Misfire under load

I have a 2001 Defender Td5 with 155,000 miles that is misfiring on hard acceleration. I have changed the battery and injector harness. When I plugged in a code reader, the following codes were logged (engine light not on): ‘Manifold pressure circuit low’, ‘Inlet air temp fault low’, ‘Multiplex input circuit fault low’, ‘1% ref voltage fault low’, ‘Driver demand 3 fault low’ and ‘Ambient pressure circuit fault low’. Mike Oakes

As you have changed the injector harness, I am assuming this is for the modified unit. The injector harness and connections were subject to problems from oil ingress, allowing the oil to seep into the ECU, and the remedy was to replace both the injector and engine harness.

Having said that, not all the codes you have read indicate a problem with the loom. It would be helpful to have the fault code numbers, but going on the description I would suspect that the EGR valve may be sticking, which may have allowed the MAP sensor to become contaminated and would account for the low pressure.

As a first step I would clean the EGR valve and MAP sensor and also try cleaning the MAF sensor. The EGR and MAP sensor can be cleaned with a carb cleaner spray and you can use either a dedicated MAF spray or an electrical cleaner for the MAF sensor. Ensure all components are fully dry before starting the engine. If cleaning the components does not remedy the situation then a new MAP or MAF sensor may be required.

The problem could also be related to a poor electrical connection to the ECU or even an instrument cluster problem. Land Rover Defender owners have reported a short circuit within the instrument cluster that results in a host of seemingly unrelated fault codes. With this in mind, I would pay close attention to the connecting plugs and ensure the loom is not damaged in any area.