I have just done a timing belt replacement (today) and I have noticed that I am getting a much worse average MPG, around 16 instead of usual 19 or so. Is this normal? Could this be due to the fact that they (possibly) had disconnected the battery and the PCM is re-learning?

I may be seeing things, but maybe someone else has noticed something similar?

I don't feel any change in power/handling.

 

How many miles have you put on after the T-belt replacement in a day?

 

Hmm. About 10.Why?

 

Feel any change in performance? i.e loss in power.Could be off a tooth on timing.

 

If you are not getting a code you are okay, as differences too far out of range would be picked up. I would reset the battery to help the entire system become accommodated.

Last week I was cleaning the IACV and like an idiot I didn't notice any difference because I hadn't disconnected the battery and then everything was back to normal.

 

A code - you mean CEL light would be lit or are there DTC codes that would not trigger a CEL light?

How long would I need to disconnect the battery for?

 

I dont believe in that disconnecting the battery stuff,not w/obd 2 systems.Get yourself a mini actron (its orange) code reader.And clear it the proper way.They are around $60.00 @ Auto-Zone.Plus any codes come around in the future,and you will be able to read them yourself.

 

I bet nothing has really changed. Your prevoius 19 mpg was built up over many miles. You cannot compare it to your 16 mpg based on only 10 miles. This assumes that they disconnected the battery or otherwsie reset your average MPG, which I bet they did. Disconnecting the battery is a good idea. It will not only reset codes but also the ecu programming based on your driving style. I have a code reader but prefer to use the battery disconnect every so often.

 

The battery disconnect really helps. Leave it off for a minute or so.

One thing also you need to keep in mind is that MPG may take a while to register more accurately.

 

What code reader do you use?

Do RX'es have any DTC codes that can be read through OBD / other means that do not set your CEL on?

I still have a Ford '93 (my son's car now, hehe) and some of the DTCs would show up (minor ones) when there was no check engine light.

 

Pending errors won't trip the CEL light.

 

Myth 2: You can clear the codes by disconnecting the battery.
This is true on pre-96 vehicles and very few, if any, OBD2 vehicles. Some folks will say, "I disconnected the battery for 15 minutes and the light went out, so it cleared the codes". No, it didn't. It may have reset the ECM and the light is no longer present, but the code is still there and if the problem has not been repaired, the light will come back on. The next time you have a problem, now you or the mechanic who is working on the vehicle are going to have to contend with that code as well as any other that is present.

 

Will any cheap OBDII scan tool do or does Lexus require something special?

 

Yes sir,dont need anything fancy.

 

Quite true.

Why I recommend the battery disconnect is to give a chance to clear any variance that might have been detected by the ECM and then caused the ECM to change some parameter to accommodate a given event (say a P0171 error caused by too much air entering). The example of me cleaning my IACV and then seeing no result until I disconnected the battery is an example.

Yet, we unfortunately have the case that many a time the code comes back because something is truly wrong, especially in the case of the A/F sensors.