O2 sensor question for Motronic on '84

[450knotOffice]

Sorry to bring this up but I've been on this forum looking around in the archives for over an hour and I can't find a rock solid answer to my question.

Steve Wong? Lorenfb? JohnWalker's workshop? Anyone?

Besides a drop in Mileage, what would be the negative consequences of running my car with the O2 sensor unplugged? I know the DME uses the O2 input to constantly analyze the A/F ratio and make continuous adjustments in a closed loop system. I've heard that the early Euro Carrera's didn't have an O2 sensor. Is this correct?

My very, very experienced (30 years) mechanic unplugged it yesterday as part of a mixture and idle adjustment on the car. He set the mixture to .8 CO and said that the car's mileage would decrease but that the Cat would be fine at the .8 CO setting with the sensor unplugged. He said that the engine would idle better (it does) and would not have that little, almost imperceptible, surge while cruising at part throttle (it doesn't).

Should I plug it back in or not?

Thanks guys.

[Rick Lee]

This is like asking what kind of oil to use. A lot of folks leave it unplugged, some just unplug it for the track and some leave it in. It will definitely run richer unplugged, assuming it worked correctly before.

[kepperly]

Porsche wouldn't have put it there if the DME didn't need it to give
the CORRECT fuel/air ratio. The leaner a car runs ( to a point) the better they run. Hook it back up. Unhooking the o2 sensor on an MFI
is permisable but don't fool with The DME sensors.

Keith Epperly 87 slant nose turbo look carrera cabriolet

[ausdilecce]

I dissagree kepperly.

Porsche put the O2 sensor there to satisfy USA ( and other countries ) regulations. Some countries ( ie Hong Kong, Saudi Arabia, South Africa - 930/20 engine - ROW) did NOT require an o2 sensor and so Porsche did not put o2 sensors in those cars.. I have a ROW 911 and it does not/did not ever have a cat or o2..

Without an o2 sensor, the car will run an 'open loop' program ( no feedback loop ) and therefore depend heavily on the manual A/F mixture setting

With an o2 sensor, the car will run a 'closed loop' program ( feedback from the o2 ) and therefore depend on the manual A/F mixture for course A/F setting and then read the o2 sensor output for fine A/F adjustments ( and motronic brain will adjust automatically )

So, if the o2 sensor is unplugged AND the manual A/F mixture screw is re-adjusted properly (to compensate for motronic seeing no o2 output and richening the mixture ) - there should be very minor changes in gas milage/performance UNLESS there are *major* changes in other external conditions ( foul weather, very very cold, very very hot, very high altitude etc. )

If you plug it back in, remember to get the car's A/F mixure re-adjusted ( or the o2 circuit will be constantly attempting to richen the mixture )

[450knotOffice]

Thank you Ausdilecce. That was the kind of info I was looking for.

[rattlsnak]

Actually, the US cars have a seperate LAMDA control unit for O2 control. Not sure if the part numbers are the same for the DME with O2, vs the DME without it.
But either way, I wouldnt do it.

[stlrj]

A very experienced (30 years) mechanic who would unplug the 02 sensor on an engine designed to run with one has obiously not been keeping up with technology and needs to replaced (fired).

I would look for a mechanic that could make my car run right with all the sensors connected and able to pass smog at the same time.

No excuses...please.

Cheers,

Joe

[Steve W]

Unless you are running a ROW Carrera, or have installed a performance chip designed to run open loop, it is best to leave the sensor connected. With the stock chip, or a performance chip designed to run closed loop with a sensor, there is almost no advantage to running with the sensor disconnected.

[ausdilecce]

stlrj,

In actual fact, it takes a *more competent* mechanic to tune a car to pass smog without the o2 sensor plugged in. Think about it..

Also, we have had o2 sensors in 911s since 1980, so the last 25 years of said mechanics experience has been with o2 equipped cars.. He must have been wrenching under a rock to not 'keep up' with that technology

[ausdilecce]

Steve,

You of all people would know for sure..

Can you say what the disadvantages are to running a non-ROW Carrera with the sensor unplugged ? Just curious

[450knotOffice]

The mechanic I am referring to knows more about ALL of the different 911's than most of the people on this board combined. He knows what he is doing. He works on everything from ancient mid sixties 911's to 914's to 996 Twin turbos to the shop's GT-3 Cup cars. The latter two have much more sophisticated injection systems than my old '84. I do not question his skill.

He told me why he disconnected the sensor - to get rid of some of the little annoyances that the Carrera has always had because of that damned closed loop lambda sensor system that the Motronic uses to try to keep the A/F ratio at the optimum level (annoyances like a very subtle part throttle hesitation and slightly unsteady idle). That system continuously adjusts the A/F ratio up and down very slightly. It doesn't hold it steady. It goes from slightly too rich to slightly too lean and back again. Maybe .1%. But that .1% seems to be enough for people to notice it because it's one of the things the Carrera is known for.

For example, I have the February 1984 Car and Driver review of the then new Carrera and even then they were complaining about the system’s part throttle no-load hesitation tendencies (pg.35). Hmm. Brand new car from Germany and it was exhibiting the same issues we've been complaining about for years.

The reason I even asked was I wanted the EXPERTS' insights into any possible negative ramifications other than slightly decreased fuel efficiency. Most of the answers have simply been that there is no real reason to keep it disconnected - not that there is a problem with it being disconnected.

As it turns out, I re-connected it. I simply like to keep things as they were designed, for the most part. I will say however that the car did run slightly smoother at part throttle with no load and it had an absolutely rock steady idle. I do suspect that my O2 sensor is getting a little tired however, so I'll probably replace it next week.

[Steve W]

The U.S. lambda controlled 84-86 Carreras with the stock 2k software are most apt to have the off idle load load part throttle hesitation, because the 1) the part throttle fuel maps are mapped almost exactly to the stoichiometric 14.7:1 air fuel ratio. This is far and away from the ratio of optimum power, and it runs this ratio with or without the O2 sensor connected. With the sensor connected, it just assures this ratio is locked. If you disconnect the sensor, and run the mix slightly richer, the part throttle response may improve and the hesitation may go away, but it's nothing major in a performance increase, and emissions and fuel economy will most likely get worse.

The other reason for the flat throttle response of the U.S. cars is that the early software does not allow for any acceleration enrichment on throttle transients, especially in closed loop. If you watch an air fuel ratio monitor, you will see the afrs are constantly at 14.7:1, no matter how you jab your throttle. All this because this was Porsche's first attempt with programmable electronic fuel injection, and in trying to make a emissions compliant U.S. car, they almost made it burn too lean in sacrifice of performance. In the later 4k and 8k software of the 87-89 Carreras, Porsche changed this by allowing some some momentary acceleration fuel enrichment on throttle transients, the result being improved throttle response.

If you have a chip designed for a car equipped with an O2 sensor, all maps and acceleration fuel parameters are already programmed into the software taking into the account the lambda feedback. If you disconnect the sensor, you would be undoing what the original programmer intended, and the disadvantages would most basically be a decrease in fuel economy, and an increase in emissions. Of course you can run open loop with a O2 sensor equipped car, but that would only work optimally with a chip mapped to run open loop.

[450knotOffice]

Thanks Steve,

I still have your chip on my to do list, along with a cat bypass (test pipe) and maybe a better air filter setup.

Take care.

-Scott