Recently I have just changed my timing belt and cleaned up all the throttle body. And I am getting worse fuel efficiency. I don't want to send the car back to the gauge before I know what's the problem.

Is there any self diagnosis system on the RX300? My CEL won't turn on.
I am now thinking to buy a OBD II Scanner. However, before buying the scanner, is there any way to have a self diagnosis on this RX300 system?

 

The self diagnoses is through the OBDII port. Lexus may have their own diagnosis machine. Check Lexmex thread on the scanners.

If the CEL is not coming on, I would check mechanical drag. By the way it takes about 10 days of driving for things to settle and engine to fine tune itself. [No hard data, but after I did the clean up, that is how long it seemed to me]

Salim

 

10 days is about right for things to fully settle themselves out. I saw that firsthand in Mexico and even back here in Florida after I'd fiddle with my vehicle. The thing I mention about 50/50 highway/city drive for a bit is to deal with the readiness monitors, but the vehicle still continues to 'learn' about your driving habits for a bit longer than that. I think we talked about this maybe more than 5 years ago on the forum about how long it took in miles, and we conjectured about 100 to 200 miles max before we saw things smooth out, but 10 days of driving is about right.

Even if the CEL doesn't turn on, there may be what is called a pending code in the computer indicating a condition that is there but not serious enough to merit a CEL, meaning more or less the condition is sporadic or not common in rough terms.

OBD2 scanner will tell some things, but more advanced laptop software or even a more advanced scanning software would tell a bit more about the status of the A/F sensors and fuel trims.

It is conceivable that one's own driving pattern can affect the fuel mileage.

Now that I don't race anymore, whenever I reset the battery after modifications, I take it very easy on the vehicle to ensure it learns a driving pattern that is more favorable to better mpg or km/l for those where that applies.

I feel I personally had a tendency even recently to race or pull off at the light after doing modifications and resetting the battery and I wasn't doing anything to improve mpg, so I had to force myself to take it easy and slow.

 

Is your new TB off a tooth or two?

 

It could be a whole host of things but I would check this (the timing) first.

 

Hello there,

Thank you very much for the replies.
I have tried to reset the ECU by pulling out the 20A EFI fuse.
However, I am quite confused with Lexmex's advice, by keeping the key on the ignition. Does that mean the engine must be running when I pull out the fuse?

I have tried to pull out the fuse with the key set to "ON". Once I have pulled out the fuse, the fan near the radiator started turning immediately. Is that a correct sign?

Many thanks.

 

Key "OFF" or no key in the ignition then pull the fuse out.
Wait about a minute then put it back in.

I'm kinda confuse when you state that "check engine" light doesn't come on.
With the key in the "ON" position, does your "Check engine" light illuminate?

 

I am confused.
Does the lamp turn on when you move the key to run position (before start)? [this checks that the bulb is fine?
When the engine is running, does the CEL turn ON.
If you are trying to reset the light, it needs to be set "on".

Salim

 

OKAY, let me try key off first.

And the CEL doesn't turn on, and it should have pending code though.
I have just bought a OBD II Scanner GS500 like the one below.
http://cgi.ebay.com/New-OBDII-Code-R...item4aa1ae6a08

And then I will try to plug it into my car tonight to see what is happening. The fuel efficiency is now just like my O2 sensor blown off last time.

 

With the scanner connected, you need to turn the ign key to ON or run ... one past the ACC and before start.

Instructions should be with the scanner. Pending code is a diagnostic aid ... the on board computer has encountered the condition, but the the condition is either beeing further accumulated or it happened and went away.

Typically the scanner will have a mechanism to erase the set code or pending codes.

Salim

 

You can try a test of the CEL by temporarily disconnecting the MAF sensor (don't turn on the car though), but it should register a P0100.

Keep in mind there may not be any pending codes. That can happen even if the A/F sensors are degraded, but sometimes the codes won't come up, not even a pending code for them.

If that scanner you have can read the fuel trims or the A/F sensor readings on both banks, that information would be very valuable for me to see.

Just the other week I was helping one of my neighbors with an older Acura TL that was not reading any codes (even pending) diagnose and idle issue we traced back to his IACV by reading the diagnostic software. The OBD system doesn't catch everything even in the 21st century.

 

I have just used the OBD II Scanner to check my car.
No stored code and no pending code was found.

Here are the readings:
Intake Air Temperature = 86F (30c)
Mass Air Flow Sensor = 0.669 (lb/min)
Absolute Throttle Position = 11.4
O2 Sensor Output Voltage (B1S1) = 0.640V
Short Term Fuel Trim (B1S1) = -5.5%
O2 Sensor Output Voltage (B1S2) = 0.120V
O2 Sensor Output Voltage (B2S1) = 0.630V
Short Term Fuel Trim (B2S1) = -3.9%
Short Term Fuel Trim - Bank 1 = -3.1%
Long Term Fuel Trim - Bank 1 = 0.0%
Long Term Fuel Trim - Bank 2 = -7.0%
RPM = 881 / min
VSS = 0 mph
Ignition Timing Advance = 16
Calculated Load Value = 23.5%
Engine Coolant Temperature = 145F



Seems like O2 Sensor Output Voltage (B1S2) has a significant lower value than B1S1 and B2S2. Is that the sign that B1S2 is not functioning correctly?

 

B1S2 is the O2 sensor located after the catalytic converter located midway through the exhaust. (under driver seat)
That reading is fine.

B1S1 and B2S1 are A/F sensors(sensors at each exhaust manifold). They should be reading about the same value, which they are, so no fault there.
Fuel trims are about right. Still well below threshold value.

I just don't understand the value of B1/B2-S1 it's showing (0.630V)
This reading reminds me of O2 sensors which fluctuates between 0.8-0.3v at idle.
RX300's have A/F sensors and have voltages around 3.30v at idle and no fluctations.

What kind of MPG are you currently getting?
What were you getting before the "work" was done?

 

Hello Nad1370,

My fuel efficiency has dropped from ~6.4 km/L (15.05 MPG) down to ~5.3 km/L (12.46 MPG)
Before that, I have just changed the all the belts including A/C and timing.

After driving for around 10-15 days, my fuel efficiency drops significant. 5.3 km/L is the same as last time one of my O2 sensor has gone bad.

That's why I have bought a OBD II Scanner this time to see if that is the O2 sensor problem.

 

Those readings on the two banks of the A/F sensors and O2 sensor are inline with what I've had. The O2 sensor reading I wouldn't pay too much attention to, that's an emissions thing.

I'd be interested to know, and in line with Nad1370's comments, about what fluctuation you see on the A/F sensor readings, how low do you see that number go and also how high you see it go.