Slowily declining MPG

[dammad]

My 1999 C2 Cab Tip runs well, but I've noticed that according to the OBC that the MPG of my vehicle has been slowily dropping over the last 6 months. I do the same commute with it everyday, so my mileage is super reproducible. It used to average around 16.7 MPG (US), but have slowily drifted down to 15.4 MPG (US) now (regular 30 minute commute with 60% freeway). I changed the air filter (Mahler) and cleaned the MAF 1.5 weeks ago, however since it has continued to drop in MPG (from 15.5 -> 15.4 in that time).

The car runs a little rough at idle (passenger seat back vibrates a little) and the tailpipes are grey.. What else should I be looking for?

[Loren]

Did you clean the throttle body too?

[bmohr]

aren't these puppies supposed to get mid 20's on mileage? Generall speaking for hwy driving.

[RF5BPilot]

Has your area switched to mandated alcohol in the fuel? That will reduce mileage.

[dammad]

Did you clean the throttle body too?

I cleaned around the throttle butterfly 6 months ago and also the idle valve. Perhaps I should take the whole throttle body off and clean.

[dammad]

Has your area switched to mandated alcohol in the fuel? That will reduce mileage.

Yes, I believe we have ethanol mix until March (80/20?). But I would expect the MPG to drop in one go and then stay low all winter rather than continue to drop slowily over time.

Edited March 3, 2010 by dammad

[Stefan]

I had declining MPG and the problem turned out to be worn out primary O2 sensors. They didn't throw a code but replacing them caused the MPG to jump up significantly. What's the mileage on your car?

[dammad]

I had declining MPG and the problem turned out to be worn out primary O2 sensors. They didn't throw a code but replacing them caused the MPG to jump up significantly. What's the mileage on your car?

good question. 115k miles, but I changed all the O2 sensors at 90k miles.

[kbull88]

I noticed this too. I thought it was just me, I replaced my spark plugs and checked the coil packs hoping it would make a difference but it didn't. I know that California is now 10% Ethanol all year round now. All of the majors starting switching last year and now its mandatory for everybody in California to be at 10% Ethanol. Figured that it was because of the increase from 5.7% to 10% that caused my gas mileage to drop.

[Stefan]

Another possibility is that your fuel injectors are dirty/clogged. Try using Techron fuel system cleaner.

[bmohr]

anyone have info on whether or not there are alternatives to the porsche OEM oxygen sensors? 120 bucks a piece for christ Sakes is crazy for such a sensor. Also, if you wanted to switch out sensors for improved fuel economy which ones are more important, Pre- or After cat? Or does it matter in which case all 4 need be replaced to see any benefits?

[dammad]

I noticed this too. I thought it was just me, I replaced my spark plugs and checked the coil packs hoping it would make a difference but it didn't. I know that California is now 10% Ethanol all year round now. All of the majors starting switching last year and now its mandatory for everybody in California to be at 10% Ethanol. Figured that it was because of the increase from 5.7% to 10% that caused my gas mileage to drop.

kbull88: What MPG do you get these days? I figured that the Ethanol content would cause the MPG to drop and stay dropped rather than decline steadily.

Would an Octane Booster help restore the MPG/running?

[dammad]

Another possibility is that your fuel injectors are dirty/clogged. Try using Techron fuel system cleaner.

Yeap, trying that already didn't seem to help. I gave it two 'doses' to be sure.

I'm thinking that if the MPG is dropping then the mixture in the car must be getting richer (e.g. using more fuel). If it was a throttle body (too much air) problem then that would seem to lead to it running lean and thus increased MPG?

[kbull88]

I am getting 14.8 combined. I can barely get it up to 17 mpg on the higway when I take it on trips that are 60 miles or longer. I know that alot of the majors staggered up the ethanol over the last year. 5.7 -> 7.8 -> 10.0. I also changed my 02 sensors at 80k. I have not had any CEL's and I just had my 90k done without any faults.