Unplugged the O2, messed with the mix - mileage sucks! - what's yours like?

[VaSteve]

The title pretty much says it all. A couple of a weeks ago I unplugged the 02 and fooled with the mix so I'm at 3%. The car runs better than I have ever had it (bought it last fall). I can finally get it up into the 5K range and don't worry about pulling away at a light. However, I thought my mileage was going bad.

Today I filled up and drove 105 miles - a combination of 75mph highway and some 3rd/4th gear blasts around the country roads. It was probably 1/2 and and half of each.

I could literally watch the gas guage drop to 3/4 of a tank. When I refilled, I took on 6 gallons for a combined MPG of 17.5. Kinda sucks. I don't recall what it was like before, but certainly better. What kind of MPG are you all getting?

[fireant911]

VaSteve,
I keep very detailed records of all activity with my car. My records started when I bought my car - I put 3,425 miles on it with the Oxygen Sensor disconnected (it was disconnected when I purchased) and averaged 17.50 miles per gallon that entire time. Upon finally discovering the problem with the disconnected Oxygen Sensor I have reconnected and have put 3,847 more miles on the car with my mileage averaging 21.30 miles per gallon. I realized a 21.8% improvement in mileage when I reconnected and I would conservatively theorize that you saw a 17.8% decrease in your mileage when you disconnected.

I did make an electronically controlled switch recently that allows me to go from closed loop to open loop at the flip of a button (the same as disconnecting the Oxygen sensor on the fly). This allows both economical performance and better performance when required.

[Por_sha911]

Originally posted by fireant911
VaSteve,
I did make an electronically controlled switch recently that allows me to go from closed loop to open loop at the flip of a button (the same as disconnecting the Oxygen sensor on the fly). This allows both economical performance and better performance when required.

I would love to see what it looks like. Did you build it or adapt a purchased switch? Where did you mount the switch. Details please.

[fireant911]

The switch was built from components from Radio Shack - total cost around $12. I am relying on my memory at this point so please be understanding. I choose not to wire the switch into the cars electrical system but instead to use two 9V batteries to power the switching device. One battery is used to switch the 9V electrical relay (DT/DP - wired to the Normally Closed pole) and the other battery is used to illuminate the switch when I am in open loop mode. My goal was to add as little wiring to the Oxygen Sensor circuit as possible since I did not wish to 'taint' this signal - I added a total of about 2 extra inches which should not effect the readings from the Oxygen sensor to the Lambda computer. The switch will eventually be mounted right next to my A/F monitor for ease of activating. I used an empty turning tool container to neatly house all of the components and mounted this on my top of the Lambda computer. My A/F ratio monitor is always in the circuit which greatly helps in seeing what is going on with the fuel mixture - whether in open or closed loop. I will dig up the part numbers I used, the schematic I developed for this project, and, of course, pictures if you are interested - just let me know.

[sagacis]

Wow, that would be great. I know I am running rich, but my mileage numbers are way worse than that. My numbers run between 14 and 16 mpg for every tank since I bought her this summer. Granted, she runs a lot of cold mornings, and my drives are always too short to really get things moving...

I replaced the O2 sensor after purchase to pass the inspection, and have run with the sensor connected and disconnected. I haven't noticed any change in the mileage either way. I would love to get 17 mpg. I guess I had better have the mixture checked...

--sagacis

[aigel]

In my book, 17mpg is very good with a disconnected 02 sensor and richer mixture. Do you know that in 1981, 17mpg was the mixed driving EPA mileage on the SC window sticker?

I never had my 02 sensor connected. I get about 15mpg in mixed driving. On the interstate I run with 4 tires on the roof when going to the track and otherwise I can't keep the foot off the gas when going any place else. So, I think I probably could get 17 if I tried harder.

Cheers, George

[fireant911]

sagacis,
Although you stated that did not see any difference in mileage between connected versus disconnected Oxygen Sensor did you feel any difference in the way the engine ran? If everything is connected properly and to quote John Walker, "You will feel some light surging from the oxy sensor doing it's thing". When I first purchased my car it made no difference in the mileage or in the way the engine ran whether or not the O2 was connected or disconnected - and taking the oil cap off had absolutely no effect on the running of the engine. Finally, I discovered that the Oxygen Sensor wire(s) was grounding off to itself since it is a co-axial type of wire. This fixed my problem.

aigel,
I looked in the original sales brochere for my 1981 car and saw that the 1981 models were estimated at 16 mpg combination and 26 mpg highway (EPA number were not available at the time the brochere was published) - I have done repeated tests and have never acheived 26 mpg... This made me feel better since Randy Leffingwell's book, Porsche 911 Buyer's Guide states on page 126 that the milegeage "29 miles per gallon highway / 24 urban EPA rating" - that's very wishful thinking.

[sagacis]

After replacing my Oxygen Sensor, I was able to feel a difference in how the engine ran. I just expected to get some sort of improvement in mileage, too.

Now that the weather is finally turning colder here in Dallas, the cold starts are a little slower. I am planning to lean the mixture just a bit to see if it helps. I unplugged the O2 sensor last week to keep the engine from dying during the surging process right after starting.

I guess I should be happy with my low mileage, given the hard (i.e. short) drives I put the engine through. I baby her until the temp indicator gets well above the white box, but that is more than half of my daily drive.

Either way, I love the car and could never turn back. My wife's year old XTerra (supercharged) only gets 15-16 mpg on the highway, so I'm doing OK with a much older engine. I guess I was spoiled by the RSX type-S I used to have.

--sagacis

[fstar]

I only dropped a mile or two when i disconnected the O2. Combined driving i usually see 17/18 and on long freeway trips 24-26. Wish it was better but i don't complain...the car runs 100% better now!

[1fastredsc]

I think most of the fuel is wasted during the warm up enrichment portion of the driving. Before the weather actually started getting cold here, i was averaging somewhere in the low 20s, now i'm in the high teens. I think the fact that my motor spends a much longer amount of time warming up has something to do with it.

[aigel]

Originally posted by fireant911

aigel,
I looked in the original sales brochere for my 1981 car and saw that the 1981 models were estimated at 16 mpg combination and 26 mpg highway (EPA number were not available at the time the brochere was published)

While the sales brochure may not have had it, my 81 window sticker has the EPA fuel consumption number. Just like on the new cars today, there is the EPA fuel consumption section on the sticker, with the EPA logo on top, only with a mixed number (they did not give you city and interstate numbers). They also included yearly fuel price assuming 17mpg and 15k miles / year. This may be a CA thing also. I wasn't around buying cars in 81, so I am unsure if this was the norm.

Cheers, George

[john walker's workshop]

you can't get any sort of decent mileage around town, or doing 50% spirited driving. i know it would be hard to just do all freeway driving at 60 mph with never any more throttle than needed to keep the speed up, but i'm sure a properly tuned SC would get low 20's if you did, with the sensor disconnected.

[fred cook]

I have the O2 sensor unplugged on my '80 SC coupe and the injection mixture set at 2.5% CO. In town it gets 17-18 mpg, on the road it gets 25-27 mpg. On the interstate I can cruise at 75-80 and still get the 27 mpg figure. Bottom end torque is good and the engine pulls well to red line.

Fred Cook
'80 911SC coupe