tick over yoo high

ive got a 97 disco 3.9 v8 auto,in neutral the tickover is 850/900 rpm. whilst driving tickover at 1250 rpm so im riding the brakes all the time . ive checked all the hoses in + out of the plenum + they are fine , ive cleaned the stepper valve at the back of the plenum . she has a cone shaped air filter fitted cos the lpg regulater sits where the air box was. tickover the same on lpg or petrol does anyone know whats at fault here , cheers steve

That is a symptom I have had ... and no power. Got leak in plenum chamber... see thread on Help please all else stumped...

as badger said air leak or the base idle needs reseting