ECU, Deceleration, and You!

[Justin]

What does the ECU do with spark and fuel when you're decelerating in gear?

Ex: 3rd gear, 4,000 RPMs, no throttle

What's happening with fuel and spark?

No fuel, I safely say, I know, but what about spark?



I know its a pretty basic question but I just want to make sure I understand exactly what's happening.

My car, which has a issue on Cyl #3, decelerates PERFECT, it sounds beautiful as it rumbles down, so... yeah.


Answers?

 

[HommerSimpson]

no it has spark and fuel.. dont believe me just turn the key off when you decelerate... same as turning the fuel and spark off.. lose either one and motor shuts off...but because you have in gear its still turning....

 

[Dan_Gyoba]

As soon as the throttle closes (IDL and E2 are connected at the throttle position sensor) the ECU shuts off the engine, same as fuel cut, provided that the RPM is above about 2200. Once the RPM drops below 1200 or so, fuel and spark are returned, so that the engine doesn't stall.

This is actually the TSRM method for determining if the ECU is properly handling fuel cut. Disconnect the TPS, short IDL to E2, and rev the engine. It should hunt between the fuel cut point (2200 RPM?) and the fuel return point (1200 RPM?) I don't recall the exact RPM points where this happens.

To the best of my knowledge, FCO shuts down both fuel and spark.

 

[HommerSimpson]

As soon as the throttle closes (IDL and E2 are connected at the throttle position sensor) the ECU shuts off the engine, same as fuel cut, provided that the RPM is above about 2200. Once the RPM drops below 1200 or so, fuel and spark are returned, so that the engine doesn't stall.

This is actually the TSRM method for determining if the ECU is properly handling fuel cut. Disconnect the TPS, short IDL to E2, and rev the engine. It should hunt between the fuel cut point (2200 RPM?) and the fuel return point (1200 RPM?) I don't recall the exact RPM points where this happens.

To the best of my knowledge, FCO shuts down both fuel and spark.

disregard what i said.. obviously i had no clue

 

[mazzer]

many misfires occur under load. esp secondary ign problems

 

[jetjock]

disregard what i said.. obviously i had no clue

Then why did you post?

Dan has it right except ignition is not killed and the cut off and resumption speeds are not fixed. They're higher when the coolant temp is low or the AC on. Without going into detail the relevant signals used in the cut off calculation are IDL, engine speed (NE), coolant temp, AC on, and vehicle speed with IDL and NE having the highest priority.

 

[Dan_Gyoba]

Okay. I was under the impression that fuel cut also killed ignition (Couldn't that be bad if the ignition lights a lean mixture?)

I had a problem with my TPS once, where the IDL and E2 were shorted all the time. It was a real pain, 'cause I had to get it home without taking it above ~2000 RPM. (This was when I was NA, and the points as defined by the TSRM are slightly different.

Actually, according to this, the fuel cut RPM should be ~1600 RPM for turbo, and 1800RPM for NA, 1200 RPM for return.

Now, I was operating under assumptions from the TSRM procedure, however, it does say to start and warm up the engine first, so it does make sense that this would be temperature dependant. It doesn't mention A/C, but I think it's safe to say that they assumed that A/C would be off for this test. Oh yeah. It also says the vehicle should be stopped. I suppose that vehicle speed could be taken into account as well.

 

[HommerSimpson]

Then why did you post?

Dan has it right except ignition is not killed and the cut off and resumption speeds are not fixed. They're higher when the coolant temp is low or the AC on. Without going into detail the relevant signals used in the cut off calculation are IDL, engine speed (NE), coolant temp, AC on, and vehicle speed with IDL and NE having the highest priority.

beacause i did not think the fuel and spark was cut.. still dont either.. or you would have backfires out the ass

 

[Doward]

Fuel is killed under deceleration.

You don't light a 'lean' mixture by any means - you are just firing on air. Timing couldn't care less what you set it at, there is no fuel to fire under zero throttle deceleration.

1200-1600 rpm is the normal window that an EFI engine will resume operation of fuel/spark.

You don't get backfires where there is no fuel

None of this applies to a carb'd engine, btw.

 

[Dan_Gyoba]

beacause i did not think the fuel and spark was cut.. still dont either.. or you would have backfires out the ass

I'm curious as to why you'd expect backfires. If there's no fuel, there's nothing to backfire. The engine still takes in fuel as normal, because it's still turning, The difference is that at the top of the compression stroke, there's nothing to combust. The fuel injector didn't open for that round, so there's no fuel. The air charge then goes into the exhaust, and out.

