rjm0521

I have a 2000 model RX300 with 81,000 miles. It started running a little rough and the CEL came on. I took it to Auto Zone and their diagnostic computer said the O2 sensor was bad. I replaced the O2 sensor (pre-cat) and still had problems. The engine code stated Random Multiple Misfires and the guy at Auto Zone recommended new spark plugs. I installed a new set of NGK Irridium plugs. CEL still on and still running rough. Then the engine code stated bad MAF sensor. I replaced that and I'm still having trouble. Now I get P0300, P0301, P0304, P0305 error codes. These are Random Multiple misfires on cylinders 1,3,4, & 5. Now the O/D won't engage at highway speeds.

The Auto Zone guys are scratching their heads now. One said coil packs, another said high fuel pressure, and another said IACV module. I've already spent $240 O2 sensor, $50 plugs, and $85 MAF sensor. What's next?

thomas1

You could try cleaning the IACV: http://us.lexusownersclub.com/forums...c=7408&hl=iacv
I wonder if your Oxygen sensor is bad or the heater is open in one of them. the two precat sensors are wired in series on the heaters. What kind of plugs did you put in. I hope not Bosch, They are crap. Also the coil packs could have corrosion on the springs that touch the plugs. Did you see any corrosion? Look with a flashlight inside the coil pack. I want to repeat the Bosch plugs are garbage, had a lot of problems on a lot of different cars with those plugs. Only use NGK or Nippendenso plugs. supposed to be iridium.

rjm0521

NGK plugs. I originally bought Bosch plugs and when I returned with no improvement one of the Auto Zone guys told me that Bosch plugs are crap. They refunded me the $72 for the Bosch plugs and I replaced them with NGK for $50. Even though it's not running right, I could detect a slight improvement with the NGK plugs. I've already cleaned the IACV.

I noticed the post: [URL="https://www.clublexus.com/forums/showthread.php?t=295592&highlight=knock+sensor"]

But if this is the problem why didn't this code show up?

onepointon

just my 2 cents: you could start with using factory parts.

you really need to see what cylinder is misfiiring consistantly. i have seen vehicles that throw codes for misfires on cylinder that arent misfiring simply because 1 cylinder has a bad one. you should clear the codes, go drive the vehicle till a code comes back and recheck. when you find the cylinder(s) that is/are misfiring, try switching your coil packs around with cylinders that arent misfiring. if the misfire follows the coil pack, theres your problem. you can do the same thing with spark plugs, but do it one at a time. based on what you have told me, i believe that to be the problem. ive seen that on many rx/es300s.

so basically your major problem is just the misfire codes, the misfiring itself, and no O/D? without a scan tool and simple tools, you may just want to take it to the dealership. however, if you do have access to them, its a good starting point. more than likely you arent getting O/D because the car is running in safemode due to the CEL/misifire.

hope i helped..

salimshah

Add one more crank shaft position sensor (this one controls the timing),

Also remember the good advice by Lexmex to clear the stored codes (to learn afresh) and start off in corner.

Salim