Dumb Question About Car Gas Mileage

This may be the dumbest car-related question ever asked on the SDMB, but I know very little about cars. Please forgive me. :o

Anyway, can you improve your gas mileage by disconnecting a spark plug wire? Mrs. HeyHomie drives a 2001 Chevrolet Tracker SUV with a V6. It passes everything but the gas station. If we disconnect two of the spark plug wires, would we be effectively turning it into a four-cylinder? Or would we be turning into an SUV that doesn’t run until the wires are re-connected?

IF we can run the SUV on four cylinders, doubtless the horsepower would decrease, but would it increase our gas mileage? If so, by how much?


If you disconnect a spark plug wire, the mixture of air and gasoline is still being injected into that cylinder - it’s just not being ignited. So, you’re using the same amount of gasoline as before, but getting less power from it. A losing proposition.

You’re gonna get worse gas mileage, since (as far as I know) disconnecting a spark plug does not stop fuel from being blown through the non-firing cylinder. IF the vehicle had multi-port fuel injection maybe you could disconnect the ruel injector for that one cylinder, but very few “normal” cars have that type of fuel system–most regular cars use a single throttle-body with one injector for all the cylinders. …And I think that pistons/cylinders overheat if they don’t get enough fuel into them anyway.

  • Apparently it is possible, I don’t know all the details but I think Chrysler had a car out in the US one year that had a V-8 engine, that could run in economy mode on only four cylinders. Northstar? Or something. I remember reading about it in a magazine a few years back, but I don’t know if the retail vehicles included that feature.

If you disconnect spark plug wired, the fuel is still going into the cylinders. You’ll be using (or, rather losing) the gas, and decreasing the horsepower. This means that the engine will have to work harder to reach and maintain a given speed, so your mileage will decrease.

Anecdote: A former co-worker bought a 4-cylinder Jeep Wrangler because he wanted to save gas. It sucked gas on the freeway because the little engine was working so hard to drive the vehicle at freeway speeds. On the other hand, my Cherokee with a 4.0 litre, six-cylinder engine got better mileage on the freeway because the engine was just “loafing”.

If you want to save gas, slow down. My Cherokee gets about 18 mpg at 70 mph (combined city/freeway driving). At 65 mph it gets 19-20 mpg. At 60 mph it gets 21-22 mpg. At 55 mph it can average up to 25 mph! :eek:

Also, keep your tyres at the proper inflation. Drive smoothly, without a lot of speeding up and slowing down. Cruise control helps, as long as you don’t have a lot of hills (which it can’t anticipate).

It was Cadillac, and they did, indeed, market them for a while back in the mid-1980s - it was called the “V-8-6-4.” They never got it to work reliably, however, and most owners of these turkeys had their mechanics disable the system, so that it just ran on 8 cylinders all the time.

It wasn’t “Northstar” - that’s just Cadillac’s designation for one of their engine types (a 32-valve V-8), and that name first appeared, I believe, in 1993.

There are also some cars and trucks with big engines that shut off half the cylinders if the engine starts to overheat; I don’t know if they shut off the supply of gas when that happens.

Huh. That’s too bad. If my plan would have worked I was going to disconnect one of the wires in my Geo Metro, effectively turning it into a glorified lawn mower that got 70 MPG :cool: .

Generally, alot of trickery will cause havoc to your emmisions system. You really want the engine in good tune as close to specs. You do have room for modest improvements in many cars, but they shouldn’t involve passing unburned fuel, rather BETTER burned fuel.

Seek out a good air cleaner and/or air intake set up and the best exhaust system for your car. Allowing a car to breath in/out better improves mileage, power and emmisions.

Some vehicles just don’t have the choice of ‘aftermarket’ parts, but many do. High flow intake and exhausts are practical and proven methods though.

But you also have to tie in the effects of gearing to make this claim.

My little 4 cyclinder Matrix XRS (6 speed) gets worse mileage than my friend’s Corolla-S (5 speed). Both engines are 1.8L 4 cyclinder engines. Mine makes 180 HP while his is 130.

His engine gets 6-7L / 100 km while I’m lucky to get 9L/100 km.

When I’ve been in his car his tach at 100 km/h sits below 2.5K. Mine is 2950 rpm. At 120 km/h mine reaches 3400 rpm while his is still below 3K.

I’m sure the difference is all from the gearing since both vehicles are similar in weight and shape. We even drive pretty similar (crazy fast when possible).

First off DON’T DO THIS. Severe damage very likely could occur from you trying this.
Now getting back to the OP. On most cars fuel will continue to flow into the cylinders even if the plug wire is disconnected. On some modern fuel management systems when misfire is detected fuel may be shut off to the cylinder in question. Also on some systems the oxygen sensor will detect an excess of oxygen (from all the air coming out of the cylinder that is not firing) and will enrich the mixture to try and get the oxygen level down. (not a good way to improve gas mileage)
Not knowing jack shit about the fuel management system on your car I strongly suggest that you do not disconnect any plug wires for this reason: If fuel/air does continue to flow into cylinder then you are putting a flammable mixture into the exhaust. The converter runs at 550+ degrees. The fuel will burn in the converter. The fuel will burn so hot inside the converter that the converter will melt internally, and when the exhaust gets plugged you will have to
A) Fix the plug wire
B) Replace the converter
This will cost a lot more than a few gallons of gas.

As Johnny LA if you want to improve your gas mileage slow the fuck down. I drove an Volvo XC90 on a few hundred mile trip a while back. On the way to my destination I drove about 75 and got 21 MPG. I got a flat so I drove back on the space saver spare at 60 and got almost 29 MPG.

Actually, it’s the other way around – the great majority of fuel-injected engines have used multi-port injection since the beginning of electronic fuel injection, and as near as I can tell throttle-body injection systems have not even been made for several years.

Even so, it’s not a good idea to disconnect an injector. The engine will not run smoothly, and the electronic control system will try to adjust to that, possibly even going into an inefficient “limp home” mode. The systems are made to work well when everything is operating properly, and they can compensate for minor problems, but a cylinder getting no fuel and producing no power is not the kind of thing they are designed to rectify.

The Cadillac Northstar engine does have “Limp-Home mode” which allows it to run without coolant by shutting down cylinders. http://www.media.gm.com/cadillac/98cad/dev-lux/nrthstar.htm

GM has also introduced Displacement on Demand for some truck engines.