moonphase

iamdk72986;
With the key on/ engine not running, the throttle position sensor harness terminal #1(holding harness connector facing you, and the latch on top. Left to right = #1#2#3) should be supplied with 4.5 -5.5 volts positive. If the voltage is either too low or too high, the PCM should be checked by the dealer. Go key off and reconnect the harness. Go back to key on (engine not running) and back probe the other two wires, #2& #3. With throttle closed voltage should be 0.3-1.0 Then slowly open the throttle. The voltage should increase smoothly to between 3.2 and 4.9 volts on a 4 cylinder engine, or between 2.7 to 5.2 volts on a v6 engine. If readings are below, or above specified range, replace TPS. This procedure is from a 97- 01 es300 manual!
On a side note, Toyota coils go out at around 100,000 miles. You can check the coils cold for resistance and if one is much different than the others, I would replace it. I think the best way to test is to purchase a spare to swap one at a time and see if there is improved performance. I bought a used/ like new Beck Arnley coil from Amazon.com- today's deals- warehouse deals for something like $15.00, and it turned out to be a new Toyota replacement coil. Bad coil = increased oxygen seen by o2 sensor= perceived lean condition due to unburned oxygen. Good Luck

iamdk72986

Baetke, if I remember correctly, the IGN ADV was around 25-26 degrees the last time I had the car plugged in. Since I need to do the timing belt anyway, I'll be sure to take a look at the timing marks before I remove the old belt. I have to do the valve stem seals on the car as well so it'd be a good time to do that. On a side note, could a stretched timing belt cause a reading like that?

moonphase, I'll check out the TPS sensor when I get the chance. As far as the coils go, I've been under the impression that they either work or go out. I guess it's not a bad idea for me to replace all the coils anyways since the car has hit that century mark. I've also been under the impression that an unburned fuel mixture would read rich. It's pretty neat how I'm learning something new everyday.

Thank you both again for all your help and I'll be sure to report my findings.

Baetke

Daniel,
The timing advance is very much a function of engine load and rpm. Saying that the timing is 25-26 degrees all by itself doesn't provide enough information to tell whether that is a normal reading or not. Thats why I described the conditions at which I observed the timing on my car in each of the examples above. If your reading was at idle, under the conditions I described, then there is something wrong for sure. A reading that is off by 10-15 degrees could not be explained by a stretched timing belt. I would think that would only contribute a few degrees at most. Since you're going to change your timing belt anyway, you will be able to tell if the marks are "on" before you take the old one off.
Keep us posted
Baetke

iamdk72986

Baetke, that reading was at idle on a warm engine. I think that explains my spark plugs showing signs of overheating. I'll keep you posted.

Daniel

moonphase

i;amdk72986;
My understanding of incomplete combustion is that the PCM is told of an abnormally high oxygen level by the o2 sensors, It will increase fuel pulse width to return air fuel mixture to 14.7 to 1 ratio. If the exhaust oxygen level stays high, the PCM will continue increasing fuel again and again(stft then ltft) until the max threshold is reached and sets a code. Although the PCM is creating an overly rich condition, it has no way of detecting it as it can only look at unburned oxygen content in the exhaust. Obviously each componant needs to be evaluated, and tested, but you can look at O2 sensor values with your scan tool at closed loop to see what the PCM is thinking. Earlier, you had mentioned that the vehicle runs better in open loop, which would make me think that a sensor is failing, but if it isn't all that smooth running in open loop then some other engine component has trouble as closed loop means the PCM is using the sensors to reach 14.7 to 1 ratio.
Anyway, do a quick check to make sure the air tube running from the air cleaner to the throttle body is not cracked, or loose as this will let in oxygen that the PCM wouldn't see until it hits the O2 sensors. Good Luck.