Discussion Topic: Car in neutral or downshift

Hey all;
As I've been tuning I've been watching what's going on with the WBO2 and everything and noticing some things. When you let off the gas, the car runs lean because you're going from giving fuel to nearly none. But if you throw it out of gear and let idle up to a light or whatever, AFR is at 14.7. Rolling up in gear, AFR sits at --- since it's way off the board.

So, would leave the gear in gear (either downshifting to a stop or leaving in one gear) cause damage to the motor since it's making it technically run lean? Or is it just an illusion to the ECU that the car is lean?

Begin...

When you are decelerating while still in gear and not giving the car any gas the ECU cuts the injectors off. When you are above 2.5k or so the injectors cut off to save gas and thats why it goes lean. This doesn't hurt anything since there is no load on the motor so it wont knock or anything. So it can save gas by decelerating while still in gear.

The above point is correct. Also note that in most states it is illegal to coast in neutral.

Also note that in most states it is illegal to coast in neutral.

Really? How so? Alot of roads they don't want engine breaking which by default is the opposite of neutral coast + braking.

Engine braking is often restricted for large vehicles (trucks and buses) around areas where Jake-braking can lead to noise pollution.

Coasting in neutral offers no control over the car aside from steering and brake input. Check your local laws and see if this is the case for you, but I've seen posts in a similar discussion indicate this is a common thing.

how the hell do they know that you cruising in neutral, and give you ticket ? or something ........

how the hell do they know that you cruising in neutral, and give you ticket ? or something ........

I don't enforce the law, nor is this relevant. :sly:

When you are decelerating while still in gear and not giving the car any gas the ECU cuts the injectors off. When you are above 2.5k or so the injectors cut off to save gas and thats why it goes lean. This doesn't hurt anything since there is no load on the motor so it wont knock or anything. So it can save gas by decelerating while still in gear.

So I was thinking...
Say you let off the gas over 2.5k and coasting down. So the injectors are basically off, what if you suddenly go WOT? The computer is quick and all, but is there any chance that it wouldn't act quick enough and cause a little bit of leanness under load? Because any little bit of knock is bad.

And I take it from the posts and some research that no matter what, as long as the car is not under load while lean, there's no damage being done right?

Knock is caused by heat. Heat comes from fuel. No fuel no heat. :)

If there is any law about coasting in nuetral, I would have to think it's emmissions related. The whole reasons all modern cars use DFC is for emissions. It also helps with fuel economy, so it makes sense to always coast down in gear.

So I was thinking...
Say you let off the gas over 2.5k and coasting down. So the injectors are basically off, what if you suddenly go WOT? The computer is quick and all, but is there any chance that it wouldn't act quick enough and cause a little bit of leanness under load? Because any little bit of knock is bad.


You may run a tiny bit lean but it wont hurt anything. The ECU is very quick at adding fuel back. And boost wont hit anywhere near that fast so your good.

The ECU has both accel and decel fuel pumps. Based on TPS delta values, some amount of IPW is added to what it is currently calling for, modifed by absolute TPS, temp, and RPM (among other things) instantly. The changes cab be made at the injector before the air can get there from the throttle body. ;)

It's also worth mentioning that running lean only causes a problem because it creates heat. Heat build up takes time. Running lean for one or a handful of cycles doesn't break anything. Running lean for seconds at a time can do damage. In most cases a motor can take even long periods of running lean. I've had half track runs in the RWD at 40+ psi and 13.5 to 14.2 AFRs. I made full track runs in the EVO with the WG wired shut with a 75 shot on top of it at 14:1 (EVOs have awesome combustion chambers). In contrast, most motors will fail quickly with excessive timing. In other words, any amount of lean burn at low load is completely safe. In newer cars, the ECU is tuned from the factory to run very lean in some conditions for emissions and economy reasons. Etc.

Yea I have heard of a few people who run MS tune to 15 a/f ratios when cruising in town so try and get better gas milage.

That's only .3 leaner than stock, I was thinking more like 20:1. When DGI technology takes off, we'll be seeing a lot more lean burn strategy paying off. ;)

When cruising my AFRs constantly vary from 15 to 17...thats about right right?

When cruising my AFRs constantly vary from 15 to 17...thats about right right?

Mine cycles from 14.0-15.4, which puts 14.7 at about dead center. 17afr seems a little lean for cruise. Only time I see 17 is when starting to let off the gas or between shifts... then again, different ECUs.

My car seems to run shitty when the AFs go leaner than 16-17. So turning to 15.5-16.0 at cruising speeds would be a good idea?

My car seems to run shitty when the AFs go leaner than 16-17. So turning to 15.5-16.0 at cruising speeds would be a good idea?

Again, it could just be the difference in ECU.

The ECU has both accel and decel fuel pumps. Based on TPS delta values, some amount of IPW is added to what it is currently calling for, modifed by absolute TPS, temp, and RPM (among other things) instantly. The changes cab be made at the injector before the air can get there from the throttle body.

I'm confused on this part Kevin. I thought that the fuel pump was running 100% load all the time.

Again, it could just be the difference in ECU.



I'm confused on this part Kevin. I thought that the fuel pump was running 100% load all the time.


The fuel pump is running 100% of the time. Not sure what he meant by that but when you get on the throttle quick the ECU adds a little more fuel and when you let off the gas it takes a certain percentage of the fuel out to help the motor de rev quicker.

It kinda seems that any adjustments i make to correct the lean condition when cruising, dont help. Maybe, thats just what the ECU is meant to run at?

It kinda seems that any adjustments i make to correct the lean condition when cruising, dont help. Maybe, thats just what the ECU is meant to run at?

Same here. I think full stand alone ECUs you can set the adjustments. But the stock ECUs are always setting anything under WOT to 14.7 by using the STFT. That's why you can make adjustments all you want, but it's always around 14.7 unless WOT.

The stock ECU has a map of what the AF ratio should be under certain conditions. When under 80% throttle or so the ECU is in closed loop and its going to adjust your short term fuel trims to make the AF ratio what ever it wants.

So say the ECU is trying to run 14.7 at idle and you take out 5% fuel. The ECU will add fuel back in till it reaches 14.7. But when you tune for bigger injectors you dont want the ECU to have to add or subtract lots of fuel because this will narrow its range it can add/remove fuel in. Say you run bigger injectors but dont remove fuel. The stock ECU cant take out or add up to 20%. So the ECU sees your running rich and takes out 18% of fuel to make your AF ratio correct. Well thats all good but now your ECU can only take out 2% more fuel if it needs to. This is why you try to keep your STFT close to 0%. Long term fuel trim factors in some also but Im not getting in to all that.

Now at WOT the ECU is in open loop and now only adds fuel according to the fuel map. It no longer uses the o2 sensor to adjust the AF ratio. So when you adjust the fuel at WOT you can see the effects because the ECU does not counter act your adjustments.

With a stand alone ECU you can do what ever the hell you want to basically.

Decel and accel "pump" is just terminology stolen from the good ol carb days. Carbs had an "Accelerator pump" that would dump raw fuel in anytime throttle position increased, to cover the lag you get in waiting for the airflow through the carb to increase and pull more fuel through the jets. EFI does the same thing, electronically.

Yes, you can generally cruise as lean as 15.5-16.0 on these motors. You can run as lean as you want up to lean misfire. This only applies to standalones running on widebands. A narrow band o2 sensor limits you to 14.7, there is no "tuning" possible. Some adjustment can be made with WBO2s that have a simulated narrow band output, but mess with that at your own aggravation...