Note: Because I'm running a big exhaust with no cats, I sometimes do get a backfire in there, particularly when the engine is cold, and it's been idling at a light. My theory here being that at idle, the engine runs rich, particularly cold. Unburned fuel accumulates in the exhaust, and then the first time I shift, driving away from the light, along comes some (hot) air that still has oxygen in it. Oxygen + unburned fuel + heat. Pop!

@Doward: Sorry, I'm running with some old info. At one point in time on the supras.com mailing list someone suggested (And experimented which seemed to back him up) that fuel cut also cut spark. It does occur to me though that this cannot be the case. Since the tach runs off of the igniter signal, cutting spark would stop the tach, too. Ain't hindsight great?

'nother thought: This would make a leaking injector potentially dangerous hey?

 

[IJ.]

If it backfires on decel cut chances are the TPS is set incorrectly.

 

[GrimJack]

Okay. I was under the impression that fuel cut also killed ignition (Couldn't that be bad if the ignition lights a lean mixture?)

I had a problem with my TPS once, where the IDL and E2 were shorted all the time. It was a real pain, 'cause I had to get it home without taking it above ~2000 RPM. (This was when I was NA, and the points as defined by the TSRM are slightly different.

Actually, according to this, the fuel cut RPM should be ~1600 RPM for turbo, and 1800RPM for NA, 1200 RPM for return.

Now, I was operating under assumptions from the TSRM procedure, however, it does say to start and warm up the engine first, so it does make sense that this would be temperature dependant. It doesn't mention A/C, but I think it's safe to say that they assumed that A/C would be off for this test. Oh yeah. It also says the vehicle should be stopped. I suppose that vehicle speed could be taken into account as well.

Ya, same here, Dan, I've seen this debate rock back and forth quite a few times over the years.

 

[jetjock]

Dan: Everything I said refers to how decel cut normally operates, not when doing the TSRM test.

beacause i did not think the fuel and spark was cut.. still dont either..

Stop being an idiot. I'm not stating opinion, I'm stating facts based on factory documentation backed up by metrology. Measuring what happens under these conditions may be far beyond your abilities but it's a cake walk for me. That and I own gear that can "run" the ECU in any configuration I choose whether on or off the car. Not to mention decel fuel cut in some form has been an emissions strategy employed in just about every injected engine for the last 25 or so years. Bottom line is you ought to stfu unless you know what you're talking about. I know things about this box many here would love to learn but I hold back because of people like you.

 

[Justin]

Stop being an idiot.

:rofl::rofl:

Fuel is killed under deceleration.

You don't light a 'lean' mixture by any means - you are just firing on air.

That's what I was thinking. The throttle plate is closed, the TPS is shut, I thought no fuel.

Here's a really stupid question. at least in my eyes. I feel a little silly asking

If there's no combustion, why does it sound like there is? You can hear a beautiful purr out the exhaust...

See, foolish question, but I'm really curious


Thanks for the help guys. Maybe someone could clean this thread up a little so there's only correct information in it?

 

[jetjock]

Simply because injection is halted doesn't mean there isn't any fuel. A bit is left over. It's the reason you have a dashpot, to allow air through to burn that left over fuel and prevent a spike of HC. That's what you're hearing.

 

[Justin]

Simply because injection is halted doesn't mean there isn't any fuel. A bit is left over. It's the reason you have a dashpot, to allow air through to burn that left over fuel and prevent a spike of HC. That's what you're hearing.

That seems like that 'little bit' that is left over sure burns a long time.

As the answer isn't something blatantly obvious I don't feel foolish anymore

 

[Quin]

Thanks for the new sig JJ

 

[Doward]

JJ hit it on the head (as usual ). Also reduces CO emissions, according to the TSRM:

http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?Section=EC&P=8

 

[jetjock]

Justin: All that happens pretty quickly and then fueling is resumed. Some engines get a burble or slight pop if things aren't set up just right. Sounds as if everything on yours is though.

Quin:

 

[tissimo]

That seems like that 'little bit' that is left over sure burns a long time.

As the answer isn't something blatantly obvious I don't feel foolish anymore

thats just the sound of the engine pushing air through the exhaust. Pull the efi fuse, and have someone crank the car while your listening at the muffler, you will hear it.. and thats only like 300 rpm