Drop In Mpg

[NormLS400]

Hi, I've been experiencing a drop in MPG over the past few months. I've
owned the car for about 3 years and regularly do a trip from the Scottish
Borders to Aberdeen. Same route, same petrol and same speed but I'm about 3-4
MPG off. Any idea what could be causing thing? I know it's not a massive amount
but just wondered.

[aviator888]

Cold weather (engine takes longer to warm to working temperature) sticking brakes, incorrect tyre pressure. Check if any of the alloy wheels get hot after driving for 20 minutes or so.

[NormLS400]

Yeah the cold weather does have an impact but I'm comparing against the last few winters as well. Wheels seem to be turning freely, I did jack the car up and check. Tyre pressure is also fine.

[cruisermark]

engine coolant temperature sensor?

although havent noticed many complaints here but in the USA the guys change this part alot:

http://www.clublexus.com/forums/ls400/377171-diy-coolant-temperature-sensor-change-pics.html

[zoran]

Have you had new tyres recently? i changed my tyres (all 4) and my mpg dropped by about 5mpg just with a different tyre brand on

[NormLS400]

No same tyres as well. Oddly it seemed to start around the time I changed the air filter but surely that can't be the cause?

[Colin M]

I would follow up Mark's suggestion of engine coolant temperature sensor.

I have been reading the thread Mark mentioned recently and am thinking of replacing mine in the spring when the better weather comes.

From what I have read, if the sensor is not working properly, the engine ECU will think the engine is not at the correct working temperature. It will according deliver more fuel to the engine, which will result in less mpg. The engine may run at higher revs, as it does when it is cold, for longer than it should.

I think this is what is happening with one of my cars. The revs do seem to be higher for longer than I would expect, although they do eventually drop down to the normal level. This is why I am thinking of changing the sensor.

[scudney]

When did it last have a service,when where spark plugs last changed??

[ColinBarber]

Ideally you want diagnostic equipment to give you the coolant temperature reading, or failing that just measure the sensor's resistance when hot and compare to another vehicle - seems a waste of time and effort changing parts if you don't know they are faulty.

It could also be worn oxygen sensors making the engine run a little richer than it should. MAF sensor needs cleaning:

http://www.lexusownersclub.co.uk/forum/tutorials/article/103-rx300-series-i-cleaning-maf-sensor/

Check the temperature of each wheel after a run, is one hotter than another which would point towards a sticking calliper.

[NormLS400]

Thanks guys. I think it may need spark plugs actually as it's not had new ones in my ownership anyway. May be a good place to start.

[Malc]

Spark plugs change is at 60,000 miles btw

Malc

[Titch LS]

I've noticed that I definitely get less mpg in the winter months. It drops about 2 mpg. Having said that, the coolant temperature sensor is not expensve and looks a doddle to fit if you decided to replace it. (I havn't replaced mine yet).