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Ls400 Mk4 - Idle Air Control Valve

frankie boy

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Can anyone tell me whereabouts the Idle Air Control Valve is situated on a Mk4 Ls400 and what it looks like?

I've had a couple of occasions recently (when starting from cold) where the engine fires up then stalls and is reluctant to restart. When it does restart, it then runs absolutely fine. I've cleaned out the EGR (PCV) valve and want to do the same with the IAC valve but I'm not sure where it is, - I've got a picture from a 'Car Mechanics' feature on LS400's which shows the engine bay layout, but this is for an earlier version LS400 and mine doesn't have the IAC valve where it's shown in this.

While I think of it, - if I remove the valve to clean it, would I then need to reset the ECU?

Any help with this would be greatly appreciated.


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The Air Control valve is on the actual power steering pump itself. If you are facing the pump pully it is on the front just above the pully. The correct way to check if the valve is working is to start the engine and turn the A/C OFF, turn the steering to full lock and pinch the vacuum hose of the air control valve and the engine revs should drop. When it is released the engine revs should rise again to normal idle.

As for re-setting the ECU, remove the Battery terminals for 30 mins (I think thats how long it is but not 100% sure) and re-connect. This will re-set the ECU. The disadvantage of re-setting the ECU is that you will lose any diagnostic fault codes which may give a clue as to what the problem is. You can get these read. The advantage of this is that re-setting the ECU oftem helps to return the managment system to normal.

The correct way to remove the Battery from this Mk4 is to place the key in the ignition and turn it to the ON position, disconnect the terminals and re-conncet after 10 or 30 mins etc. After it is re-connected turn the key off and turn it to the ON position again for at least 6 mins before you are able to start the engine. Also make sure you have your security code for the sat nav before you do this :)

Let us know how you get on.

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Aaahh, - a case of mistaken identity I'm afraid.

I meant the idle air control valve - which is part of the fuelling system, - not the idle valve for the power steering. I've had a couple of cold start problems and wanted to clean out the IAC valve to see if this fixed things. Presumably its in close proximity to the throttle body, but I couldn't readily identify it.

Thanks for the response, anyway.

By the way, I believe you can reset the ECU without disconnecting the Battery. I've got a 'Car Mechanics' feature on the LS400 which advises you to pull a particular fuse to achieve a reset without losing security codes. If anyone is interested I can (hopefully) post a picture. Also, I think you can still get back issues of this edition of Car Mechanics.


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