There has got to be more to it than this...

GM has some vehicles that can shut off a cylinder or two to conserve gas. Earlier today I fired up the jeep, and unpluged injectors 4, 5, and 6 - it ran a little rougher but did not die....

So is this all it takes to improve fuel economy? As long as the jeep still has enough power to cruise at 45 mph, I can't think of any harm it will do to the engine.

Obviously I can't leave the injectors unplugged all the time, so I'm thinking of riggin up a switch on the dash that will turn them on and off.

Someone tell me the reasons why this WONT work!

Ummm yeah I'm pretty sure there is more to it than that...

I think the engine leaves those cylinder valves open all the time or something? I can't remember there was a similar thread to this a while back where someone was using a resistor to do this somehow.

But yeah, I'm almost certain that there is a lot more to it than that...

It also cuts fuel. Your jeep is still spraying fuel into the cylinders when you unplug the spark plugs. It also cuts the cylinders in a way that it remains balanced... I can't think of which cylinders that would be on the I6 though.

You could unplug injectors as well, but what your effectively doing is making your Jeep an inline 4 that has to throw around two extra pistons. You might see benefits if youre going on a long highway trip, but the GM vehicles do it on the fly.

EDIT: Ooops OP said he removed plugs.

The newer GM systems alternate which cylinders do not recieve fuel unlike when the system was first put into cadilacs. The Caddy system always shut down the same cylinders which lead some problems, can't remember them off the top of my head.

~Alex

Edit: misread the OP - thought he was cutting spark. Cutting the fuel may work; but I wouldn't do it.

He said he unplugged the injectors...

Edit: Beat me to it.

alex22 said:

The newer GM systems alternate which cylinders do not recieve fuel unlike when the system was first put into cadilacs. The Caddy system always shut down the same cylinders which lead some problems, can't remember them off the top of my head.

~Alex

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Are we talking problems in the short term, or after 50,000+ miles?

I'll ask tomorow at work, I'm pretty sure it was one of those "over time" kind of problems and not an instant issue.
The First fuel managment attempt was by Cadillac in 1981 where it moved the rocker arms in order to not open the valves, it failed. The newer ones just shut down the injector pulse.
It would be kinda interesting to have a switch in the cab you can hit to turn off a few injectors for highway crusing.

~Alex

There was just a thread on this exact thing a few weeks ago. Some fellow published up a schematic and said his Teacher tried it and doubled his mileage, or some such thing.

We all doubted it.

The schematic is around here somewhere. IIRC, he wired relays to cut the injectors.

Ron

There is much more too it.............

http://www.greencar.com/features/features46/

I bet it will decrease the mileage for the city at least. With the valves closed you might as well be spraying fuel and spark in. The engine has to do the same amount of work to compress the cylinder with none of the gain from combustion.

Unplugging the injectors just makes an air compressor unless you can control the valves!

cut the fuel pull the plugs and rig up a fitting and air line to an air tank- bingo maybe better fuel but you got OBA which is way cooler than a full tank a gas

wolfpackjeeper said:

I bet it will decrease the mileage for the city at least. With the valves closed you might as well be spraying fuel and spark in. The engine has to do the same amount of work to compress the cylinder with none of the gain from combustion.

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When I first read your post, I thought, "ah ha! this has got to be it!" But then I read the link posted by XJEEPER and it looks like GM, Chrysler, and Honda are deactivating cylinders by preventing valves from opening (and presumably turning off injectors although it was not mentioned). In this instance, the engine is still compressing air in the cylinder, but it is not pumping air through the intake and exauhst manifolds. I must be missing something here because wouldn't it take more energy to compress air in the cylinder than it would to pump air through the intake and exhaust?

Does anyone know for sure if one of the wires running to the injectors is ground? Of course I can check with a multimeter when I get a chance... If one wire is ground, than I can deactivate 3 injectors with 1 switch without using relays.

Oh, and if anyone else is thinking of trying this be warned: after unplugging an injector the check engine light comes on and won't turn off - even after hooking the injector back up.

Thanks for everyones input so far!!

teookie said:

Oh, and if anyone else is thinking of trying this be warned: after unplugging an injector the check engine light comes on and won't turn off - even after hooking the injector back up.

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i believe that is because the ecu has to reset itself. it will go away on most cars after you drive it for about 100 miles.

The only other thing I question is spark. Won't the spark plugs begin to foul if they are firing for no reason?

I like where this is going but I'm just not sure its truly feasible on these older technology engines.

I think its a cool idea the only problem is the pistons are still draging and i think that the GM system does open the valves. your jeep is still compressing the air which is a waste, thats alot of drag in the engine. Another thing the engine block would heat up a little strange, the back cylinder isnt making any heat. i dont know if tat would do any thing. But as far as spark goes a few new engines have a waste spark so it wouldent hurt anything.

to eliminate spark, you could put the plug wires on the same relays as the injectors... if it was a big deal.

and the heating thing-agreed. i think pulling every other cyl might be a better idea...

I think its a cool idea the only problem is the pistons are still draging and i think that the GM system does open the valves.

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Definetly makes more sense to me (and the others here at my work) to open the valves.

and the heating thing-agreed. i think pulling every other cyl might be a better idea...

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I don't think heating will be to big of a deal. The coolant flow should keep everything more or less around the same temp. can't pull every other cylinder due the 4L's firing pattern. To run smooth you have to pull every other cylinder in the firing sequence; 1, 2, and 3, or 4, 5, and 6. It has to go hit miss hit miss hit miss, not hit hit miss hit hit miss.

Another probelm that someone pointed out to me is the potential for mixture getting pulled from the other injectors through the intake manifold into the "deactivated" cylinders. If this happend, it would almost surely be a lean mix and could lead to major problems, ie holes in pistons. Any experts out there want to comment on this?

I've pulled my intake before and cleaned it out, but I can't remember if the sludge was only in the individual runners to each cylinder, or if it was all over the inside of the plenum. If it was only in the runners than that would indicate the gas does not "backflow" into runners for adjacent cylinders. This would be good for my case here, but like I said i can't remember if just the runners were dirty...

Just found a Wikipedia articale on this: http://en.wikipedia.org/wiki/Cylinder_deactivation