Just thought of this tonight..... (while coming down the mountain with my foot mostly off the gas)


Most fuel injected engines cutoff the fuel if the TPS sensor indicates idle and the engine is above a certain RPM.

But if your foot is off the gas, you get engine braking.

What if you could fake out the system by telling the ECU the throttle is closed when it's not.

The fuel would be cut off, and the throttle will vary the amount of engine braking (closed throttle the most, WOT the least).

So you end up using the throttle to control downhill speed without any use of gas........

Some TPS's have a separate contact for idle position (which would make it easy to tell the ECU your foot is off the gas), with the throttle vane position.

So I'm wondering if the idle position sensor overrides the throttle vane position. (Will the fuel be cut off if the idle position sensor is on and the throttle vane position says it's not)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

That would depend on the ECU type. On some, it might. Many systems don't have a separate closed throttle switch.

Another possibility would be to add a solenoid valve that opens at closed throttle when the fuel cut is active, only when it's active.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

That would also depend on the vehicle. My fuel injected conversion van weights in a just shy of 7000lbs. There is no engine braking, in fact it accelerates going down inclines!

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Chris'
Autobody
Restoration
Service

Factory systems with idle deceleration fuel cutoff also do things like cycle the fuel on periodically to keep engine temperature and catalyst temperature from falling too much. On my Jeep, you can actually see the temperature gauge drop below normal on a long downgrade, and if you pay very careful attention you can feel/hear it turning the fuel on and off to prevent the temp from dropping further. Its a pretty neat feature, but I'd be leery of rigging one up so that it could leave fuel completely off for extended periods because the thermal cycling stress on the valves- particularly the exhaust valve- might be excessive.

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