Is it possible to selectively shut off injectors to save gas?

[newyorkin]

I'm thinking about wiring 4 injectors through a switch on my '98 Tahoe, so I can shut off 4 injectors when I'm on long stretches of highway. I was thinking about how my dad's Caddy shuts off 4 cylinders at highway cruising, and thinking I could do the same. There would still be valve cycling, a spark, and a power-robbing compression stroke, but the key would be no fuel entering the cylinder. I'm sure I'd have to figure out which injectors would keep it running smoothly...

I'm sure this would throw a code, but what other effects do you guys think I could expect?

Maybe uneven ring and bearing wear...

 

[jcg]

That's exactly what GM's Active Fuel Management (known as Displacement on Demand originally) does, but it also deactivates the valves to create an "air spring" in the cylinder to cut down on pumping losses. First the injectors shut off (1, 4, 6, 7) and the engine continues through a couple cycles to burn off any residual gas in the intake, then the exhaust valves shut off to trap air in the cylinders and after a couple crank revolutions to build up pressure the intake valves shut off. This trapped air helps act like a spring and cuts down on the "drag" of the dead cylinders. The valves cycle every so often to replenish the air charge.

That being said... you could try shutting off injectors but I don't think you'll get the gains you hope to since you can't control the valves too. You might actually get lower mileage because your putting more load on the remaining 4 cylinders having to drag the others along for the ride. The trapped air charge is key to reducing pumping losses. The newer generation of V8's has dramatically lower internal friction than older motors did too, which helps a lot.

If you do decide to give it a shot, make sure you wire the injectors through the switch correctly or you could end up shorting them all together and possibly fry your ECM.

 

[dyeager535]

Without breaking my brain, I don't think it's a good idea to try. If I'm not mistaken the newer cylinder deactivation varies which cylinders are deactivated. Pumping loss is going to be huge, I doubt making the other cylinders work harder to overcome the "dead" ones is a route to economy. Again my rudimentary understanding of these systems, solenoids working the valves allow the pumping losses to be eliminated vs a "normal" mecahnical cam/valve relationship.

 

[newyorkin]

That's one thing I was wondering, if only on the highway, would the dead cylinder compression load be that much higher on the live cylinders. I know the modern systems control the valves electronically, which is not an option here...

I would wire them through a switch as only an open or close for each injector line, no crossing wires anywhere, just interrupting them. The injectors are supposed to be 15-18ohm, so there's the possibility I could add a few ohms with the wire I'd use and screw up normal operation, too. Obviously, it's going far outside the norm, so I would expect some risk being involved.
But that's why I asked... To get an idea of what risk to expect...

 

[jhellwig]

The solution is buying a smaller vehicle or a newer vehicle not shutting off injectors.

 

[newyorkin]

I save gas on this when I don't need 4wd or cargo room.

It kinda sucks to take 4 kids, a wife, their luggage, and a camper on it, though.

My company pays most of my fuel costs when I need to drive the truck, but I still don't feel great about burning 13-14mpg almost daily. I guess another option is find a job that's good with a smaller vehicle.

Newer large SUVS are only getting 13-16 mpg anyway, so what would it gain me to add a car payment to my transportation costs?

 

[dyeager535]

The other thing to look at, is what are these vehicles getting for the effort? I really don't think it's a "huge" difference. I could be wrong, but it doesn't seem like the systems are getting the press they would if it was as large as say, doubling the mileage.

 

[newyorkin]

That's the whole reason I want to pursue it. My dad's caddy is getting 30-35mpg highway in a big ole V8. I don't know what he gets around town, but I asked him why he takes his big caddy on long trips instead of my mom's newer Impala, and we got to talking about the benefits he's getting from the cylinder shutoff. His older Caddy's without the deactivation were getting around 16-18mpg highway. I'll ask him tomorrow what he gets around town, but he almost exclusively uses the caddy to go out of state now, and his minivan around town.

If the difference is something like 5mpg, then I agree, it's probably not worth the effort.

 

[GsxrMike]

I don't think it would hurt to try but I think the best way to do it would be to add a 12v relay inline with the injector and use the switch(s) to run the relay. Maybe start off with 2 cylinders. And if you use relays you can run 1 switch and 1 wire.

 

[Blue85]

If you could get 5mpg more in a K5 it would be a miracle solution, wouldn't it? Nobody claims that DOD "doubles" mileage, but 5-10% is enough to make it worth doing for OEMs. If you could get 25% improvement, you would probably own that segment.

It's really not practical to retrofit DOD. Please don't turn off some of your injectors with a switch . You need a whole new engine management system. Think about what would happen. As soon as half of the injectors turn off, you have 4 cylinders pumping straight air to your oxygen sensor. To correct for the excess oxygen, the ECU has to turn up the fuel (not really what you were trying to accomplish), proably to the point of rich misfire. So the engine will either stall or run terribly. The 4 rich cylinders will foul the plugs and belch smoke. So you could disconnect the O2 sensor as well and run open loop, but again this is going AWAY from good economy. Unplug 1 or 2 injectors sometime and go for a drive. If it's not bad enough for you, unplug 4 .

