DIY fuel flow/injector gauge results
 

See https://www.gassavers.org/showthread.php?t=7345 for the equipment/hookup I'm talking about. Briefly, I'm using a dwell meter to measure duty cycle of my injectors, effectively telling me exactly how much gasoline is in use at any given moment.

Here's what I've learned in a short drive with my 2002 GMC 5.3l v8:

- Gas usage really DOES change a lot with RPM, no matter what you do with your foot. That's not to say that the gas pedal doesn't have an effect, just that given the same gas pedal input and load, higher RPM will use more gas.

- DFCO on this vehicle is difficult to trigger, but once you trigger it, it will continue down to ~1200rpm. To trigger DFCO requires >2000 rpm and foot off the gas pedal for a few seconds.

- Neutral at stoplights has almost no measurable effect in this vehicle. In D it reads 1.1 to 1.5, in N it reads 1.1 to rare, quick 1.7. It's within the margin of error, I think.

I'm hoping to use the same setup in my VW.

That's what I noticed, too. WOT at low revs doesn't use much gas (less than moderate throttle at high revs). Then the gas rises steadily as revs rise. I think one of the best overall FE tips is to do whatever you can to keep revs low. And I think that cuts engine wear, too.

Although Honda recommends revving it up occasionally, to blow out carbon deposits.

I think with an automatic the revs (and gas use) are very low when you're in D, and the vehicle stopped. So I don't picture any gain for shifting into neutral.

It'll be interesting to hear about what you notice.

Well, extra load from having to fight the transmission through the torque converter is supposed to use a little extra gas, hence the traditional tip for automatic drivers to shift to N at stoplights. I suspect that other vehicles will differ. Some probably idle higher and push the TC more, and N would help. Others probably idle lower in D and higher in N and could be worse off in N.

I wonder if maybe that traditional tip is sort of an old-wive's tale, and not really grounded in fact. Or maybe it makes sense on a carb engine, but not an EFI engine.

It seems to me that putting drag on the engine via the TC does not in itself consume gas. What counts is how much the injectors are open.

The point I'm making has to do with the confusing fact that "drag" and "load" don't mean the same thing. The former means outside forces that try to keep the engine from spinning. The latter is a measure of how much air/gas mix is moving through the engine, as determined by the position of the throttle plate.

Drag and load together are what determine actual RPM, at any moment.

Anyway, extra drag imposed by the TC won't cause higher fuel consumption unless the ECU is deciding to keep revs up by operating at a higher load (i.e., greater throttle opening).

Okay, my previous findings were from a test drive. I was able to refine them after a real drive. I was a little bit off on all my observations, and I will probably find that to be more true as I drive with it more. Here's yesterday's modifications to my previous observations:

- GPH vs. RPM: Not quite as bad as I thought, but still more true than most people who have really thought about it seem to believe.

- DFCO: I observed it happening a couple times where it started closer to 1500 rpm. I realized that it takes a long time to come on, and previously I thought it wasn't coming on below 2000 because I didn't have the right opportunities. I counted approximately 6 seconds for DFCO to come on.

- "N" while stopped: It's worth a little bit, the reading hunts around a little but definitely averages slightly lower.

So, for that model year and engine, so far I recommend P&G or at least N whenever reasonable. For FE purposes, engine braking should be avoided unless you can do it for a decently long time. 10 seconds of engine braking will cost more of it keeping the fuel up with RPM than it will save in DFCO. Going down a long hill (maybe >1/2 mile) and you want to keep your speed down, fine. For short rolling hills, use N.

Cool, got the VW wired up for it too, but I probably won't be driving it until Wednesday.

After another more complete day driving the truck with it, I've been able to pay more attention to the actual numbers.

At highway speeds, coasting in gear uses twice as much gas as neutral (and, additionally, slows the vehicle down of course). 2.7 to 2.9 % duty cycle vs. 1.1 to 1.6%. This is even with my truck's tall gearing and GM's general freewheeling torque converter. (I was going to provide a link to someone else's post about that but I can't find the post now. Anyway, he said that when you let off the gas in a GM with an automatic it coasts a lot better than other vehicles.)

I'd like to compare driving strategies and transmission differences between my truck and my VW, and I think FI duty cycle will be a pretty decent way to compare. I was thinking that I'd like to measure how well it stays in gear compared to very wide throttle in high gears in the VW.

Interesting news

Today I drove with the meter in my 2008 Volkswagen Rabbit 2.5l 5 speed manual. Here's some very interesting data:

- DFCO: Once in DFCO mode, it will ride down to 1000rpm at any speed. However, entering DFCO is another story. Above 60mph, it's instant and dependable. 40 to 60, it's very quick (maybe takes 1.5 seconds, and it doesn't hold steady in that time but drops off pretty sharply) and dependable. 30 to 40 it takes a few seconds and is not entirely predictable.

- Gas pedal input: With DBW, the gas pedal position is NOT related to the throttle position, as I observed using VW-specific Vag-Com hardware and software. That said, 100% gas pedal (flooring it) actually uses less fuel than 90%. If I floor it, then back off a little bit, injectory duty cycle jumps by an absolute 2% (or by 10% of previous duty cycle) -- so, if duty cycle is reading 20%, it jumps to 22%. (What's the proper way to express this change in percentage clearly?) OTOH, if I am at 90% first and then floor it, fuel rate goes down.

- ...I know I had a third interesting observation but I no longer remember what it was. :(

Are you using the HF meter? Where are you attaching the leads, under the hood or at the OBD port?

I am using the HF meter (which is now on sale for $23 in-store...I wish I could justify investing in a couple more). I tapped a fuel injector wire right at the injector and extended it into the dash, which is where I attach the positive lead. The negative lead is grounded at a power outlet.

I would have tapped the FI wire at the PCM (ECM, ECU, whatever your manufacturer calls it) but in both vehicles it's too difficult to do that way. In the GMC I can't get to the well-protected wires at the connector, and in the VW I can't get to the unit without disassembling way too much car.