ScanGauge Questions for a VW Diesel.

[Deezler]

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:
Deezler's 2003 Jetta TDI Build Thread

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!

[tasdrouille]

The Scangauge uses the MAF signal for the MPG calculation. To prove it unplug your MAF and look at you instant MPG, it'll be really high. It also uses the IQ value, as you noted from your tune.

I would suspect a failing or low reading MAF or a bad connection. Some people fill the MAF connector with dielectric grease and that resolves the problem. But your car was apparently pulling smoothly so I don't know.

The timing value is relative to BTDC. A value of 3 is BTDC. Minus values are ATDC.

[Deezler]

Tas, knew I could count on you for a helpful reply. Thanks. Someone else on tdiclub echoed your thoughts about the scan gauge using MAF readings for mpg calculation.
Air Intake Upgrades and MAF questions. - TDIClub Forums

I switched the MAF sensor to the one I received with the 3" MAF Housing I ordered from vwvortex classifieds (from VR6 gas engine). I didn't expect the sensor to come with the housing, but the guy threw one in for some reason. Figured it would be d.o.a. if he just threw it in, but thats what I am still running and the engine runs awesome.

New problem today though, is a P0102 CEL code on cold startup (MAF reading low). If I clear it with the scan gauge it doesn't come back until the next startup. The car is pulling crazy strong and idles super smooth. I like the way it runs a lot. Not at all sure how my mileage has been affected though. If I get a MAF low trouble code, then the sensor must be under-reporting a low air flow number, but the acceleration has increased! Thats cool.

Driving to SAE World congress in Detroit this morning, it read 109 mpg. Too bad its a completely fictitious # though, since I observed the fuel gauge decline noticeably once I made the full round-trip.

[Deezler]

Oh and thanks for the timing info. phew. I normally see values of around 1 to 5 during highway cruising. If that was ATDC I'd have some serious fuel waste going on. WOT through 4th gear shows up to 16. Sounds about right.

[tasdrouille]

So you got a MAF from a VR6, it probably is operating properly, but not at the exact voltage the TDI ECU is expecting for a given airflow.

It might be something like the 2.0 MAF, but reporting lower voltages instead. See http://www.geocities.com/tech4tdi/pages/maf2l.html

The weird thing is that if you're reading low, fueling should be cut back. But then you got a tune so who knows what the new maps look like but the tuner.

[Blackbomber]

Correct me if I'm mistaken, but I was under the impression that when injector nozzle size in an ALH is altered, the ECU no longer has an accurate knowledge of the ammount of fuel going through the engine. Regardless of the MAF used, it's my understanding that calculating by the tankful will be your only reliable means by which to measure consumption. Of course my understanding does not factor in an external measuring device (i.e. Scanguage). Somone straighten me out, please.

[Old Tele man]

...interestingly, according to the "data" tables, the stoichiometric A/F-ratios are the same for gasoline (14.7:1) and diesel#2 (14.7:1) fuels, so MPG-calculations based on air-flow rates only should be almost the same.

[dcb]

Problem is diesels rarely are running in stoichiometric. Usually they run "lean" as the compression ignition will ignite just about any fuel in the chamber regardless of the ratio to air.

[rmay635703]

I have talked to an old mech and he says that reading maf on a diesel is the most useless method of determining fuel consumption he could think of. He said that would just make random numbers that have no bearing on anything.

He said most diesels (especially older non-turbo models) have a static fuel amount directly tied to the throttle position and RPM assuming of coarse the motor is warm.

If this is true why isn't the fuel rate computed using a different method such as by displacement, RPM and throttle position? Most OBDII diesels have TPS right?

Would this be possible to impliment?

[mikeross]

I could not agree more with you on that.. Why don't they implement that kind of computation