Overrun fuel shut-off delay

[jiri_j]

Hello all, I have a question regarding an engine management system of D2 V8. I have found out that when accelarating (or reving the engine) to e.g. 3000rpm and quickly releasing throttle, there is approximatelly 1 second interval of fueling even though the throttle is in a closed position. I checked this connecting the ECM injector signal (I tried a random injector from each bank) into an osciloscope. My understanding was that ECM should cut-off fuel just when the overrun condition is going on, this is immediately after the throttle is closed. Does anyone have an idea if the extra 1 second fuelling interval should be expected? And why? I checked TP and MAF and they react instantaneously. Tried also a new TP sensor and this didn't cure the problem.


My concern is that during deacceleration there is backfiring in both exhaust banks for approximately 1 second and I worry about a possible demage to catalytic converters.

[jiri_j]

So guys, any thoughts? Is a 1 second fueling switch-off delay expected after releasing throttle?

[PhilipA]

I have to say it has me stumped.
A possibility I can see is if it is a deliberate slowing of the throttle closing process by the ECU to reduce emissions. You have checked that the TPS moves freely.
Have you checked that there is nothing slowing the retraction of the accelerator cable. I have found that on my Thor manifold the rubber cover at the throttle end sometimes comes loose and slows the closing of the throttle blade.
Also check that the throttle cable is not slightly overadjusted, as it could just hold the throttle open and thus the tPS above the limit for idle cut off.
The other thing maybe to check is that the IAC is not hanging up a bit, although the TPS should send the idle signal.
Regards Philip A

[bee utey]

I can't see why 1 second delay to cut-off is abnormal, it is probably part of the strategy to manage gear shift smoothness or something. I suspect the reason you are investigating the issue is because you are hearing the backfiring on over-run. I would suggest it is far more profitable for you to hunt down any small leaks around the exhaust manifolds. They introduce air to the exhaust on decel and promote backfiring. I haven't had any evidence that the small pop noises in the exhaust can seriously damage a cat converter.

With LPG converting I see this kind of thing all the time, and on injection systems you see the cut-off as it happens, on screen. I haven't taken note of the exact interval to cut-off on the D2 but I might have a look next time I plug one in. I suspect it is as PhilipA says an emissions strategy. It will be linked to air flow as the IAC is adjusted down by the ECU on over-run.:)

[PhilipA]

BTW, Motronic has a very powerful diagnostic which can pick up most sensor errors , except MAF errors which can show as o2 sensor errors.

Just a thought. O2 sensor errors can do lots of strange things and can cause richness.
So you may find if the car has done say 140KK plus that new O2 sensors will help fuel economy and maybe fix the problem. You can buy generic Zirconia sensors very cheaply and all you have to do is join the wires. I am pretty sure the motronics have zirconia sensors(with 17MM nuts) and not the more expensive and harder to get titania (12mm nut) as earlier models had
Regards Philip A

[jiri_j]

I have to say it has me stumped.
A possibility I can see is if it is a deliberate slowing of the throttle closing process by the ECU to reduce emissions. You have checked that the TPS moves freely.
Have you checked that there is nothing slowing the retraction of the accelerator cable. I have found that on my Thor manifold the rubber cover at the throttle end sometimes comes loose and slows the closing of the throttle blade.
Also check that the throttle cable is not slightly overadjusted, as it could just hold the throttle open and thus the tPS above the limit for idle cut off.
The other thing maybe to check is that the IAC is not hanging up a bit, although the TPS should send the idle signal.
Regards Philip A

Thank you for comments, mate! I checked the TP and MAF sensors response with oscilloscope and there is no delay at all. If there would be a mechanical problem, I would see it on the TP sensor signal directly.

I am also wondering if the one second delay is there for a purpose (like you said, ECM reducing emmisions, or smoothing deaccelaration) or if there is a delay due to a slow response from a particular sensor or valve.
Is there a way to clean IAC valve with carb cleaner?