The reason you have to cycle through the cylinders is to keep all of them warm.

 

[Blue85]

To save fuel, I run a wideband oxygen sensor and tune the engine to run lean (up to 16.5:1) under low load conditions. This isn't quite as good as DOD, but it IS a realistic retrofit. I don't know how many miles you drive to pay off the conversion, but in my case I was switching from a carb to EFI at the same time, so it wasn't a big deal.

 

[Zmaster265]

Blue85- Wow, that is a very interesting setup you describe. I'd love to know more. What sort of EFI setup are you using and what mileage do you consistently achieve?

 

[newyorkin]

If you could get 5mpg more in a K5 it would be a miracle solution, wouldn't it? Nobody claims that DOD "doubles" mileage, but 5-10% is enough to make it worth doing for OEMs. If you could get 25% improvement, you would probably own that segment.

It's really not practical to retrofit DOD. Please don't turn off some of your injectors with a switch . You need a whole new engine management system. Think about what would happen. As soon as half of the injectors turn off, you have 4 cylinders pumping straight air to your oxygen sensor. To correct for the excess oxygen, the ECU has to turn up the fuel (not really what you were trying to accomplish), proably to the point of rich misfire. So the engine will either stall or run terribly. The 4 rich cylinders will foul the plugs and belch smoke. So you could disconnect the O2 sensor as well and run open loop, but again this is going AWAY from good economy. Unplug 1 or 2 injectors sometime and go for a drive. If it's not bad enough for you, unplug 4 .

The reason you have to cycle through the cylinders is to keep all of them warm.

Thanks, that's why I asked here first.

 

[newyorkin]

To save fuel, I run a wideband oxygen sensor and tune the engine to run lean (up to 16.5:1) under low load conditions. This isn't quite as good as DOD, but it IS a realistic retrofit. I don't know how many miles you drive to pay off the conversion, but in my case I was switching from a carb to EFI at the same time, so it wasn't a big deal.

I do about 70 a day, some days up to 150. The truck has 4 o2 sensors, so I need to save a good amount of gas to swap them all out. I need to swap them out at interval soon anyway, though... Where could I find a wideband sensor?
How complicated was reprogramming?

 

[Thunder]

I dont see how running wide band 02 on a stock ECM will do anything. Widebands are mainly used to tune an engine. It is used to percisly monitor the AF ratio Then the ECM is flased for the tune you want. Once that is done you are stuck with the tune untill it is reflashed
You can run a wide band on aftermatket EFI. where you can use it for data logging F/A ratios and make the changes you want and have more accurate control over your AF ratio.

Your cheapest route is to find someone locally or on line that will wideband tune and reflash your ECM. And install a good cat back exhaust system.

Dont shut off injectors. very bad move.

 

[dyeager535]

Wideband lets you actually tune the engine outside of stoich. GM used "lean burn" on their camaro's (at least) in the late 80's, but didn't tell the EPA, and was forced to quit using it, even though the coding is still in the ECM's. GM was able to run the engines at something like 16:1 AFR under the right cruise conditions as I recall. You can only guess outside of 14.7:1 on a narrowband, and that's dangerous when talking about a lean running engine at cruise.

He's talking OBD1 I'm sure, it's a simpler matter (as I understand it) in the OBD1 systems to do things like tune for lean cruise than it would be on the OBD2 systems where you've got post-cat O2 sensor feedback.

 

[newyorkin]

ok, I get it guys...

I'll go back to the inline driveshaft traction motor and regenerative braking retrofit idea...

 

[Blue85]

I run a MegaSquirt ECU on a TPI setup. It includes a table for "Target AFR", based on load (vacuum) and rpm.

Innovate Motorsports has made widebands reasonable to own. The LC-1 sensor + controller is about $200.

Sorry, I don't have consistent fuel logs. A tank of gas does last me 2-3 weeks (even longer when I don't drive it!), but I live 2.5 miles from work, so that doesn't mean too much. I started taking advantage of wideband right from the start, so I don't know what economy would have been with EFI and a narrowband, which would be the most valuable information here.

 

[6.2Blazer]

As mentioned in a previous post, there is a lot more to it than just turning off the injectors. Besides controlling the valves, without being able to actively change the engine calibration I'm pretty sure the engine would run quite poorly. I do automotive testing for a living and we've had issues with faulty injectors or bad wiring in the past and the engines run horrible when this happens, it's about the same as if you started removing spark plug wires.

From what I have seen in advertisements, companies like Dodge and Chevy who offer this option on new trucks only show a relatively small improvement. Chevy states the 5.3 in a 4x4 truck with the active fuel managements is rated at 14 city / 19 highway. I couldn't find right off hand the ratings without fuel management but I know they are not that much lower.

 

[dyeager535]

That's not even a 20% improvement. As easy as it is to manage with the new ECU's its probably worth it for the across the board improvements in economy, but individually it would be a lot of work to engineer.