Shunting

[jimmyV8]

I have some quite a lot of shunting going on at low speeds in my 98 400 Chimaera. I suspect it could be the rear diff bushes but just wondered if there are any other things i may consider doing before diff removal? CV joints maybe? Cheers

[fenderbender]

Assuming this is play in mechanical items and not something more exotic** then you just need to check all joints in the transmission system for play. Start at the gearbox output shaft and move backwards.

• My own car shunted quite badly at low speeds til I discovered the intermittent short in the circuit to the injectors.

[blitzracing]

Its a combination of factors both mechanical (as mentioned) and fuel related. Basically the cars cycle lean at around 1500 RPM, due partly to the way the catalyst fueling is arranged. This constantly sweeps the mixture rich / lean to minimise the emissions, and needs feedback from the lambda probes to do this. Unfortuatly the pre cats TVR fit alter the signal the probes see, and distorts the timing and amount of this cycle, that ends up as shunt. This then wears the mechanics so it gets worse over time. A simple test to see if it is down to the catalyst map is to change the tune resistor under the passenger dash to 400 ohms and try the non catalyst fuel map. This runs quite a bit richer, without all the cycling process, but is like to damage the catalysts over time. I have been working on various fixes with "That daddy", but these need very carefull setting up to get it spot on, so not ideal as yet. Still tinkering though.

Mark

[jimmyV8]

Some of this is low speed shunting i gues from the mapping issus as mentioned but there is constant knocking noise during this which i assume to be bushes or drive shafts? Or both?

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Just normal backlash in the cv joints / diff can cause knocking noises.
Depending how bad it is depends if it is normal or not.
Mine has shunted from when I bought it at 3 years old and 7k miles, and when it shunts i get a slight knocking occasionally.

I just try not to let it shunt now by driving in a lower gear than normal and thus keeping the engine revs up a bit more.