[PhilipA]

AFAIK those IACs give few problems. They rotate rather than "step" and are like a rotary valvewhich uncovers a port. They can be cleaned easily AFAIK , but I have never actually cleaned one.
I think my suggestion on my most recent post is most likely the issue.ie o2 sensors
Regards Philip A

[jiri_j]

I can't see why 1 second delay to cut-off is abnormal, it is probably part of the strategy to manage gear shift smoothness or something. I suspect the reason you are investigating the issue is because you are hearing the backfiring on over-run. I would suggest it is far more profitable for you to hunt down any small leaks around the exhaust manifolds. They introduce air to the exhaust on decel and promote backfiring. I haven't had any evidence that the small pop noises in the exhaust can seriously damage a cat converter.

With LPG converting I see this kind of thing all the time, and on injection systems you see the cut-off as it happens, on screen. I haven't taken note of the exact interval to cut-off on the D2 but I might have a look next time I plug one in. I suspect it is as PhilipA says an emissions strategy. It will be linked to air flow as the IAC is adjusted down by the ECU on over-run.:)

Yes, that is exactly the problem - backfiring on overrun. It is a pretty loud backfiring, and since it burns in exhaust I am afraid it could melt the cats in a long term.

I don't see the point why all that additional fuel on over-run should be wasted into exhaust. Then, of course, any exhaust leak would provide air for burning all the fuel in the exhaust. No matter what, I would expect all the fuel to be burned at a proper combustion. Am I wrong?

Is IAC responsible for letting enough air to enter the engine on over-run? I might check and clean the valve.

Regarding o2 sensors: they seem to work perfectly, swinging from 0.1V to 0.8V, responding quickly even at high rpm. I get 0.45Hz at idle. I get both long term fuel trims close to zero. Anyway, with 170t km I might consider replacing them as well...

Thank you guys for your help! Happy to hear more opinions :)

And yes, bee_utey, if you get a minute and could measure the injectors response on over-run, that would be perfect! Thank you again!

[bee utey]

As I said before, more likely your problems will be with exhaust leaks. The IAC won't cause the problem, as more air in the cylinder would improve combustion. It is air introduced after the exhaust stroke that causes this issue. I have heard this kind of backfiring hundreds of times and it always dissapears when the exhaust is air tight, from front to back. Back in the days of air pumps they used to backfire something shocking, the only way to fix it was to plug the air injection pipe. It acted like an air leak into the exhaust. Forget the 1 second over-run, you are losing 0.:cool::cool::cool:1% of your trip fuel here.

[jiri_j]

I had all my exhaust gaskets replaced recently. Both manifold gaskets and y-pipe gaskets. This didn't cure the problem. Backfiring is on both banks.

I went to clean the IAC valve yesterday and it was full of carbon. Used a full can of carb cleaner and now the valve looks nice and shiny :-) Idle has improved, however backfiring didn't. I noticed that the valve is opening as RPM increases. The valve went maximum to 50% position at 4000 RPM. The movement was continuous. The valve didn't open more then halve even when reving the engine very quickly. Should the valve open completely at high RPM? This would confirm unsufficient supply of air into the engine on overrun.

I also measured manifold vacuum and after releasing throttle, vacuum spikes, then stays at this value for 3-5 seconds and then slowly returns to the idle value. Having a perfectly working engine, I wouldn't expect the vacuum to stay at a high value for such long. This again confirms unsufficient air supply on overrun in my opinion.

Last test I did was to unplug a brake servo vacuum line on (actually just before) overrun. No backfiring. Does this all confirm a faulty IAC valve?

[GunMetalRover]

Did this issue end up being resolved?

My backfiring appears on overrun also. I have ordered new exhaust manifold to exhaust gaskets. Hope this sorts it for me.

Cheers

[PhilipA]

I just read through this post again.
Jiri_J have you had a diagnostic read out on the Motronic?
If so what did it say ?
I had another less common thought that your injectors may be drooling.
Regards Philip A

[jiri_j]

I just read through this post again.
Jiri_J have you had a diagnostic read out on the Motronic?
If so what did it say ?
I had another less common thought that your injectors may be drooling.
Regards Philip A

Hi PhilipA, I went through a couple of things and wasn't able to resolve the issue. Still there. I have a scan diagnostic tool and checked every sensor of the engine management system. Everything works normal. Thanks to my mate who had also v8 I tried to swap CKP sensor, IAC valve, TP sensor, MAF, O2 sensors; checking them using a scan tool or using an oscilloscope plugged directly into the ECU didn't show anything wrong. Long term fuel trims +/- 0.8 %.

