lean from intake leaks, out of adjustment air flow meter, etc then disconnecting the O2 sensor would richen the mixture and mask the true problem.
-Chris

Joe Bob · Joe Bob's Garage

I just got the Bosch one from Pelican.....helped out on my 3.6....

Dan Owens · 01-07-2003, 11:33 AM

I think Ted said was that the o2 sensor was already replaced. If that's the case I would be looking elsewhere for the cause of the hesitation. I stick with air flow meter needs adjustment if it's hesitating and has flat spots in normal acceleration.

Dan O

stlrj · 01-07-2003, 05:10 PM

I would be looking elsewhere too because the symptoms point to ignition, not 02 or mixture related. Plugs? Ignition cables? Cap?

Cheers,

Joe

john walker's workshop · 01-07-2003, 05:45 PM

see if any local shops have a test airflow meter that they can let you run around on for a day or two. i put mine on customer cars a lot, resulting in the sale of quite a few new airflow meters.

speeder · speeder's Garage

I think that it sounds like the wrench-guy is "sub optimal", or at least not familiar w/ 911s and their various fuel delivery systems. The "disconnect the O2 sensor, they run better that way" advice applies to CIS cars w/ lambda, (80-83), and is not universally agreed upon for those cars, but no motronic car runs better w/o sensor working. I agree that your problem might not even be related to fuel mixture, you need someone good to diagnose it correctly. That is cheaper than the "throw parts/labor at it until we find the squirel in there" method.

Is it normal procedure to mess with the settings on air flow meters? I always thought that the reason for the tamper-proof plugs on them is because they are not "user servicable". Every one I've ever had w/ that plug removed was FUBAR by some genius who thought that he could tweak it.

Dan Owens · 01-08-2003, 04:59 AM

Speeder,

They do have the service plugs in them and aren't meant to be fiddled with just for fun. They are adjustable to a point to adjust between a lean and rich mixture. Also they are elctronic coupled to a mechanical air flow door if you will. As the door opens and closes to allow air flow to increase and decrease the signal is transmitted to the DME to adjust fuel. This is a real rough explanation. The electronics can wear in spots and loose signal in only part of the RPM curve causing the hesitation and flat accelleration. If it's a lean or rich adjust it's ok. It the contacts are wear or electronics are ging out then it's the whole meter that needs to be replaced.

Dan

doug petty · 01-08-2003, 08:25 AM

Just replaced the air flow meter on my 85 w/145000 miles on it & now my rebuilt 3.2
has a lot more throttle response, smoother idle, mid range like it never has before. Now i've got one problem my little 205's won't hook up when you hit the gas, but I can live with that until I find some 16 x 8's for the rear. lol.

Steve W · Steve W's Garage

ChrisBennet also has a valid point. Many 3.2s suffer from intake air leaks as they age around the intake manifold gaskets. The 12 gaskets shrink and harden, causing massive amounts of unmetered air into the motor - read lean. The loose paper gaskets often times also get sucked into the intake port, making things worse. Test by spraying engine starting fluid (ether) around those areas, and if your idle shoots up, you have air leaks. If so, try retorquing the manifold bolts, and if that doesn't work, you will have to pull the manifolds off and replace the gaskets. The reason unplugging the O2 sensor makes your car run better, is because unplugging it make your fuel injection run in full time rich mode, compensating for your air leaks.