alankehoe

If you do end up replacing it make sure it’s the denso part. I did replace it first with a spurious alternator which lasted about a week. The denso part has been on the car now for 60000km and still going strong..

My alternator sounded exactly like that for a good few months before it finally died and left me stranded, so that would be my hunch.

I am sure someone will be along to help diagnose this properly.

Sounds like the alternator to me mine went at about 200,000 KM and just before it completely died was behaving as you describe. I ended up sourcing a denso alternator for it for a little over €200 

It does seem strange that it has been behaving like that for over a year however but still my bet would be alternator

if you do end up going to replace the alternator beware of spurious parts - 

Try Amayama (they can be a little slow for delivery),

RH Seat cushion (SEAT-SMOOTH LEATHER) -  https://www.amayama.com/en/part/toyota/7151153080

RH Seat cushion (SEAT-SEMIANILINE VENTILATED LEATHER) -  https://www.amayama.com/en/part/toyota/7151153110

You can probably get the correct part number for your car from a dealer if you are not sure. 

Mine has averaged 28 MPG (UK) over the last 3 years,

you can see in the attached image the first year or so was mainly city driving where it can be quite thirsty however on the motorway it is in its element. 

whats the voltage of the Battery now ? the car wont start if it has already been depleted !

To conclude what the issue was here: 

After the alternator failed it was replaced with a spurious part which was faulty and surged when the engine was hot, this alternator since failed and was replaced by a Denso part. No problems with the car since then.

Best thing to do is check the voltage of the Battery when the engine is running if its not around 14v then I would be almost certain your alternator has failed thus your Battery is being depleted.

I had a failed alternator on my is250 and when it happened everything on the car shutdown [Steering, Brakes, Engine Misfiring] so I am not sure you can trust the diagnostic error code at this point.

2 minutes ago, m4rkw said:

Is the steering hydraulic or electric on those?

electric

Sounds like the alternator could have failed ? was the Battery light showing on the dash when you broke down ? 

A bad crankshaft sensor would be more likely to make the engine run rough than not run at all.

Been thinking about getting the alloys powder coated for almost 3 years now, I needed new tyres so took the opportunity to get them done. Should have done it earlier not bad for an 11 year old car

Does anyone have any idea what might be causing the car to idle rough, see details below.

Details

2008 Lexus is250, 120,000 miles / 200,000 km

Description

Idling rough when the engine is hot and in either drive or reverse. when in park or neutral idle is fine, see the video here https://photos.app.goo.gl/CuDuxKGNB64WwuuM7

Recent History

This seems to have started after the alternator was recently replaced due to failure, also worth noting that the air-con currently is empty of gas however I am not sure that makes any difference. Initial thoughts on the issue was that the crankshaft sensor may be bad but we have replaced that and the issue remains.

On a trip from Dublin to Belfast yesterday [100kmh cruise control on], its amazing what this car can do on the open road 5.9 l/100km equates to about 48MPG which from a 2.5 v6 is very impressive. however it also has a very impressive way of easily achieving 20MPG or so when driving around town :)