clovehitch

Hey guys, new member. Any help is appreciated!!!

I have a 2004 GS300 with 180k miles. I paid just over 3k for it last week. It was running fine when I test drove it. It appears to have aftermarket headers. VSC OFF lights stay on.

Originally, it had codes for the MAF sensor. P0171 and P0174. I took the easy route first and cleaned the MAF sensor with MAF sensor cleaner and cleared the codes. These codes disappeared, engine began idling worse.

Codes P0133 and P0153 came up, slow response from both sensor 1's. I checked wiring and briefly looked for leaks, didn't find anything. I replaced both sensors with the correct Denso item numbers and cleared the codes. Engine REALLY likes to stall now and can occasionally backfire. But throughout all this, it runs fine from 1,500 rpm's and up.

Now I have codes:
P0133 B1S1 O2 SENSOR SLOW RESPONSE
P0153 B2S1 O2 SENSOR SLOW RESPONSE
P0171 B1 LEAN
P0172 B1 RICH
P0175 B2 RICH

I have a new Denso MAF sensor on the way. But what else could be the culprit? I don't see any obvious leaks. What should I look at? I am only somewhat car-saavy. I can replace accessories like water pump, alternator, etc. but I am not a mechanic. I'm not afraid to dig into anything but let's just say I've never built or rebuilt and engine or taken heads off. I'm just a guy that loves lexus (I own a ES300 as well) that can fix small problems normally but this one is aggravating. Thanks in advance for any advice.

clovehitch

Well I can add that my valve cover gasket has certainly leaked oil in the past. I cleaned it off so that I can check for leaks but I'll need a new gasket soon. But I don't think this would be related to the issue.

I noticed the clamp that attaches the intake tube to the throttle body wasn't extremely tight. I tightened the clamp and checked for codes. It has p0172 and p0175. I cleared them and started it up. It is idling much better now. Hasn't stalled in 10 minutes. Update when I can.

Still idling rough. Waiting for codes to pop up.

Here is a video of the idle and stalling
https://streamable.com/c01pi

clovehitch

Unfortunately work called and off I go for a week. I decided to leave the GS at home and take my 98 ES300 to work. I didn't want to kill the cats on my GS by driving it when it has this problem. Also getting 18mpg highway when paying for 93 octane is atrocious.

Update in 1 week when I install new MAF sensor.

Feel free to drop a helpful comment during this time.

KTGS300

If the new MAF sensor doesn't resolve your problem, another easy thing to check for is the air intake resonator. That's the box sitting after the MAF sensor and before the throttle body. Specifically, check the underside of the box as the plastic tends to get brittle and cracks as it sits right on top of the exhaust manifold.