The FINAL word on DFCO (sorta)

[ProDarwin]

Originally Posted by davelobi

Anyone ever beat this high idle at higher vehicle speed?

I made a IAC blockoff for Saturns quite a while ago. I sold a bunch on sixthsphere.com, and I also gave the drawings to someone else to make more. There are a few floating around on there if you want to pick one up.

The main reason I made it was so the car was way less annoying to shift - RPM drops like it should when you let off the gas and clutch in.

Also, given that you are in Michigan, you could run the car with an MS, or OBD-1 swap the car and tune the OBD1 computer.

[Thenorm]

complicated logic on DFSO is definately a reason why i like neutral coasting so much.

[Blacktree]

Originally Posted by davelobi

Maybe time for the warm air intake I've been thinking about trying. I'll update it I get around to it and if it makes a difference or not as far as dfco is concerned.

Anyone have ideas about this? Anyone have evidence that a wai may help here?
thanks,
dave

I already have a warm air intake, although it isn't as "hard-core" as some I've seen. I don't have the intake sucking hot air straight off the exhaust. But the air cleaner box is modified to suck in air from the engine compartment instead of outside. Combined with a partial grille block, it does increase intake temps, although not drastically.

Does that affect DFCO? I have no freaking idea. And to be brutally honest, I don't think it's worth the effort to install a stock air cleaner box and find out. But don't let that stop you from trying it yourself.

[user removed]

It was probably 25 years ago when I first listened to an injector and experienced DFCO.
The car was an early fuel injected Datsun 280 Z. They started using FI in 1975, under license from Bosch. You could hear DFCO with a automotive stethoscope while revving the engine with the car sitting in front of the main door to my shop.

Basically Nissan used DFCO in their first FI cars to eliminate the unburned hydrocarbon spike when the throttle was released. The throttle position sensor had to be at idle position and any imput would automatically stop DFCO. The early Z cars (75-78) had no catalytic converters in the Federal versions, while California versions had converters. My 76 even had no EGR system from the factory while still passing Federal emissions.

I believe the initial purpose of DFCO was to eliminate the air injection systems that were necessary to burn the otherwise unburned fuel when the throttle was released without DFCO. They even had fuel shutoff in some of the last carbureted versions of Nissan-Datsuns of the same era as the first FI Z cars. DFCO accomplished the same function as air injection had done before, but with the added advantage of 0 fuel delivery.

regards
Mech

[Blacktree]

Interesting. The Saturns do have air injection. That might explain why their DFCO parameters are so stingy.

[davelobi]

Originally Posted by Blacktree

Interesting. The Saturns do have air injection. That might explain why their DFCO parameters are so stingy.

The air injection didn't start until around 2000 on the "S" bodies.
My SL is a 1997 so no air pump.

Oldmechanic, How did they accomplish fuel cut off on the carberated cars?
I loved the "Z" cars. I had a 72 240, a 74 260 and several of the 280Z's

[user removed]

They put a solenoid in the idle-low speed circuit which shut off fuel flow when the throttle was closed until engine RPM dropped to a little over idle speed. The last of the NAP-Z 4 cylinders (200SX and trucks) had a heated screen in the carb to help with atomization (early mid 1980s). Also a "gulp" valve that had reed valves to allow air to pass through the engine when decelerating, without any fuel.

regards
Mech

[cptsideways]

Most older throttle cable efi engines have a throttle damper, I believe for emissions & something to do with reducing cylinder temps slowly.

These are a simple device attached to the throttle body that dampens the closing action. They are dead easy to adjust for a faster closing action. Might be worth have a play.

[davelobi]

Originally Posted by ProDarwin

I made a IAC blockoff for Saturns quite a while ago. I sold a bunch on sixthsphere.com, and I also gave the drawings to someone else to make more. There are a few floating around on there if you want to pick one up.

The main reason I made it was so the car was way less annoying to shift - RPM drops like it should when you let off the gas and clutch in.

Also, given that you are in Michigan, you could run the car with an MS, or OBD-1 swap the car and tune the OBD1 computer.

ProDarwin, what did the iac cutoff do to your throttle plate closed idle? I also hate when it revs up a little between gears, small but a waste of fuel and the rpm will be lower when next higher gear engages anyway.

Also what did you mean about running my car with an ms?

[Arragonis]

My Aygo dropped into DFCO with no throttle at any engine speed over 1400. It would then allow RPM to drop to about 900 before engaging the idle circuit.

At idle it would rev at about 0.85K rpm.

If I selected a gear (1st or reverse) at standstill the RPM would raise to about 0.95K which I assumed meant the tiny engine would not stall when drivers pottered about a car park and would also not stall when pensioners held their steering wheels hard over causing the PAS pump to draw extra energy from the engine.