Hey all!

Just a quick question about the fuel injectors on these cars. I have been reading on how fuel-injectors work if you supply no throttle-input while going down the highway ("coasting"). I have read in multiple places that "most modern" fuel injection systems only supply enough gas to maintain the idle RPM if you take your foot off the gas. They said that this means if you're going down the highway at, for example, 2000 RPM, and you take your foot off the gas, the fuel injection system will simply shut off since the wheels and gears are pushing the engine at 2000 RPM, which is over the idle-RPM.

My question is, is the fuel-injection system in the 1996 Z28 included in the "most modern fuel injectors" group that does this when you let off the gas?

Just a question I have out of curiosity. If you don't understand my explanation above, let me know and I'll try to explain it better.

Thanks!

Nick

 

The LT1 PCM has a feature called "decel fuel cutoff" (DFCO) that is part of the program, and does exactly what you describe - cuts the injector pulse width to "0" under most decel conditions. It does this not only to improve fuel economy, but to minimize emissions, and prevent fuel buildup on the cat, which will eventually combust and melt the substrate.

Next time it's coasting down, pay attention to the engine, and you should feel the injectors turn back on as RPM comes down to maybe 1300-1500. There will be a slight surge in the engine. It was very noticeable with the T56 manual... not sure if it feels the same with the automatic trans.

 

It does, Fred (at least with my modified car). I feel it every day. I can clearly tell when they shut off, then reapply.

 

DFCO is an interesting set of tables if you have small or moderately sized injectors (under ~60 lb/hr in my exp).

Fred might find this interesting.

GM > Engine > Fuel Control > Fuel Cutoff, DFCO

 

Interesting. Does that mean that when I slip the tranny into neutral and coast I am not gaining any benefit?

 

That's exactly what it means. I was actually looking online to find out which is better: coast in drive or neutral, and that's how I found out that fuel-injectors did this.

You will burn more gas because in drive, the gears/speed is turning the engine until you hit low 1000s where the engine starts to give gas to itself to maintain idle-rpm. If you slip it into neutral and coast, the gears will "unhook" from the engine and the gears/speed will no longer turn the engine, so the engine will have to supply itself power in order to maintain idle-rpm.