Focus Zetec bad TPS = better econ

[LocoJason]

Just figured I would share...

A few months ago, my TPS developed a dead band from about 25-50% throttle. This is fairly normal in higher miles cars and generally sets a P0122 code. As I use a scangauge I noticed almost immediately that when the TPS value fell off, my fuel economy jumped up substantially. I was very skeptical at first (as I'm sure you are), but verified after several tank-fills that the increase in economy was real.

The only conclusion I could come to was the ECM did not disregard TPS input despite setting the code. So there's a corresponding reduction in calculated load and most likely a notable fuel cut taking place because the ECM 'believes' the driver has let off the throttle. Ignition timing advance would also be reduced from ~25 to ~15 degrees BTDC when in the dead band :-( The end result while steady-state cruising the dead band, it gained 3-5 MPG according to the scangage and almost 3MPG overall per fillup.

So I decided to wire a small switch into the TPS so I could manually trigger the dead band at any point. Not surprisingly, at most higher speeds and loads there was a substantial reduction in power and a lazy feeling to throttle inputs. Still, my fuel economy was up and I didn't have to spend anything...

I do not have any instrumentation on the car such as an EGT thermocouple or a wideband O2 sensor, so I can't confirm the amount of fuel cut taking place or the increase in EGT. Running lean and with less ignition timing advance raises EGT substantially. Still having a working catalytic converter, I am mostly concerned with damaging it due to the high EGT's so I plan to install one. I've been driving this way for a month now and I have had no issues yet. I wonder how many other vehicles have logic that allows this to be a 'mod', essentially enabling a lean-mode for better fuel economy.

[Joggernot]

The program is to squirt a bit of extra gas when you start acceleration. Do you suppose that with the "broken" TPS it isn't giving that extra squirt of gas? The purpose of holding the TPS constant is to keep from squirting gas every time you up the TPS. This alone could give you better gas mileage.

With naturally aspirated cars we would disconnect the small pump attached to the throttle to prevent the extra gas from going in. Acceleration had a "hesitation" before coming up because the vacuum had to get the gas started, not the pump. No problem when in the country.

[LocoJason]

It's a good thought, thanks. I don't think it's entirely related to 'accelerator pump' or acceleration enrichment from tip-in because it's generally taking place during steady-state cruise. IE if you hold throttle angle constant and kill the TPS signal, it still cuts fueling slightly. Lack of acceleration enrichment is likely still a factor in the overall fuel economy increase though, as the enrichment won't take place (or is diminished) without seeing the sudden change in throttle angle.

I've worked on a few older systems without any pot in the TPS; they would look at sudden changes in MAF/AFM delta voltage over a timeframe for 'accelerator pump' behavior. This system seems to only use the TPS.