have just developed a rough idle on my 300zx tt top end power is the same there apears to be no water loss but the exhaust seems more niosy than ususal temp seems ok could this be fuel related any help please

Check your maf, injector connectors and the temp sensor on the front water pipes, mine started doing this again recently and cleaned up the plugs and terminals, seemed to cure it. Check for a vac leak or any pipes off under the throttle bodies and on the balance bar.

cheers for that will check tommorow in daylight

Also check around the rear drivers side engine bay bulkhead and listen for hissing. The small vacuum pipes to sensors and even the boost guage can split and leak causing rough idle. Often shows as boost guage not going right over to the minus side when idling. Another area to check is the connector below the TPS. When mine had an issue there a wiggle of the connector would suddenly have the idling go smooth again. And as Groover says an injector connector can act up but then you should have 5/6th power and possibly a hot cat light coming on due to unburnt fuel.