Exactly. All cars should go into a fuel cut under decel for emission, economy and drivability purposes. Without the fuel cut, you will get pops/backfires through the exhaust. Even an A/F of 17-18 can still result in such backfires (but this will also depend on what type of exhaust you have).
As for what’s better, being in neutral or leaving it in gear. Well… I personally think throwing it in neutral is overall better on ecomomy because when in decel, the engine is being used to slow the car down. Eventually, you’ll have to re-apply the gas and as a result use more fuel. If you left it in neutral, you would coast farther and only use a small amount of fuel to keep the engine running.
Drive around a BMW or any other vehicle that has an instantaneous MPG gauge in the car. This will give you a much better idea how your driving habits affect fuel economy. I was able to go from 28mpg average to 32mpg average just by changing my driving habits. It is obvious the car decels when letting off the gas - you can see the gauge shoot beyond 50mpg. When I leave it in neutral and am going 20+mph, the gauge will also shoot beyond 50mpg.
im sorry my friend…
i just graduated the STEP program, a program based completely on the education in BMW, this “shoot beyond 50mpg” is the computers way of saying, i cannot calculate, its not the actual mpg… good try though
Cool… imma gonna have to look into this more, its a subject that i am now very intrested in, thanks!
and for the “displacement on demand” or cars that shut off cylinders, they do this in sequence to the vehicles timing, hence, cylinder 1 spark shut off, the injector is also nill… remember the old caddy 4-6-8… what a joke, this engine did not shut off timing, and we all knows what happened to those motors =)
great topic, i like this kind of conversation, it also helps out in my job, gets you thinking more… :tup:
You may be right, but do the math manually and I bet it’ll still come out to over 50mpg. Obviously you need to do some logging to calculate.
So enlighten me… why can’t it calculate mpg in these “I cannot calculate” situations? There’s still injector duty and mph. Do you actually know the equation or what variables the computer uses to do such calculations?
One more thing, I also tested the mpg computer to be 1-2mpg off from my actual average that I calculate every time I fill up. So if the computer couldn’t calculate properly as you said, wouldn’t the computer mpg be far more optimistic???
i know for a fact that in all 3rd gen rx7’s (93-95) when coasting in gear the injectors don’t fire at all… and my wideband (which reads up to 20.0) reads 20.0 when i am coasting
“On an EFI car, when the revs are above 1000 and the TPS reads zero, and the car is in motion and in gear, the ECU cuts the fuel completely. The engine keeps spinning because it’s being dragged by the gearbox, which is being dragged by the rear wheels, which are in turn being dragged by the intertia of the car. If you push the clutch and let the revs drop, the computer senses this and turns the fuel back on. You can watch it happen if you have an air/fuel ratio gauge in the cockpit.”
do i know a formula. no
is there a formula, possibly
do i care, no
is it in the STEP training manual that i looked at before i typed my previous response, yes
am i an Engineer; no
am i a mechanic, yes
do i need to know formulas, no
the fact of the matter is when you take your car out of gear you loose a variable… hence the spike, thats the best i can explain, for further info contact germany, not me… but thats the facts sorry if i cannot back my statement up with math,… if i had a scanner i would upload the page… sorry
Not True. When you are at 3000rpm and in gear with the throttle closed, you engine is pumping only air through it, thus engine braking, which is why if you are coasting with closed throttle, you are slowing down.
Your engine does not burn gas at any rpm with the throttle closed, with the exception of idle.
Unless (There always is an unless) you have a pesky EVO like I do, which if you are above a certain RPM, not sure where the cutoff is, and you close the throttle, it will burn a small amount of gas so that you don’t get bucked by letting off the gas suddenly. I know that if I am in say, 3rd gear, about 6,500rpm’s and you suddenly close the throttle, it will come down gently because the car is burning gas to keep it from reving down fast. At a certain RPM, it will shut the gas off, and you will see the car start to slow even more than before. I think the window is somewhere between about 4,000 and redline…
Sorry if I asked for too much… just looking for some evidence to back up a statement you just made from a training manual. If you (and the manual) were right, then taking my car out of gear would always make the gauge “shoot past 50”, right? That is not the case. It appears to me that the computer is doing a fine job at calculating the mpg so before you make statements be sure you can back yourself up more than just saying “i read it in a training manual”.
the end, i dont want to see or hear about it again
back on topic.
there is NEVER at any point other than you turning the key off that injectors stop firing on a piston engine. there is always air pumping and a small amount of fuel.
how the hell do you people think the engine idles? its certianly not magic.
the first column is full vacuum (that means foot off the gas) you can see that it gets close to, but never ever never ever never is it 0.
to calculate mpg the ecu only needs mph and pulse time. if the speedo is mechanical and the ecu isnt fed a signal, then you could use gear and rpm to come up with the same number, but i dont know of any ecus doing that. usually it uses rpm vs speed to figure out what gears its in to use per gear fuel corrections.
I really believe some cars completely shut off the injectors, so I wouldn’t say “never”. Some vehicles may leave a little fuel to burn under decel to keep catalytic converter temps up, but to me that would be the only reason. My standalone has the ability to completely shut off the injectors on decel. Fuel is needed to keep an engine idling, but not needed while an engine decelerates.
Just found this on the web… other searches yield similar results.
DECEL EMISSIONS. When decelerating, the engine will typically either lean out the fuel mixture or shut the fuel off completely (some fuel-injected engines). The computer typically uses inputs from the Vehicle Speed Sensor, TPS, Airflow sensor, and engine rpm to determine when this occurs. When the throttle closes and manifold vacuum shoots up, the computer cuts back on the fuel. Normally, HC, CO and NOX emissions drop during deceleration because the engine is no longer under load and is receiving little or no fuel.
If CO emissions remain high during deceleration, the engine is receiving too much fuel.
Causes may include:
• Leaky fuel injectors; and
• Faulty VSS, TPS, or airflow senso
it looks like in newer cars (much newer like post OBD2B) they are cutting fuel on decel and turing it back on at 2000 rpm to “catch” idle again, and they pulse it (so its not completely off) to keep excess oxy out of the cat.
i could only see this working on autos, and cars in gear. if you pulled the engine out of gear the revs would drop too quickly.
Actualy you can look at a scan from my car from a WOT run to letting off the gas and costing. You can see were TP% goes to 0, timming drops to 0, Injector pulse drop to 0 all while the maf is still reading air
