Off Throttle Tuning Problem

[260DET]

Basically the problem occurs when slightly lifting off the throttle, the transmission then shunts badly, bang clang. The sort of situation which often occurs when driving in city traffic where the throttle has to be feathered to reduce speed slightly rather than fully lifted off. Problem does not appear to be clutch related.

Any suggestions? I would do some research myself but can't find any detail as to what potentially comes into play with a M600 when a partial throttle lift off occurs.

[Holmz]

Basically the problem occurs when slightly lifting off the throttle, the transmission then shunts badly, bang clang. The sort of situation which often occurs when driving in city traffic where the throttle has to be feathered to reduce speed slightly rather than fully lifted off. Problem does not appear to be clutch related.

Any suggestions? I would do some research myself but can't find any detail as to what potentially comes into play with a M600 when a partial throttle lift off occurs.

It would likely be fuel or ignition... Asif there is much left to choose from.
So not really m600 related per se.

If it is still providing positive output (power), then the transmission gears should stay loaded in the same 'direction'.
If you come off the throttle to the point where the power is going into the engine (rather than coming out of it), then the gears become loaded in the other direction.

So if it banging away, it sounds like the power is going around the 'net-zero' mark <in-out-in-out>.
Is it misfiring around there?
Do you have any Lambda measurements?

Let's assuming that your steady state tuning is spot-on.
Transient/dynamic tuning is probably a more involved and difficult thing to get right.
Maybe it going rich with wetted out runners and airflow going to zero??

Does it do this bang-clang just for a short time after the initial lift-off, or continually at that throttle position?
If for a short time, then how long?
And does it decrease in intensity in some fashion?

Perhaps the most important question is:
Is the throttle cable or DBW?

[260DET]

Thanks H, it sounds like your power on power off analysis may be applicable, it makes sense anyway. As does someone elses suggestion that it is likely to be tuned too lean in a light load situation. When it occurs the shunting is continuous and consistent, no DBW. As mentioned in the heading it seems likely to be just a tuning problem but there is no general tuning forum here.

Just remembered that the injectors have just been cleaned and needed it, the spray patterns were not good before.

[FoxPerformance]

deceleration fuel cutoff on cars with very high duration cams can cause a bucking at low speeds and light loads on/off throttle, maybe not exactly what your feeling but may possible be part of it?

whats your AFR doing during this problem

you may also want to check that your throttle blade isn't going completely shut and sealing off air flow during this period and the engine is solely relying on the idle air passage way through the idle stepper motor

[Holmz]

deceleration fuel cutoff on cars with very high duration cams can cause a bucking at low speeds and light loads on/off throttle, maybe not exactly what your feeling but may possible be part of it?

whats your AFR doing during this problem

FoxPerf is right.
260DET> Even without the fuel cutoff you need to know the AFR, but the fuel cutoff would be hard to work around (f not impossible, then difficult)
Do you have fuel cutoff enabled?

...As does someone elses suggestion that it is likely to be tuned too lean in a light load situation. When it occurs the shunting is continuous and consistent...

Well that is good news.
1) Non-continuous means that you would need to work on the dynamic aspect.
2) Continuous means that you just need to tune for the steady state part.

This could be difficult on a dyno if it is a condition where the car is rolling faster than the engine is producing power to enable it to maintain that speed... I am not sure if you can do this on a dyno - but I am no dyno expert - so maybe you can???

If you have Lambda measurement capability then you could get it into that state <bucking and balking> and measure the AFR to get that sorted out.
You may be able to do something with the advance as well? I am not too sure, and decelleration might be a different condition than acceleration. Probably just some tuning the higher RPM cells with low throttle position (or MAP) to enrichen the AFR and maybe adjust the advance there too.

If you know the RPM where is happens you also know it is at light throttle, so then you could probably even adjust it blindly without AFR, just by hitting those cells directly... (It is probably not currently too rich there, but if it gets worse then go the other way)
You can play this while you are working on it...
http://www.youtube.com/watch?v=KXewIR7Y7cc

[MarkMc]

Probably better mention what type of car, gearbox, mods, etc. Is the overrun fuel cut function activated, how sensitive is your Acceleration Enrichment, etc?

LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING, LOGGING,

It is almost pointless trying to do things without it these days. The ECU will remember 64 channels at 200Hz, I have not met a driver that can.

[SprinterTRD]

More than likely it is too much ignition timing at light load with bigger than standard cams, so its simply a tuning issue.

[Scott@FP]

Blow off valves with strong springs will also cause a similar issue, especially when fuelled MAF.

[Chris Wilson]

It could also be nothing to do with the ECU tune, for example, fitting a one piece flywheel to a car that had an OE dual mass, and even worse, also fitting to it a none dampened clutch driven plate, when the OE set up also used one. Such a combination can make a perfectly tuned and healthy engine and drive train sound like a broken cement mixer at certain loads and RPM.