I had the exhaus gaskets replaced, including the y-pipe to manifold gasket. I inspected cats (using a camera) - perfect. I checked for vacuum leaks, exhaust leaks by pressuring inlet manifold/exhaust system by air. Everything is perfect. Inspected camshaft. The only thing I found was that after releasing throttle on overrun, the injectors fire for 1s (checking the injectors' signal on oscilloscope). Since there is a little air present for combustion (provided by IAC valve), remaining fuel is wasted into the exhaust where it explodes. Hence backfiring...

I talked to Mark Adams (v8performance) about it and he told me that this definetely shouldn't be happening. He could look into the ECU and tell me if it is a programming bug. However, sending the ECU into UK and back would take 2 weeks and I use my Disco as everyday driver...

[nobbyclrk]

Hi


The only thing I found was that after releasing throttle on overrun, the injectors fire for 1s (checking the injectors' signal on oscilloscope).

How much is the injector duration changing?


Since there is a little air present for combustion (provided by IAC valve), remaining fuel is wasted into the exhaust where it explodes. Hence backfiring...



This bit is puzzleling. If there is not enough air in the cylinder to burn the fuel, under compression with a spark. Where does it find the extra air to explode in the exhaust?

[jiri_j]

Hi



How much is the injector duration changing?



This bit is puzzleling. If there is not enough air in the cylinder to burn the fuel, under compression with a spark. Where does it find the extra air to explode in the exhaust?


I am not able to measure duration of the pulses, the resolution of my oscilloscope is not big enough at this time scale. I would guess that the duration is decreasing though.

The extra air most likely comes from IAC valve after the injectors stop firing. I tried to block the valve on over-run (no backfiring), also tried to open it fully (again no backfiring). In the first case the fuel is wasted into exhaust, no air, no backfiring. In the second case there is enough air for combustion and the fuel is burned in cylinders.

So far this is my understanding of the whole thing, but I am not perfectly sure why this happens. I am a bit confused as you, mate... :-)

It is most noticable during engine braking, after a quick release of throttle. Also if I rev the engine up to 3000rpm and release the throttle quickly.

[PhilipA]

Haven't got the three amigos ?

Engine Drag Torque Control
Torque control occurs when wheel slip occurs during deceleration. Engine Drag Torque Control is also integrated with ABS and
is located in the ABS Electronic Control Unit. The rotational speed of the wheels is monitored by the ABS ECU via the ABS wheel
sensors. If the rotational speed of the driven wheels is vastly different to the non-driven wheels during deceleration, the ECU
interprets this as wheel spin. Depending on the degree of wheel spin, the ABS ECU may take one or more of the following
actions.
Instruct the Motronic ECU to de-activate the deceleration fuel cut-off function.
Instruct the Electronic Throttle Control to adapt the throttle opening angle until wheel spin is reduced.
Regards Philip A

[jiri_j]

Haven't got the three amigos ?

Engine Drag Torque Control
Torque control occurs when wheel slip occurs during deceleration. Engine Drag Torque Control is also integrated with ABS and
is located in the ABS Electronic Control Unit. The rotational speed of the wheels is monitored by the ABS ECU via the ABS wheel
sensors. If the rotational speed of the driven wheels is vastly different to the non-driven wheels during deceleration, the ECU
interprets this as wheel spin. Depending on the degree of wheel spin, the ABS ECU may take one or more of the following
actions.
Instruct the Motronic ECU to de-activate the deceleration fuel cut-off function.
Instruct the Electronic Throttle Control to adapt the throttle opening angle until wheel spin is reduced.
Regards Philip A


Thanks Philip. No, no three amigos. I think this is not related to ABS since the backfiring occurs also when the car is parked and you just rev the engine and quickly release throttle.

From discussions with Mark Adams, the ECU should cut fuel immediately after releasing throttle, and when rpm drops below certain rpm (not sure, but as far as I remember it was 1100rpm) injectors should start firing for taking control of idle.

I think I have to live with that for now, I was only worried about burning cats in a long run...