Engine Off at Stop Lights - Page 2 - Fuelly Forums

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Old 05-29-2008, 08:16 AM   #11
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Originally Posted by almightybmw View Post
random thought about pre OBDII cars... my protege fires 2 injectors at a time. seems fairly wasteful, since one is on the intake stroke and the other is on the power stroke. all it does is blow fuel onto the intake valve, and when wrapped out under WOT, that's a lot of wasted fuel. but that's not idle. Even at idle it still wastes some. small savings add up over time.
Actually, it's doing better when it sprays onto a closed valve, gives the gas a short period to absorb heat from the head and vaporise before being inducted. When it sprays straight in, larger droplets can make it out the exhaust unburned. Good reason to run a 195* thermostat on MPFI batch fire cars. Although the way the injectors are paired on some V6es with batch fire, it seems they spray on closed valves up until you demand more than about 30% of the fuelling. That's probably why older systems take more to hot start. The fuel just puddles there unless the injector sprays longer through an open valve, due to the ports cooling off. With SEFI the timing can be controlled better to spray into an open port at startup.
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Old 05-29-2008, 11:09 AM   #12
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My last 274 miles, I lowered my peak speed to 52, employed DFSO to the max, and probably idled less than 20 minutes total. 4 gals-68.5 MPG. Coasting when possible sometimes more than .5 mile. Engine on. I would estimate total engine on time as .2 gal. Thats not worth the effort it would take to get from 68 to 72 MPG. In my situation it could be dangerous.

In one case when the green turn signal wasn't an option. I just take a different route. Even though there are probably ten more traffic lights, I avoid an equal number of lights later.

The VX at 45 steady is probably getting better than 65 MPG maybe close to 70.

This is more than 6 hours driving with very little wasted idling. Braking was equally low.

Thats 12 above the old highway rating.


I might have been in DFSO for as much as 20% of the time. Understand this is on a road where you have to maintain an average of 45 or better to catch the lights. I miss better than 95% of the 56 traffic lights in a 40 mile ride. On one 4 mile stretch at 45 I time a light 4 miles ahead of me, to coast through that one light and then 4 more miles without another light. 55 limit on this stretch, but now I am at 47. I used to P&G this stretch up to 65 MPH coasting to 50, wasted too much gas.


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Old 05-29-2008, 08:39 PM   #13
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I probably would shut off the engine at red lights, but my starter can be a bit persnickety. Sometimes it will start right up, sometimes the solenoid will click up to a dozen times before I restart. That being said, I'll shut the engine off when driving through the burger place, getting gas at Costco, or when waiting for a freight train to pass by.
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Old 05-29-2008, 08:52 PM   #14
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Fuel cut off was incorporated into fuel injection to do the same job an air pump was used for in the carburetor era.

Your HC emissions went ballistic when you closed the throttle in a carbureted car. Air pumps were first used, then gulp valves.

With FI you kill the fuel on deceleration when it does you no good and the only thing that is pumping through the engine is air.

No fuel=no emissions

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Old 05-29-2008, 10:32 PM   #15
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On OBD-II motors and later it might be... There's some earlier motors though that turn all the injectors on until the ECU realises the motor is running, then it fires 2 at a time at high idle until it reads that the motor is up to temperature, and then it kicks into closed loop and starts metering fuel properly. This can take 30 seconds to go through even on a warm engine. So 10 second break even point might be true for OBD-II and newer, but when it's an older motor that's using like 12x as much fuel for 5 seconds, 6x as much fuel for 25 seconds, you can begin to see where it's thought the breakeven point might be 2-5 mins. I'm going on a fuel timing table and discussion I saw for the non-turbo 6 cyl Mitsubishi Eclipse.
I can't vouch for all cars, buy my 1987 Nissan Bluebird with CA18ET would use no extra fuel on warm start - the VEMS verified it wasn't running rich too. It had MPFI with all 4 injectors firing simultaneously (although the injectors were wires in 2 groups of 2, the software was just set up to fire them simultaneously, twice per engine cycle). I used to turn off the engine and restart all the time. Look at the gaslog for Bluey I, particularly the last few tanks. The last tank was maybe 38-42% engine ON during short 10mile commutes, with dozens of cutoffs per journey

Also in my CA20e Bluebird I have now, I switch off if it is for more than 4 seconds, again it doesn't seem to use extra fuel on restart (as this car is Auto I use startermotor-start which is more wasteful than bump-start, I think).
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Old 05-30-2008, 06:19 AM   #16
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Common misconception. It's hard to believe that DFCO exists the first time you hear about it; I thought the same thing. In fact, fuel gets completely cut in both my 2008 VW Rabbit and my 2002 GMC Sierra under certain conditions, and the engine does not stall or have any problems.
You are correct, however I was responding to his thought about shutting off the engine while stopped. If your car is stopped with the engine running, it is using fuel.
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Old 05-30-2008, 07:09 AM   #17
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Fuel cut off was incorporated into fuel injection to do the same job an air pump was used for in the carburetor era.
Maybe DFCO is so underused in my truck because it's equipped with an air pump? I read something about the air pump in the Chilton book for my truck the other day. The book I have doesn't cover any carbureted models, I'm pretty sure. I'll have to check again when I get home from work today.
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Old 05-30-2008, 07:38 AM   #18
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To answer the question I use a 20-second minimum as a rule, meaning if I don't foresee at least 20 seconds engine off time it's as well leaving it idle... Most intersection light cycles take 30-45-60+ seconds to work themselves around, I like to use the tactic mostly on known intersections where I have a better chance of predicting it accurately.

I do make a few mistakes, but to me if it's not 20+ seconds then it's just not worth it.
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Old 05-31-2008, 09:21 AM   #19
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I'd like to point out:

It's important what year your car is. 1996 (IIRC) was the onset of OBD-II.

OBD-II cars use NO FUEL when you downshift and don't have your foot on the gas. Pre-OBD-II cars still meter out fuel when you do that.

Also, I believe the pre-OBD-II cars are also the ones where the ECU takes a long time to realize the engine was already warm; therefore they are not very good to turn off at the lights.

Modern ECUs can turn the car over, and quickly realize it was just turned off. I've read the fuel savings timetable for this is about 10seconds; restarting the car is a savings if you are off for more than 10 seconds.


My OBD-I car and I pop the clutch and coast in neutral at about 500-700RPM when I'm approaching a light. Seems to work best for me. And sometimes I just put it into neutral because I don't want to excessively wear my throw-out bearing. Seems like the best compromise for my pre-OBD-II car: I'm not keeping the RPM up high, and I'm not restarting the car.
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Old 05-31-2008, 10:42 AM   #20
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It's important what year your car is. 1996 (IIRC) was the onset of OBD-II.
1996 is the first year they were all required to have OBDII. For a few years before, just a few cars had it.

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OBD-II cars use NO FUEL when you downshift and don't have your foot on the gas. Pre-OBD-II cars still meter out fuel when you do that.
That's called DCFO (Deceleration Fuel Cut Off) and it's not that simple. I've got a meter hooked to fuel injectors on two vehicles, and each vehicle will definitely vary. My 2008 VW is quite aggressive about it above 40mph; but if you're doing below 35mph when you downshift (or just take your foot off the gas), you're better off shifting to neutral. My 2002 GMC, on the other hand, only goes into DFCO mode after a long wait, and will only really save gas if you can stay in DFCO for 30 seconds or more. On either vehicle, if not in DFCO, it must meter fuel out just like older vehicles -- and it really does, it uses more fuel for more RPM even while engine braking when it's not in DFCO.
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