Using too much fuel 911SC-is the missing O2 sensor the reason?

[Harvs11]

I have a US spec 1980 911SC with about 42000 miles on the clock. It has a bypass pipe fitted so the catalytic converter and oxygen sensor were removed at the same time. The engine is otherwise completely stock and is in very good condition. The fuel system and ignition were all recently serviced so it is running really nicely. CO readings were about 2 -2.5 % so it is in the recommended range for air/fuel mixture.
Here in Australia the fuel prices are on the way up ($1.60/litre). As this is my daily drive, it's getting a bit costly. I am getting really poor fuel economy combined city/hwy of about about 17l/100km or 15mpg. I think I should get closer to 23-25mpg.

My question is would the removal of the O2 sensor cause the poor fuel economy?

[cab83_750]

Before we proceed, I would like to say Welcome and ......nice car.

I have an '83 and before the removal of the cat, I was getting high 20's. I disconnected the 02 and the car ran smoother. Perhaps, I dropped a mile or 2 per gallon.

When I installed SSI and adjusted the AFR, my mileage is now down to 20. I guess to answer your question, I would say "no." If there is a drop, it would be minimal.

[Mr.Puff]

Hey, nice 911! Seems like there are a lot of aussie's on this board . I was in Melbourne in May, loved the place. It's much easier to navigate than Sydney (I got your back).

[tt_targa]

Hi,

Nice SC.

I get a disappointing combined 16 mpg and have never gotten the car to run well leaned out. I am completely stock.

My original sticker says 16 city and 21 highway.

Good luck.

[SCrescue]

Vandha,
It would be easy enough to drill a hole in the bypass pipe, weld a bung in place, and reinstall the O2 sensor.

Prior to 1980, the CIS motors didn't have an O2 sensor and they were good for 20+ mpg also, so I'd have to agree with cab_83 that 15 mpg seems too low with just removing the cat and the O2 sensor.

In more modern FI systems, the computer can operate in open loop mode if the O2 sensor fails or is removed, albeit with a loss of performance and efficiency. The lambda system on the 80 & up CIS cars is much less sophisticated than modern computer controlled EFI like Motronic, so it should be fairly easy to troubleshoot the cause of your missing mpg...I'm sure one of the CIS experts can point you in the right direction, but I'd start by putting the O2 back in the loop.

Btw, I like your car too, great picture.

Mike...

[JV911SYDNEY]

welcome aboard vandah...thats a sweet looking car!

Originally Posted by Mr.Puff

It's much easier to navigate than Sydney (I got your back).

them's fightin words

[Harvs11]

Thanks for the feedback guys.

After reading thru some of the tech articles it looks like I need to look at how the warm-up reg and start-up valve are working. Replacing the O2 sensor seems like a good idea too as I think the fuel meter is set up for it, unlike pre '80 models.

I'll post an update if I work out a fix.

Gotta spend some $$ to save some $$

[fireant911]

vandha,
My mileage improved 22.8% after reconnecting the oxygen sensor. I keep meticulous records regarding all Porsche activities (including of course refilling and distance traveled). Your pocketbook will appreciate 'closing the loop'.

[fred cook]

I have an "80 SC coupe with SSI exhaust. I have driven the car both with the O2 sensor connected and disconnected. Either way it gets about 17-18 mpg in town and 25-29 mpg on the road. Perhaps you should check the air/fuel mixture to see if it is set too rich. I have mine at about 3.5% CO. At this level, the engine runs well and pulls hard all the way to red line. Good luck with your car!