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I can't get consistent results on EITHER of my VWs. Both of them are wildly inaccurate depending on conditions, and in different ways:

The Golf GDI

The SG is just plain non-linear in its reporting of FE on the Golf TDI. On the highway the mpg reading is way low (38-42 mpg on trips where I'm actually getting close to 50) and in town it's way high (generally reading 48-60 when I'm clearly not doing as well). My steady state testing shows the SG consistently overstating fuel consumption as the rpms push beyond 1800 or so.

This is NOT a calibration issue. If it were, BOTH highway and city driving would either be high or low. SG is set to DieselA, of course.

Some others on the tdiclub forums have seen similar results, but others vehemently deny that the SG is inaccurate. Is there anyone here with a TDI VW who IS getting consistent, dependable numbers out of their SG, and if so what are your settings?

The Jetta (gas) wagon

This one's a little different. It actually seems to be pretty linear, and on highway trips it's quite accurate.

But around town (most of my driving on this car) it's just useless. Any change in throttle position takes 2-3 seconds or more to register on the gauge. As a result if often just completely fails to record my accelerations. Early on in a trip I'll look down at the SG after getting up to cruising speed and it will show 90 mpg. Starting from a stop I can get up to 15-20 mph and it will still be showing idle fuel consumption. Then it bounces all over the place, showing 2-3gph for a few seconds, idle FC for a few seconds, etc. Seems like it's just polling the ECU for a reading every few seconds and then holding that reading until the next one. Only after several seconds of steady state cruising will it settle down to a reading that makes sense. Likewise, when I get off the gas it continues the reading for several seconds before finally showing idle fuel consumption. Maddening. The one good thing is the LP display is relatively instantaneous, so at least I get quick feedback on that.

I've got the thing set to FAST update (also have tried NORMAL - doesn't seem to make any difference), and engine type set to Hybrid. I know this car is capable of better mpg than I'm getting (not that it's anything to complain about) but I'm driving blind.

Anyone with a gas VW either (a) experienced this or (b) NOT experience this and care to share their SG settings?

I may just have to contact the scangauge folks and beg for help.

Hi Dan:

___Here is the problem with CI-ICE’s… Their A/F ratio is variable just like the Lean-burn Insight’s and Civics. At idle, I can bet you are down near 64:1 whereas at WOT, I can almost bet you are closer to stoich. No way to compensate for that swing although I am not exactly sure what Linear does for the diesels different than the gasser’s which run at close to stoich all of the time.

___For the gas Jetta, are you swapping that thing form vehicle to vehicle as it sounds like a Bus issue. Reset everything back to default and pull it. Reinstall and see what bus it reads. Probably a default but I am not sure. 2 to 3 seconds for a change is something going on inside the data stream rather than the SG-II more than likely but I have no idea what that could be?

___Good Luck

___Wayne

My ScanGauge has been very close for my last 4 fills, but that could be because my drive is pretty much the same for each tank? Over the last 4 fillup adjustments, I changed the value by less than 0.5%.

I do agree that the fuel economy does seem to take a large hit when rpm's get above 2000. I'm not sure if it's the SG or the engine but am leaning towards it being the SG as it seems it would take a wild change in engine operation at high rpm's to cause that. There is something a bit different going on: IGN seems to increase as rpm's rise above 2000, but who knows how much of an effect that has? I have just started to try to figure out if IGN correlates at all to changes in fuel consumption.

FWIW, here are my current settings:

Fuel: Diesel A
Engine Size: 1.9L
Fillup: +1.4%
Speed: +4%

Wayne: that's as good an explanation for the SG's TDI irregularities as I've heard. I guess that's just the way it is. I think the reason I notice this more than other TDI owners is my pattern of driving: our tanks are usually EITHER all-city (SHORT 1-5 mile commute and in-city trips) or almost all-highway (400-1000 mile roadtrip every few weeks), whereas I suspect most TDI owners have a more consistent city/highway mix from one tank to the next.

Mike, I agree there's something about (relatively) higher RPM that throws the calibration off. Indicated fuel consumption climbs rapidly with RPMs (regardless of load) from 1600-2500 even though the published BFSC maps indicate a broad efficiency plateau in this range.

Wayne, thanks for the tip about resetting the SG on the gasser. I'll try it. I'm not swapping (I have a separate SG for each car), but I'll try resetting everything and see if that helps. If that doesn't help I'll swap the SGs and see if the other one has the same problem on the gasser.

At idle, I can bet you are down near 64:1 whereas at WOT, I can almost bet you are closer to stoich.


Wayne - I thought the VW's sold in the US didn't allow the 64:1 ratio due to issues with US fuel quality

Wayne, thanks for the tip about resetting the SG on the gasser. I'll try it. I'm not swapping (I have a separate SG for each car), but I'll try resetting everything and see if that helps. If that doesn't help I'll swap the SGs and see if the other one has the same problem on the gasser.Hey, thanks for the idea! I think that worked. Now I only have one car with an SG problem, and the consolation for that is I don't seem to be the only one.

Can't believe I didn't think of that. I'm in IT. Reminds me of my favorite engineer joke. Four engineers go on a ski trip together: a mechanical engineer, a chemical engineer, an electrical engineer and a software engineer. Along the way their car starts to run rough, to the point that they're wondering whether to bail.

The mechanical engineer goes, "aw crap, we threw a rod. Trip's over. Let's see if we can get back home, swap in a new engine and maybe we can try the trip again next weekend."

The chemical engineer goes, "no, I don't think it's that serious. We just got some bad gas at that off-brand gas station on the way out of town. Let's siphon it out and fill up again at a better station. No big deal."

The electrical engineer goes, "no, it's an ignition problem. We need to go back and install new plugs and wires."

The software engineer says, "look you guys are all blowing this out of proportion. Let's just get out of the car, open and close the doors, get back in and restart. The problem will go away."

I get very accurate readings with my '01 TDI. My speed correction is 3%, which comes out to 2.5 miles difference over 700. Pretty close. My FE correction is around 17%. Try those numbers and you should be close.