Thread: Overrun fuel shut-off delay

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

Originally Posted by 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

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...

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?

Originally Posted by nobbyclrk

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.

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