14 MPG...what?!

UTKarmann_Ghia · 03-12-2003, 10:53 AM

Ok, when I bought this car a few weeks ago and drove it back from Little Rock, AR, it was getting about 24mpg on the highway back to Denver. Since then, I've gotten pretty poor gas mileage to the tune (no pun intended) of about 14mpg. I'm thinkin this kinda sucks. I've replaced the o2 sensor and hooked that system back up. I'm getting all of the tuneup parts in and such, and next week I'm going to get the mixture adjusted for emissions. Now, it has been very cold in denver (at times around zero), but the car has run FLAWLESSLY during this time. Even at startup it's great. Will a simple idle mixture adjustment bring this back to mid-20's as I think it should be? Any ideas?

rsr808 · rsr808's Garage

That's pretty standard gas mileage for a 911... 15 street and 25 highway.

Slider79SC · Slider79SC's Garage

Plus we have the good ol crappy gas additives that affect it during the winter months in Colorado.

speeder · speeder's Garage

Yeah, if you got 24mpg on the highway, it's running muy perfecto. 14 in the city is as good as I ever got w/ my stock '82, and about 22 on the highway, but I drive fast.

UTKarmann_Ghia · 03-12-2003, 11:18 AM

Now I feel bad; I've been rippin the SUV owners for their gas guzzlers

Oh well, I guess I didnt buy the car for its mileage.

cab83_750 · 03-12-2003, 11:21 AM

Don't forget to disconnect the O2 sensor after your smog check!

UTKarmann_Ghia · 03-12-2003, 11:23 AM

Ok, I've heard some conflicting "opinions" about the whole o2 sensor thing. Whether to disconnect or reconnect. What's the beef? Can anyone give me good arguments for and against?

Slider79SC · Slider79SC's Garage

Try running around with a short geared tranny and then watch your gas go away fast. My 5th is everyone elses 4th.......

dean · 03-12-2003, 11:37 AM

I averaged 19 mpg with my SC. 14 mpg sucks. It should do better than that. With the 930 I can't get over 14 mpg.

Rick Lee · Rick Lee's Garage

Some cheap insurance would be to have a pro check your mixture. Running rich and letting it go too long is what killed my last 3.2. My new one has about 400 miles (since install) on it and I've gone through almost two full tanks of gas. Took it in for a check and - voila - running very rich due to bad AFM. I caught it early enough this time. It's less than one hour shop labor to confirm your mixture is right. Learn from my mistake.

UTKarmann_Ghia · 03-12-2003, 11:53 AM

Thanks, next week I'm taking it in a having them check the mixture. And living in Denver, the mile-high city, almost everything runs rich.

Anybody wanna get on a soap box about the o2 sensor? Anybody, anybody?

masraum · 03-12-2003, 12:37 PM

Hm, my '88 runs around 20-25 on the highway depending upon my foot, and then has run anywhere from 14 to 19 around town depending upon my foot

masraum · 03-12-2003, 12:38 PM

Personally I say leave the O2 plugged in.

targa80 · targa80's Garage

The O2 sensor when heated sends a signal to a Lambda control box. The lambda control box will depending on the signal it receives will control the duty cycle of the frequency valve. The frequency valve adds fuel to the control valve in the distributor this effects the fuel flow to the fuel/air mixture. This is occuring all the time to maintain a supposedly perfect mixture ratio of Lambda 1. Disconnect the O2 sensor and it goes into an open loop mode and the frequency valve is either at 50 or 60% duty cycle for all conditions. I can't remember which off of the top of my head. At the bottom of the page on my web site are 2 files one is CIS theory and the other is CIS troubleshooting. in pdf format. They might help you with the understanding of the system .

CamB · CamB's Garage

Don't feel too bad about excess consumption. I've got a '78 Jag which I only drive around the city/motorway and I get.... wait for it... approx 12mpg if I'm lucky.

And it is tuned as lean as it will (safely) go...

speeder · speeder's Garage

Well, this could start a real food-fight around here, with several members, (including John Walker), telling everyone to disconnect the O2 sensor on these cars and set them rich. I disagree, one of the reasons for disconnecting is to solve a running problem that I've never had, (bucking under steady load), and guess what? The setting where the car is getting best gas milage should be where it runs best. (Most complete/efficient burn of fuel). If you are running richer than optimum, you are puking unburned fuel out the tail pipe.

If all of the CIS components are functioning properly, and there is no other motor or tuning problem, a stock SC should run great w/ lambda, (O2), hooked up+ get best gas milage since this system micro-adjusts mixture while running. I will concede that the optimum mixture setting may be slightly richer than the factory(emission) spec., which is .08%.

The benchmark for me w/ my SC is that it gets good gas milage and runs excellent on 87 octane, as it was designed to. Gas milage is important not only from a $$ standpoint, (especially now), but also tells me how efficient my power plant is. I am in the process of rebuilding/resealing my motor right now and installing a more efficient exhaust system, ('74 style w/ SSIs), the exhaust is a win-win upgrade, performance+gas milage. Making car lighter is another one.

BernieP · BernieP's Garage

I have a 1980 Euro SC (no O2 sensor) I'm in Pittsburgh and I drive mine every day. I have noted a dramatic drop in MPG when the weather is cold. (Below 20), it comes back as the weather warms up.

Targa Dude · Targa Dude's Garage

Would the high Altitude have anything to do with your gas milage?
Just a thought..

Jorge (Targa Dude)

stlrj · 03-12-2003, 01:02 PM

On paper it sounds good but in reality the Lambda system only works as advertised when the base mixture is set to it's correct lean spec, otherwise the system will not be able to compensate for the over rich mixture induced by the high Colorado altitude and crappy mileage will result. CIS was never famous for altitude compensation and with the Lambda system on board, having the mixture on the money is even more critical.

Cheers,

Joe

UTKarmann_Ghia · 02:03 PM

Yep Targa Dood, the altitude is definitely a factor and so is the crappy additives they put in the fuel in the winter time. Cold weather over the last month didnt help either I'm sure. I was expecting 20mpg+ overall and was concerned when I was gettin real crappy mileage. But from the sounds of it, it's typical.

I was just being an instagater by asking the o2 question and trying to solicit some opinions on the matter. What I'm taking out of this is 1) I have no bucking problems with my car with the o2 sensor pulgged in so it's fine to keep it plugged in, 2) especially at my altitude, I want to have the o2 sensor to make the appropriate mixture adjustments, 3) my gas mileage is typical of a 3.0 911SC.

p.s. Targa Dood--that is one FINE ride! /me is very impressed