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THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCH02 996 C2 - OBC MPG and Real World Discrepancy
Leading reported evidence
Check odometers (main and trip) and they appear
Performed · outcome unconfirmed
No successful repair reported for this finding.
0
Reported fixes
0
Unsuccessful attempts
1
Discussions
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Performed · outcome unconfirmed · Original poster
I have checked both odometers (main and trip) and they appear to be accurate around 0.1 miles.
Leading evidence: 'Check odometers (main and trip) and they appear' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, the most frequently mentioned possible cause is 'fuel sender gives the computer inaccurate information' (1 discussions; 0 report identifying it and 1 suggest it). The most frequently discussed action is 'Check odometers (main and trip) and they appear' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Performed · outcome unconfirmed · Original poster
There are brand-new O2 sensors installed as of today and the battery was disconnected for quite some time with some other maintenance.
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Performed · outcome unconfirmed · Original poster
A few choice words and about an hour later, the vacuum lines were all inspected and repaired (utilizing the hard tube to some new rubber hose).
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Performed · outcome unconfirmed · Original poster
I replaced the MAF sensor, and smoke tested the intake in multiple locations, with no indication of a leak (this was done on a cold engine).
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Performed · outcome unconfirmed · Original poster
I have replaced plugs, coils, and injectors, but am having a discrepancy between the OBC MPG calculation and the gallons in/gallons out at the gas pump math.
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Suggestion
This is very close to the optional factory size of 265/35/18 but it�s still worth checking with the gps on your phone because at 40 mph 2 mph variation is 5% and may account for some of your discrepancy.
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Suggestion · Original poster
I'm unsure if this is related or not, but I had to change injector number 6 due to a misfire from what looks like excessive carbon build up on the plug.
Discussions that performed work without a confirmed outcome
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Performed · outcome unconfirmed · Original poster
I have replaced plugs, coils, and injectors, but am having a discrepancy between the OBC MPG calculation and the gallons in/gallons out at the gas pump math.
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18 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
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I have searched the forums, but can't seem to locate a solution. I have replaced plugs, coils, and injectors, but am having a discrepancy between the OBC MPG calculation and the gallons in/gallons out at the gas pump math. According to the math at the pump (tracking each fill up with Fuelly) I am averaging 16.6 MPG of city driving. The OBC is reporting 20.6. I have checked both odometers (main and trip) and they appear to be accurate around 0.1 miles. I will/can continue to monitor fuel consumption manually, but didn't know if there were any tips/tricks to get these numbers to line up closer. I am resetting the OBC at each fill up prior to restarting the engine to ensure there is no fuel consumed prior to the reset.
Interesting problem. Short answer, I'd try resetting the DME. Porsche refers to it as "Adaptation." Given the work you said you did, you should do the adaptation anyway.
This is from the 7.2 DME manual:
Adaptation The DME control module must perform a learning and adaptation routine if– The battery was disconnected– The DME control module connector was unplugged– A new DME control module has been installed– The throttle part has been replaced.
To initiate adaptation:
1. Switch on the ignition for 1 minute without starting the engine. Do not press the accelerator. 2. Switch off the ignition for at least 10 seconds.
The following conditions must be fulfilled, otherwise learning is not possible:– Stationary vehicle– Accelerator pedal angle < 0.8 %– Battery voltage > 10 V– Engine temperature between 5 �C and 100 �C– Intake air temperature > 10 �C
From my experience your the MPG calcs do not adjust for long term fuel adjustments. The top suspect for this descrepancy would be your air mass sensor or/and vacuum leak.
I replaced the MAF sensor, and smoke tested the intake in multiple locations, with no indication of a leak (this was done on a cold engine).
I do have aftermarket Fuel Injectors, and I wonder if they are causing the issue as I can't seem to get a stable idle.
There are brand-new O2 sensors installed as of today and the battery was disconnected for quite some time with some other maintenance.
I am going to fire the injector parts cannon at it, just to eliminate that variable (I know its a costly parts cannon).
I am also replacing:
Variocam Solenoid for Variable Cam Adjuster (Valve) x 2
Solenoid for Variable Cam Adjuster (Camshaft) x 2
Engine Coolant Temperature Sensor
and replacing all vacuum hardlines with their rubber equivalent to make sure I don't miss anything.
There is some wetness around the solenoids, so it seems like the right time to do them.
May have nothing to do with your issue, but my mpg calculations changed significantly after replacing my fuel sender. The new sender change also resulted in my estimated miles to fill up to be off by 60 miles. If I continue driving after the gauge show 60 I would run out of gas. I suspect the new fuel sender gives the computer inaccurate information on the amount of gas in the tank.
Changed the starter (the old one was squealing every third start or so, and the date code indicates 2020, so not impressed on the life of that Bosch unit) and found a broken vacuum line near the SAI. A few choice words and about an hour later, the vacuum lines were all inspected and repaired (utilizing the hard tube to some new rubber hose).
The car seems to fire much quicker and the cold stumble I was having seemed to have disappear. Could be a placebo, but tomorrow's cold start after a few days off will be a good indicator.
I still plan on replacing the other parts though.
Stay tuned!
@Justin Browning
Silly question but what wheel and tire package are you running? My Speedo is off about 4 miles per hour vs. the gps on my iPhone and definitely impacts mpg calculations. This is with stock 17” wheels and OEM size tires.
225/40-18 up front and 275/35-18 in the rear. I have the predicted 2 mph high on the gauge, but the odometer (trip and total) seem to match against mile markers on the road.
This is very close to the optional factory size of 265/35/18 but it�s still worth checking with the gps on your phone because at 40 mph 2 mph variation is 5% and may account for some of your discrepancy. Other that that I don�t have anything else for you, good luck 🍀
No worries, I have just replaced one bank of the variocam lifters and am waiting on the bracket with integral seal for the other side. After all this is done, I'll reasses and see if it helped.
@Justin Browning
I received a $21.29 virtual payment from the Porsche Gasoline and Emissions settlement. It’s possible that the discrepancy you are experiencing is real
My 02 used to report back accurately. I rebuilt the engine and ever since, the computer is optimistic by 20%. Computer registers 25 and I calculate 19 in mixed driving. I cannot figure it out. Gas gage is accurate.
Not much help to you.
I'm unsure if this is related or not, but I had to change injector number 6 due to a misfire from what looks like excessive carbon build up on the plug.
I wonder if one or more injectors are dumping too much fuel into the cylinders and the computer is unable to see it, causing the excess fuel consumption?
I'll have to monitor after maintenance is complete and see if this solves the issue.
https://rennlist.com/forums/996-foru...6-misfire.html
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