Greetings, it's been awhile since I posted, but I've been doing some thinking and needed advice from the guru's.

In an effort to improve gas mileage, I was wondering if it's possible to manually shut off the fuel delivery system. Living in central PA, there are many hilly roads where I can coast for large stretches while maintaining the speed limit. If I could shut off the fuel during these stretches, it would give a boost to the mpg's.

I was thinking about trying this on my 1991 Dodge Spirit. It has the 2.5L 4-cylinder motor with SPFI and OBDI. I initially thought about splicing into the hot wire running to the fuel pump and routing it though a simple switch that I could manually turn on and off. My question is could this ruin the fuel injector, since it would be running dry? There may also be an issue of getting air into the system and causing problems when I need to accelerate again. So I need some brainstorming on this question and possible other problems that may crop up by turning the fuel pump on and off.

A second option would be to splice into the wire that operates the single port fuel injector. If I could shut this off and allow the pump to continue to run. The unused fuel would be sent back to the tank. Problems that I could think of would be setting a code if the OBD system recognizes that the injector is not working.

Other problems that I am concerned about is wear on the internal components of the engine. Could there be damage to the valves and cylinders if the engine is still turning without the fuel being injected? I'm not sure how much lubrication occurs from the fuel entering the combustion chamber. Again, ideas are needed.

If this seems to be a workable idea, I wouldn't mind experimenting on my car and possibly trying it out on my truck, although that may be a different animal because of the multiple fuel injectors and OBDII system. I know some vehicles such as the Impala basically do this by cutting fuel to 1/2 of the cylinders when at cruising speeds, so it seems possible.

I've just updated my vehicle data shown below. My old Spirit has over 194,000 miles on it and is still running well. It's my daily driver to work and has been dependable over the last 12.5 years. I realize this thread is dealing with my car, but I don't mind if it gets applied to our 2nd Gen Ram's. I don't run my truck much with the high fuel prices, but it still hauls a load when I ask it to. It is still running great and I miss not getting behind the wheel very often.

Thanks for all your help.

 

In my opinion, there are too many variables to consider when making this kind of modification, assuming that it would even work to begin with. Coasting down hills while in drive, but no throttle, with an automatic transmission should be the best of all worlds to save fuel and remain 100% ready for acceleration when you want it. If you really wanted to go a step further, you could drop it into "neutral" when going down hill, then putting it back into "drive" when you are ready to accelerate, but I think the risk of a mis-shift would far outway the possible gain of a small amount of fuel savings, and I mean SMALL! If there isn't a load on the engine, as in coasting down a hill, it will run lean anyway, but just enough to keep it running smoothly. I'm no expert by any means, but I would strongly suggest against shutting off fuel delivery while driving. The engine will stall. It may take a few seconds for the pump to build enough pressure and refill the line with fuel before the engine would start again, not to mention that you would have to place it in park to get the starter to engage anyway.

 

My dad and i did a little experiment with the "N" coasting then switching back to "D". Gain was .02, not worth it. Everytime we went down hill we put in "N". Truck didnt seem happy to be going back into drive after it was in "N". Not worth the effort, or tranny wear.

 

I've played around with that too, mainly where there is a stop sign at the end of the hill and I can shift back into gear from a dead stop. I don't like throwing it back into gear while still moving, so I avoid those situations.

There is one spot on my drive to work where I can coast for about 2 miles down off of a low mountain before hitting the stop sign. If I can cut my fuel on this coast, it will make a difference in my fuel mileage. What I'm looking at doing is simulating the fuel savings like hybrids do. When there is no need to use any throttle, kill the fuel. Another concern that I have is how fast the engine will turn without the fuel. I don't want to risk losing the power steering and brakes.

I could always turn the ignition off to the first click, but my car doesn't have a safety lock between the shifter and ignition to keep it from being turned off all the way, which would result in locking the steering wheel!! That is something I'm not willing to risk. I may play around with doing that on some back roads at low speeds, just to see how the car reacts, but would never actually try that at driving speeds. Also, I would lose the radio when turning off the ignition, don't want to risk that either.

 

That makes NO sense, just turn the key off, its the exact same thing. However, I DO NOT recommend it, just put it in neutral, or hell, just keep it in drive and coast. If you turn off the engine, you WILL NOT have power steering, or power brakes, that would be suicide to coast down hill for 2 miles without PS or PB.

 

By keeping the engine in gear and and turning off the ignition, the engine will still be turned by the transmission, even with an automatic. This is true to a certain speed, probably around 25 - 30 mph, although I'm not sure of the exact speed when the tranny will stop rotating the engine. Since the engine is still turning, you will have power steering and power brakes since a vacuum is being created by the engine being rotated. I tested this out on a back road when I ran for lunch today. I turned the key off, and still had steering and brakes, although I only pumped once, plus the engine fired right back up when I turned on the ignition. I didn't have to hit the starter. I tried this at approximately 30 mph. When I first tried it in the parking lot at about 10 mph, I did have to restart the engine with the starter.

However, cutting the fuel is NOT the same as turning off the ignition, in that I lost all my guages, as well as the radio (the horror), when I turned off the ignition. I had no speedometer and no tachometer. This is why I'm looking at a way to cut off the fuel without turning off the ignition.

Also, this does make sense. The same as hybrids, if I can stop the fuel to then engine when I don't need it, it will save gas. Believe me, there are enough hills on the roads that I travel that I believe I can get a fuel savings. Even when coasting with the tranny in gear, the engine is still turning around 2200 rpm. Shifting out of gear, I'm around 1200 rpm. If I can cut the fuel for a total of 4 minutes in a 20 minute trip to work, it will be a small, but accumulating savings in fuel.

Thanks for the comments, but I'm still wondering if it will damage an injector if it runs without fuel plus answers to the other questions in the original post.

Let me amend this and say that I only shut my car off for a few seconds when I was experimenting earlier. Like the poster above and I stated earlier, turning off the ignition is not a good idea, since you could lock the steering. Plus I don't feel I had the engine off long enough to fully test my idea. But, this is why I'm asking questions. I want to know what I can safely do before I invest time and effort into making changes.

 

dont know if its true or not, but been told you can freeze up the engine if you run it in gear while it's not on... I think the guy telling me that was talking about oil flow and pressure... for sure you run the risk of extra wear and tear on all the drivetrain components. but if you figure out a way of doing it and have some qualified numbers to prove your saving money without excessivly killing your vehicle I would be highly interested.

 

Too much effort for little or no gain! Just COAST in drive, easiest and safest way.

 

Have you considered experimenting with shutting off two or four of the eight fuel injectors, with a switch from the cab?

 

If you shut off the fuel but leave the engine running it can damage the injectors as you are running the engine out of fuel. putting the car in neutral and coasting down the hill is also hard on the tranny as the pump is not turning and can burn up the clutches it is just like towing it which you should never do without disconnecting the drive shaft. If you have a 4wd you can throw the transfer into neutral and tow it but would not recommend that for coasting down hill. Yes hybrids turn the motor on and off but they have an electric motor that takes over during that so it is not quite the same thing. Basicly the best thing you can do for millege is make sure it is tuned up and tires are up to preasure (stock size is the best for optimum milleage) or best of all buy a econo box for every day and use the truck for weekends.