Thread: Overrun fuel shut-off delay

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

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

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.

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

Originally Posted by 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

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?

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

Originally Posted by 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.

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!

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 0.1% of your trip fuel here.