TVR Chimera

This beautiful looking car had an uneven idle that would get worse as the car got warmer eventually it would stall out when hot. It was almost undrivable and had apparently been to a number of garages and had a few parts replaced in an attempt to fix it. As a result of the problem it was unable to pass an MOT due to not running long enough to test emissions once warm.

TVR used a modified Rover V8 engine with a distributor ignition combined with a sophisticated (for the time) Lucas fuel management ECU.  It has no standard OBD (on Board Diagnostics) and can only be interrogated for error codes or live data with specialist software and hardware that was unavailable.

The car came with some wiring diagrams and we located information on a number of known/frequent issues on that engine and so had some direction. Unfortunately access to the battery and ECU inside the car was extremely limited being located in the passenger footwell.

The Lucas ECU uses a number of inputs to control fuelling. Outputs are to the Idle Air Control Valve, injectors and EVAP system.  Once we confirmed the IACV was working (a common issue) we started to look at the inputs one by one, starting with the known issues - it was clear that others had been there before, but it made sense to test them first.

Finding no problems with the usual suspects we looked at the remaining inputs.  Hooking up an oscilloscope to the newly replaced heated oxygen sensors (aka Lambda/O2 sensor) we found no signal at all from either bank.  Despite supply voltage there was also no apparent current supply to their built in heaters. The sensors should work once hot without needing the heaters active and as this was contrary to the symptoms we briefly discounted them as likely being switched off by the ECU as the engine was out of cold start.

With no obvious other reason that the O2 sensors should not be working, we reasoned that as they were a long way from the manifold in a very large exhaust pipe we should look again at the heater supply. Finding no fuse in the diagrams they appeared to be switched directly and permanently via the fuel pump relay - which was clearly working as the car was running.

Except we found it was not.  An after-market immobiliser had been fitted that bypassed the fuel pump relay leaving it's malfunction to only affect the O2 sensor's heating circuit and the EVAP circuit. It was working just enough to show voltage but not support current flow.

Due to their location and size relative to the exhaust duct the O2 sensors needed to be almost permanently heated. Their (lack of) output was also ignored during the ECU's extended cold start procedure, hence idling fine when cold.  Without proper O2 sensor input the ECU constantly assumed that the engine needed more fuel.

The fuel pump relay was replaced, smooth idle was restored at all temperatures and the car passed it's MOT.