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[PSABIN8951]

My 3 litre MV6 has just started returning much poorer fuel economy (used to give around 30mpg in town now dropped to 21mpg) it has also started idling high at 1000rpm (used to be 700ish). Ive checked throttle stop & cable all as they should be with no dirt or excessive tension. Ive blasted thottle butterflies etc with carb cleaner and done the same with IAC valve all made no difference. Any suggestions please before I go bankrupt filling her with fuel! Oh and its a manual box

[Varche]

Vacuum pipe leak somewhere?

Is the IAC seated properly? If not use a cable tie to pull it into the plenum.

Mine had a split under one of the curved vacuum pipes at the back of the plenum area. Couldn't see it.

[cem_devecioglu]

major reason for increased fuel consumption is either an air leak(into engine)  or exhaust leak(from exhaust pipes)..

and also some old sensors..

[albitz]

I wish I could get my 2.5 to do 30mpg on average, never mind around town.

[PSABIN8951]

Ok quick update - checked breather pipes, all clear and sprayed vacuum pipes and also ICV to plenum connection with carb cleaner, no sign of leaks but....
When I pull out the ICV from the plenum the engine slows down to a more normal level. Ive checked for voltage on the connector to ICV which is sitting around 11.8V at idle with warm engine. So does this point to the valve having died?

[I_want_an_Omega]

Ok quick update - checked breather pipes, all clear and sprayed vacuum pipes and also ICV to plenum connection with carb cleaner, no sign of leaks but....
When I pull out the ICV from the plenum the engine slows down to a more normal level. Ive checked for voltage on the connector to ICV which is sitting around 11.8V at idle with warm engine. So does this point to the valve having died?

Dunno - but I've got a spare one you can play with if you want.

[Kevin Wood]

Ok quick update - checked breather pipes, all clear and sprayed vacuum pipes and also ICV to plenum connection with carb cleaner, no sign of leaks but....
When I pull out the ICV from the plenum the engine slows down to a more normal level. Ive checked for voltage on the connector to ICV which is sitting around 11.8V at idle with warm engine. So does this point to the valve having died?

Is there a good seal on the rubber plug where the ICV attaches to the plenum? They can leak here.

Also, try pinching the smaller diameter pipe that feeds the ICV from the front. The engine should be starved of air and stall when you do this. If the RPM just drops it is getting air from somewhere it shouldn't.

[cem_devecioglu]

spray carb cleaner between the ecotec bit and plenum..if idle changes you have a leak there..

check the 6 o-rings under the plenum (if they are old they can also leak)

[PSABIN8951]

I have tried spraying carb cleaner at the plenum/ICV joint area and no change there. I will try pinching the small pipe though - ill let you know.

[PSABIN8951]

Pinching pipe did cause car to die. Ive now fitted a secondhand ICV and things are looking better, still idling a little high (750ish) i have done a full reset though (battery disconnected for 2 hrs) Any ideas how long it takes for ECU to fully relearn optimum settings? Driving to work today im back to 25mpg or better around town.

[Kevin Wood]

Might be worth checking that the throttles are closing completely. When the ICV gets dodgy it has been known for cars to be bodged by winding up the throttle stop so it doesn't keep stalling.

Throttle stop should be adjusted so that the throttles close completely but without them sticking in their bores.

[PSABIN8951]

Throttle stop adjusted slightly. Runs ok but lacks power on very light initial throttle then goes like a bat out of hell after that. No stalling just returning around 24-26mpg irrespective of how i drive.