.........fuel system when you take your foot off of the throttle? Mike Allen ( an automotive writer for Pop. Mech.) claims that when you take your foot off of the gas pedal while in gear that the fuel system shuts off. Thus you are able to actually save more fuel by staying in gear vs putting the car in nuetral while going downhill. Article is here...http://www.popularmechanics.com/auto...o/4230705.html

This seems to be opposite to my experience. If you come to a stop light the engine still runs at idle. How would the car know if you are coming to a stop or just cruising downhill?

Anyone able to confirm or deny this claim?

 

It could be that if your foot's completely off the throttle and the engine speed is above idle, the computer cuts off the fuel, but as the engine speed decreases to idle, it re-introduces fuel to keep the engine running.

I think if you have an air/fuel meter (either a stand-alone aftermarket gauge, or one that plugs into the OBC port), it should be easy to check. Get going up to 60-70 MPH, and then take your foot off the throttle while leaving the transmission in gear. If the computer is cutting fuel entirely, your air/fuel meter should instantly peg high. You could then repeat the test, putting the transmission in neutral, and see what the air/fuel ratio is once the RPM has decayed down to idle.

I think it varies from car to car whether the fuel is cut off entirely when you're off the throttle and RPM is above idle. If you go to some of the "hypermiler" forums on the web, you can find more information. ("Hypermilers" are people that obsess over getting the maximum possible fuel economy from their cars, even going so far as making a competition out of it.)

 

I have seen and read some of these hypermileage folks posts. Kind of a three ring circus..........

I would love to see if anyone can confirm this claim.

 

Well, I can confirm that at least *some* cars do this. Here's a link to a description of the Bosch L-Jetronic fuel injection system. As I suspected, the fuel cutoff during coasting is only active above a set engine RPM. Below that RPM, fuel is re-introduced so the engine doesn't stall.

What I suspect you're really asking is whether or not ALL modern fuel-injected cars behave this way. That, I don't know.

 

That is exactly what I was hoping to see. I did a google but did not come up with anything as concrete as that. Amazing the results you can get with the right wording.......lol

Thanks !

 

You're welcome. I got a lot of relevant hits searching for "fuel cutoff while coasting" and "fuel cutoff during overrun" (since "overrun" appears to be the technical term for lifting off the throttle while the transmission's still in gear).

 

Great Scotts! Good info guys. (sorry, couldn't resist after I read the back and forth between you two. Heh)

 

 

Our Insight has fuel cut and I think the MINI does this to a degree but not complete shut-off. We wouldn't get the burble on over-run if there wasn't any fuel getting to the engine. I say the MINI does it to a degree because there is a definite lurch around 1700 rpm when your decelerating in gear. This is when I think the ECU begins to re-introduce fuel. There wouldn't be any lurch if there wasn't some sort of fuel cut happening.

 

Now that makes sense about the lurch..........

 

Now this Bosch L-Jetronic fuel injection system would be on the 2nd gen. MINI correct? We just took delivery of our second MINI and if I recall correctly I remember seeing Bosch on the air intake somewhere on our 08....

 

No, the L-Jetronic system is an old Bosch system from the 70's and 80's. It was just the first example I found that explicitly described the "fuel cutoff on overrun" system that Scottinbend was asking about.

The system in the R56 MINI is the Bosch DI Motronic. (The "DI" stands for "Direct Injection", where the gas is injected directly into the cylinder under very high pressure, rather than being injected upstream in the intake tract and then sucked into the cylinder, as was the case with the earlier MINIs).

I'm still looking for a technical document that describes the fuel cutoff scheme for the DI Motronic system, but I'm almost certain it's there - it's too simple to pass up, especially in the face of today's ever-tightening fuel economy and emissions standards.