MDL

I've never done the math, but the faster it's burning fuel the better I like it.

Pavegeno928

Thanks for the inputs guys. Much appreciated.

I'm not really worried about the gas mileage as far as price goes...just want to make sure there is not an underlying problem that could serve to shorten the life of the engine and possibly increase performance.

old man neri

Some of us drive more than others.

Jay H

My 1984 3.2 Carrera came with full records from new. The first owner wrote down every fuel fill up since he took delivery in 1984. He averaged about 22 mpg during his ownership of 85,000 miles. I have continued his practice and write down every fill up as well and check the mileage. I'm getting 23-24 mpg on average with reformulated fuel with up to 10% ethanol. I've gotten as good as 26 mpg on a long trip with the A/C on during the summer months. Most of my driving is probably 50% to 70% freeway commuting. I do not have a cat on the car at this point, but the car has a fresh O2 sensor and is in good tune as far as I can tell.

rusnak

This thread got me thinking, "why is there such a variation of rates of fuel consumption for 3.2 911s"?

So I looked at "Bosch Fuel Injection and Engine Managment". There is a graph that show the effect of aging/depletion on the O2 sensor. Clearly, it shows a lower voltage is sent to the DME as the sensor ages, which would make the DME think that the mixture is leaner than it really is. The mixture would be richened, and that would hurt mileage. I think some 911s with poor mileage and stock motors might need the exhaust gas to be checked for correct fuel air ratio. I think a fuel air check, along with the lean stop and rich stop tests would confirm whether the O2 sensor is healthy.

Here is an article from Wikipedia, and the graph from the book:

http://en.wikipedia.org/wiki/Oxygen_sensor

Sensor failures
Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles (50,000 to 80,000 km). Heated sensor lifetime is typically 100,000 miles (160,000 km). Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burned off during operation and failure occurs due to catalyst depletion, similar to the reason a battery stops producing current. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the mileage worsens.

Leaded gasoline contaminates the oxygen sensors and catalytic converters. Most oxygen sensors are rated for some service life in the presence of leaded gasoline but sensor life will be shortened to as little as 15,000 miles depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty.

Another common cause of premature failure of lambda probes is contamination of fuel with silicones (used in some sealings and greases) or silicates (used as corrosion inhibitors in some antifreezes). In this case, the deposits on the sensor are colored between shiny white and grainy light gray.

Leaks of oil into the engine may cover the probe tip with an oily black deposit, with associated loss of response.

An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel rationing system.

Applying an external voltage to the zirconia sensors, e.g. by checking them with some types of ohmmeter, may damage them. [1]

Symptoms of a failing oxygen sensor includes:

Increased tailpipe emissions
Increased fuel consumption
Hesitation on acceleration
Stalling
Rough idling

rusnak

I'm such a retard sometimes.

I forgot that I have this article from Car and Driver.

The '84 911 was reported to get EPA 20 mpg city, 32 EPA highway, and 24 EPA combined. Car and Driver observed 17 mpg....hahahaha.....lousy leadfoots.

Pavegeno928

I'm hoping the O2 sensor change has some impact. I doubt it has ever been changed. I have spent the last three years restoring the car since I bought it. It was in great shape when I bought it so really I have been able to spend my time and money on the little stuff, which can make a huge difference. Though I should have done it sooner, I just didn't think about the O2 sensor until recently. Next project is a new stereo system and reupholstering. Restoring back to Fuchs wheels will happen after my current set of tires needs changing.

I really appreciate the great inputs from everyone.

I put in a Griffith's Air conditioning system this past winter and am looking forward to reporting on it after the summer for everyone. So far it is working great but the summer heat and humidity of Florida should test it thoroughly.

Geno

Pavegeno928

UPDATE...I took the car to the dealer to replace my O2 sensor and I was surprised at what they told me. I watched them lift the car up and look around for a bit. Than another tech came over and than another so there were three guys looking at my car. I started to get a little worried. After a little bit they came over and told me that there is no O2 sensor on my car. It looks like I have a European exhaust is what they said. I've only owned the car for three years but I have records going back to when it was new and haven't seen anything in there about a new exhaust. I guess that could explain the low fuel mileage but I'm not completely convinced. I guess when I get home I'll check in with my regular mechanic. Any thoughts?

rusnak

You have a 911SC? The O2 Sensor was added to the SC, but the early ones did not have one. Euro or not makes no difference as to whether you have an O2 sensor. Can your mechanic check the air fuel ratio with an exhaust analyzer?

BReyes

Mostly city driving more than highway would, and if you are on US1 going north all the way to say downtown would. Solution more peaceful highway miles.

old man neri

I do believe that he has an 88.

Maybe someone just unplugged it when the new exhaust went in. I bet you the connection is still there. Maybe you can weld in an O2 bung and just connect it back up? I really don't know.

Pavegeno928

It is an 88...sometimes the dealerships don't have a lot of experience with the older cars...there were about 50 newer cars in their service department and the mechanics were young guys. I had a great mechanic in Destin, FL but I think he has left the dealership and moved on. The quest for perfection continues...

rusnak

Pav,

Your system is operating in "limp home mode". It is going to be running very rich, and it will overwhelm the catalytic converter, which is not cheap to replace.

I think you should look into doing what Neri suggests. You can buy a steel O2 bung and have a muffler shop weld it in. I'd check the cat first though to make sure it is still good. It's very cheap to add a bung to an exhaust system. After the O2 sensor is replaced, it is a good idea to check the CO level and adjust if necessary.

Pavegeno928

Thanks for the great advice...I will take care of it when I get back home. I'll give you all some feedback down the road.

old man neri

You may not even have a cat, you may just have a cat bypass that doesn't have an O2 bung.