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Old 07-28-2008, 01:47 PM   #1 (permalink)
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Difference in fuel between 3/4 and WOT

First time posting, have been lurking here reading for a bit.

I have a '97 Geo 1.0l 5spd, dancing at 50mpg right now, trying to get past it with more driving technique.


Without a vacuum gauge to see for sure, I'm curious...

Say your going up a hill, 5th gear, WOT and your just holding.... lift off the pedal a little bit, it does just the same, still working it's butt off to maintain.

With Fuel Injection, is there really any more fuel going through at that point between almost and complete WOT when under such a load? No change in RPM or power noticed.

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Old 07-28-2008, 01:55 PM   #2 (permalink)
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WOT will dump in more fuel under those conditions than near WOT will, partially because the car runs open-loop at WOT. I could videotape my MPGuino, if I had a camera...

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Old 07-28-2008, 02:34 PM   #3 (permalink)
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If the additional throttle input kicks your engine into open loop mode you will use more gas. How each engine operates is going to be different though.
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Old 07-28-2008, 02:50 PM   #4 (permalink)
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Thanks for the replies... I did a search and read up on "open loop"... now that I know what it's called, it's a lot easier to find the other threads on the subject.

So is this done on the Geo by means of a Throttle Position Sensor? Since there is no way to know when you have entered open loop, when climbing, once you feel it not gain any more, any more pedal applied is a waste.... yes?
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Old 07-31-2008, 11:09 AM   #5 (permalink)
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I don't know how it's done on a Geo, but most fuel injection ECUs compute the engine "load" and above 80% load for a given RPM it will enter open loop.

At very low engine speeds engines with large-ish throttle bodies can exceed 80% load with very little throttle input, but at high speeds such load may require full WOT or nearly full.

I doubt the TPS is the sole factor determining when to cross the open loop threshold. A speed-density ECU will use a MAP sensor to determine load and when the pressure in the intake manifold begins to near atmospheric (less vacuum) it will go open-loop. Other vehicles will use a MAF sensor and when a computed value of airflow is exceeded at a given RPM the ECU will know that the load is high and will enter open-loop. Both systems might have an additional requirement that a minimum throttle position is exceeded in addition to the above requirements, but that may or may not even be there or it may vary depending on engine speed.
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Old 07-31-2008, 11:53 AM   #6 (permalink)
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A scangauge can monitor open/closed loop mode for you.
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Old 08-06-2008, 03:03 PM   #7 (permalink)
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Ditto about the Scangauge. I think it is right around 75% where it goes open loop.

I usually make a little run before going up the hill, then actually back off the throttle gradually on my way up. Do more Pulse and Glide if you aren't already, and extend your coasting to the stop signs. Slow down to 55 mph or less. That should break 50 easy. A Scangauge will show you a lot and IMO worth the investment.

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