Lexus Ls400 poor economy after timing belt change/service

[LS]

Not sure if this is under correct category but im new here.

Basically i used to average around 17 mpg in the city, which was slow start/stop driving. I would get 40 miles out of £10 petrol.

I then got timing belt/fan belt kit replaced, new water pump, idlers etc and a full service. I now get 40 miles out of £15 and have done around 70/80 miles since having the work done.

I've read a lot of conflicting information, some people saying garage may have knocked a few sensors etc out of alignment or spark plugs may be loose. However also heard that apparently as Battery would have been disconnected this resets the ECU and the car will have poor mpg until it re-learns my driving habits etc. 

One thing i noticed soon as i picked the car up was when i started it there was a split second clunk noise which sounded like it came from under drivers side at front of car. However only happens occasionally and as usual didn't happen when i told the mechanic. Not sure if related in any way.

I doubt it is the o2 sensors etc as some people have said on other forums because it was immediately after i picked the car up.

 

Also i always fuel up as soon as the petrol light comes on however when i picked the car up and put petrol in it barely budget above the limit when the light comes on. Therefore im guessing the garage must have really drained the fuel tank so i dont know if this maybe affects the ecu in any way because i know that is bad for the fuel pump. Dont know why they would have done that as i left plenty of fuel in the car for them.

 

Any advice is appreciated because i dont really know what to do 

[Bluesman]

Hi Jav and welcome to the LOC, we are a friendly bunch and some of us even know how to pull them to bits and put them back together again, with of course the one bolt and washer left over.

I cant help with your present queries but I'm sure someone will be along ASAP.

We recommend you spend a couple of mins looking over the rules of the LOC, we will be asking questions later

http://www.lexusownersclub.co.uk/terms-conditions/

 

Regards Mike.

[steve2006]

Hello and welcome to the LOC.

The bigest cause of poor fuel consumption is a defective temperature sender unit, on thE LS400 there are 2 one for the gauge and another for the ECU.

With the work carried out thIs may have been disconnected and not plugged back in or the plug damaged, the sensor is located under the front spark plug wire tray.

What is the engine idle speed when fully warmed up?

[LS]

Thanks for that mike i will have a look over the rules etc. 

 

Right i think theres a good chance its that steve, because my revs are sitting a lot higher than before.

 

Before my service:

Cold start : revs just below 1k rpm

When warmed up: revs settled around 500rpm

After the service:

Cold start: revs just below 2k rpm

When warmed up: revs settled around 1k rpm

 

 Would you recommend taking it back to the garage for that or is it fairly straight forward to do yourself? 

[Bluesman]

I would definitely take it back, remember to stay cool and be very polite. You get far more out of anyone if you remember that. Good luck let us know how you get on. Mike.

[LS]

Justt thought i would update this as problem is now resolved. It slowly improved the more i drove and now economy is back to normal...20mpg hahah. Im guessing it was maybe ecu relearning or something a few people suggested that but i have no clue. So if happens to anyone else i would suggest waiting it out a month or two and take it on few motorway runs etc

[LS]

Also the revs etc have all settled back to how it was before i had the timing belt done..

[steve2006]

Thanks for getting back to us, every bit of infomation is useful.

What may have happened was the Battery was disconnected for some reason during the service and this would cause the ECU to reset to default condition, it has now relearn the correct parameters.

Correct hot idle is 650 RPM +\- 50.