Have a look in gomog. In the Plus8 specific section. There is also a link to the Gems manual in there.
Is your MIL light coming on at all?
Do a diagnostic check using an OBD tester.
I had a 2002 Plus8 with Gems. I have an Autel Maxiscan that is a good tool. By using the gomog links and Maxiscan I sorted a couple of issues.
First was MIL light intermittent. Maxiscan diagnosed an " intermittent random misfire". Checked everything but what sorted it was moving to V-Power petrol. It only came back when reverting to 95 octane petrol. I then alternated fuel to reduce the prevalence of 95 octane.
Second MIL light trigger was for an oxygen sensor problem. Identified as a post cat sensor heater problem. As it was post cat no effect on engine running. New sensor found using the Gems manual and Maxiscan to pinpoint and confirm the sensor that failed, and part numbers found in the manual and ordered from Rimmer Bros.
A diagnostic check is a preferred first step as it might narrow down the issue and save random component changes as there are several that might be the cause. Any faults found by the Gems will be kept so the OBD tester would ( should) find them.
