Well, I've had my scan gauge for a while now and thought I was pretty familiar with it. The last couple days its been acting crazy (for mpg readings), and it is obviously due to my own actions. This has led to a couple questions, so hopefully you guys can help me out. The car is a 2003 VW Jetta TDI (ALH engine code).
Heres what happened. I originally got the mpg readings pretty well tuned to actual mileage results. Then I got new injector nozzles and had the engine calibration re-tuned accordingly. The fueling duration was reduced over most of the engine operating map, and the mpg readings went up about 20% accordingly. Not surprising. Being a fairly lazy individual I left it this way and continued to monitor my mileage day to day.
Last weekend I did the following mod:
http://ecomodder.com/forum/showthrea...html#post99025
What I didn't mention in that thread is how I had to swap out the mass airflow sensor (MAF). This undoubtedly changed the engines perception of how much air flow was entering the engine. But the engine was running great so I decided to stick with it and see how it ran for a few days.
Day one with new airbox/intake : Drive to work reported an avg of 44 mpgs. Terrible seeing as how my inaccurate, but repeatable, scangauge readings were in the 58-62 mpg range before the mod. Heavy heavy downpour of rain has me thinking that it could be real, and perhaps new MAF was causing car to overfuel.
Day two (today) with new airbox/intake : Same drive to work and back shows 98 mpg. What the heck? Dry, calm conditions. Only difference was (perhaps) the sealing of a small intake leak
after the mass airflow sensor.
So, finally, my questions:
- I thought the ScanGauge metered mpgs based upon fueling alone? Doesn't it read the injector pulse signals from the ecu? Or does it also use MAF readings?
- If this is the case, then changes to reported air flow could affect the engine calibration and thus fueling amount slightly, but not cut it in half. Thats just physically impossible, as I was driving the same way, maintaining highway speed at 70-75 mph. Turbo boost was comparable, meaning that it would simply take a given amount of fuel to produce the power necessary to maintain highway speed....
- Side question: "Timing" for a VW diesel is not spark timing (obviously) but rather main injection timing. WHat is the unit on this value? Degrees before top dead center, or degrees FROM top dead center? I can't figure out if a display value of 3 would mean 3 degrees before or after TDC. My only theory right now is that bad airflow metering could affect injection timing, and thus power production, leading to same engine power with different fuel consumption. But clearly 98 mpg is just wrong.
- Final question: If the gauge keeps reading 90+ mpg, what should I do? Just unplug it and start over?
The weirdest thing is how awesome the engine is running... if there was an apparent drive-ability problem this would make more sense.
Thanks a LOT to anyone who's read this far and has anything to offer!