Engine hunts when cold

Q My 2001 Vauxhall Vectra 1.8 petrol with the Z18XE engine starts up fine, but then hunts or misses until it has warmed up, after which it runs fine. The missing has slowly got worse, but

there is no EML lit and no fault codes stored according to two code readers – an Autel Maxiscan MS409 and Foxwell Automaster Pro NT624. I have also looked for air leaks and found none. I have changed the engine temperature sensor, which has a resistance of 2.3 kohms at around 21°C.

Wes Keeley A Having already checked for air leaks around the induction system, hopefully this can be discounted as a cause of the problem.

Although you have already replaced the coolant temperature sensor, there is a common problem related to the

CTS multi-plug on your Vectra. This can fail due to vibration and the multi-plug should be replaced. Autodata informs me that a wiring adapter kit and harness is available, and the part numbers are

93174557 for the adaptor kit, 90543875 for the harness and 90543865 for the heatshrink kit to protect the new harness.

Inspecting the plug carefully and using an ohmmeter to check the continuity of the wiring should highlight any problem in this area. If there are no problems in this area, then another possible cause is the throttle body operation. This may be contaminated with carbon deposits, so cleaning out the throttle body may rectify the fault.

Another possible source of the problem is that the heating circuit on one of the 02 sensors is not operating. This would normally flag up a code, but is another point worth checking.

Finally, the MAF sensor can cause problems by giving inaccurate readings to the ECU, but this fault is generally not confined to a cold engine.