seagull_cqx

Hi, everyone. I know that no 911 qualifies as an economy car, but my C2 is absolutely guzzling premium these days: roughly 10 mpg in mixed driving. It's not throwing any codes (save P1691, but I think that's just a wire to the CEL), the only mod is a Fabspeed cold air intake, and it certainly doesn't smell like it's running rich, but man it is just going to town on gas. Any ideas what I should tell my mechanic? Thanks!

Icelia

Is that what the computer is saying?
might want to pull the plugs and look at them. You might have a leaking injector.
you can also test using a fuel pressure gauge. Pump pressure up and then see how long it holds. Does your oil smell like gas?
don’t drive it until fixed if so.

chris8382

In addition to checking the internals as mentioned above, you may also want to check for a fuel leak. If small enough, and at warmer temps, you may not notice the leak from underneath (as with an oil leak leaving a stain). Same idea, build fuel pressure and then watch for it to drop.

Regardless, I agree with your instincts….10mpg means something is off.

seagull_cqx

Sooooo, allow me to amend my statement. It dawned on me that I never checked the OBD reader after today's drive, so I just plugged it in a moment ago. Here's what I got:

1) P1691 - check engine light fault
2) P0455 - fuel tank ventilation system (major leak)
3) P0420 - catalyst system efficiency below threshold (bank 1)

Like I said above, P1691 has been there forever. P0455 has cropped up before but clear the codes and it goes away for a while, plus it predates this 10 mpg nonsense. But P0420 is a new one. Now, it was me who installed that Fabspeed CAI; maybe something went wrong there? I wouldn't put it past me.

On the plus-side, everything smells fine.

Porschetech3

The 02 Sensors are what controls the fuel mixtures on all modern cars... THAT IS THEIR JOB !!

They control the air/fuel mixture to 14.7 AFR .( also known as Stochiometric , also known as Lamda 1 ) ...during all cruising and hot idle..virtually anywhere gas mileage will be effected ( except during WOT= Wide Open Throttle) ..IF they cannot control the fuel mixture to stay in the parametrs ( their function is monitored), a fault code will be stored . BUT they can be lazy and slow and still keep the "average control" within the parameters ..

The Federal Mandate was for them to last 8 years/80K miles, and with the reduction of ZDDP in all "new oils" they are suppose to allow the Cats and 02 Sensors to last for 120K miles. If your engine consumes more oil than the average they can die a LOT sooner..

Sounds like you need new 02 Sensors and Cats...sorry ..

Carlo D

Definitely O2 sensors!
i suggest getting the Bosch 15733 as they are cheaper. Kit comes with what you need to wire them up. Very easy just need a wire stripper.

Here

seagull_cqx

Ha, wow—that same Bosch part with a Porsche logo on the box is, like, 100% more. I suppose I need four total, right? Two before catalyst, two after?

yelcab

The before and after o2 sensors are different. If your mileage is 100K or thereabout, you should (but its up to you) replace all four sensors. Bosch or Porsche sensors, it's also up to you. Do not think that Bosch sells you the same exact sensor they sell Porsche. Very often there is a limited spec range that Porsche demands for their brand.

brontosaurus

You clearly have a fueling issue. Replace the precat widebands first. I highly doubt Bosch is binning widebands for Porsche & I have had no issues with the Bosch ones on my car.

yelcab

I also drive Ferrari which use Bosch O2 sensors. There are differences between the Ferrari branded Bosch sensors, vs the Bosch sensors non-Ferrari branded. Certainly at the edge of the performance envelope. The Ferraris tend to eat O2 sensors for lunch so they don't last anywhere near 120,000 miles. More like 20,000 miles.

I specifically bought a Bosch non-Porsche branded O2 sensor for a Porsche Boxster and the ready-to-use wiring was too short. But, heh, it's your car and your money. You should do whatever you feel like doing with your car and your money.

For all important engine related sensors, I now buy only from the factory and pay the price. MAF, Knock sensors, Cam sensors, Crank sensors, and O2 sensors.

Paul Waterloo

I replaced my pre-cat sensors years back, my notes say: Bosch parts from ECS Tuning, Part Number 99360612801

I would NOT get the Bosch 15733, they are a PITA to splice in, it's so much easier with the correct part number. Best of luck.

seagull_cqx

Fun little epilogue to my tale: it was the Fabspeed cold air intake all along. In a last ditch attempt to solve the problem before spending god only knows how much at the mechanic, I cleared the codes and reinstalled the stock intake. Presto whammo, the P-car was back to 23 mpg over the weekend.

¯\_(ツ)_/¯

yelcab

Whow... a cold air intake?

Woodman71

Well, now I understand why some folks swear by sticking with OEM stuff, but yeah, weird it would do that to your mileage. Glad you didn't break the bank trying to figure it out through other methods.

wdb

Well yay! I'm all about OEM when it comes to airboxes, but that seems excessive. Yes the Fabspeed moves the MAF, which a bad thing. Yes it adds a bunch of hot air, also bad. But 13 MPG bad? That's a lot.