dom420

Ok this is a bad first post but unfortunately I must start off like this.

Just bought a 99 es300 with 138k on the clock. Car was running beautiful when I tested it, everything was butter etc.

Was running fine for two days until it needed more gas. I filled up with regular as per the manual. And immidiately after gassing up the CEL came on and the car started running rough as hell.

Check on here, first thing I used up most of the gas and filled up with a full tank of premium and added a can of seafoam to the tank as well. Cleaned my gas cap, and cleared the CEL by disconnecting the battery. Of course the light came back on and the car started acting up again.

Checked the codes this time, got a P0171 system too lean code.

So I cleaned the maf, did a top engine seafoam cleaning. Cleaned the trottle bodies, changed the air filter. The car was running noticeably better after that but once again the CEL came back after a day or two and the car was experiencing the symptoms again.

I have the service history for the car, and the TPS was replaced at 70k(with the same symptom description from the customer), then 5k later at 75 same symptoms came up and the dealer replaced the MAF. Then there was no more issues until 120k when the car was resold to the person I bought it from. The timing belt was done at 100k.

Anyway, I already ordered a new OEM maf since I was almost positive that was it. I have yet to do the oil change and spark plugs, but if anyone has suggestions please let me know. I read up that it may have something to do with the Oil Control Valve but not sure if its related.

The roughness is on acceleration(under load) at about 1500-2000 rpm. At full throttle it clears up. At cruise under load the car starts to shake/buck. But only under light throttle as well.

Any help or suggestions are appreciated. Thanks

tomf

Sounds like a problem I just had. It turned out to be the bank 1 sensor 1 Air Fuel ratio sensor. I have a 99 es300. It would stutter/buck/jerk at cruise speeds. Later it started stuttering a low speeds. I replaced the MAF. Didn't fix the problem. Searched for vacuum leaks. Replaced the bank 1 sensor 1 Air Fuel ratio sensor and the problem went away. Here's a link to my thread if you are interested. https://www.clublexus.com/forums/es3...ed-advice.html

Part number is denso 234-9007. Got it from amazon

dom420

Ok awesome thank you. I have a strong feeling that it is the MAF, just based on the service history. The last one lasted almost exactly the same milage that is on the new one.

I have the part ordered so I will start there.

Just curious but were you only getting the bucking shaking at cruise or did you also experience the hesitation/roughness on 1500-2000 rpm?

It seems like there could be two unrelated problems causing similar symptoms.

Lastly is this the part you are talking about?

http://www.partswebsite.com/oemlexus...n=2&type=parts

Thanks again for the help.

tomf

Initially, it was noticeable when I was cruising with my foot lightly on the accelerator. It would go away when I pressed on the accelerator. As it got worse it was severe at all speeds. I stopped driving it until I replaced the sensor. Like you I replace the MAF first. Then I ended up replacing the air fuel ratio sensor. These sensors rated to 100,000 miles.

One of the things I found out as I researched my problem was that the ECM doesn't use inputs from the oxygen sensor during accelerations or until the engine is warmed up. Thus I would get stuttering at constant speeds but not during acceleration. Also my CEL light would come on at almost the exact same location when I took my boys to school. What was happening was the engine reached operating temperature and started taking inputs from the malfunctioning sensor causing the CEL to come on.

Denso is the OEM part maker that Lexus uses for these parts.

Yes those are the sensors I'm talking about. I can't tell for sure if they are the exact same parts from your link. But I think it is because that is a OEM lexus parts site. It just doesn't list the part number. To find the Denso part number go to the Denso aftermarket online catalog http://www.densoaftermarket.com/catalog/ look up oxygen sensor for your car and it will list the air fuel ratio sensor and the oxygen sensor. P0171 refers to Bank 1 sensor 1 which is the front left air fuel sensor. It's the on the engine exhaust manifold near the engine firewall. You have to get under the car to change it. I recommend you get an offset oxygen sensor socket to remove it.

here's a link to the part I bought from Amazon for $99 http://www.amazon.com/gp/product/B00...ef=oss_product

Just so you know I'm not a mechanic or an expert. These are just things I've learned keeping my cars running. Let us know what ends up fixing your problem. Thanks Tom

dom420

Ok so, its not the MAF..

Onto the o2 sensor. I did some reading and realized that the part you changed was the California spec o2 sensor, so I ordered the non cali spec o2 from amazon, part number 234-4622. Its about 30 bucks cheaper than the Cali spec too.

Its supposed to come tomorrow so Im going to change the plugs, oil, and the bank 1 o2 sensor and we weill see how it goes.

tomf

Where did you read about the cal spec? That sounds like something I would like to know. Would have saved me some money. What's the difference between cal and non-cal? Thanks

dom420

Well I got the OE part number from my dealership and cross referenced it on densos website. I notice there were two kinds. I did a bit more searching came up on this forum post as well.

http://www.carkb.com/Uwe/Forum.aspx/...590/O2-Sensors

The difference between them is that Cali had much stricter emissions standards. So Im guessing its more sensitive to a uneven AF ration. Its also more expensive..

And also the front and back o2s are the same, with the back one having a shorter wire. So I just ordered the cheaper of the two(front, longer wire). It does not make sense that the one with the longer wires would be cheaper, but go figure.

I got the sensor today. Probably gonna swap tom morning.

dom420

Ok so Im an idiot, its not the same part. The connector is different. Yet when I checked on all the website they said it would fit.

So I just chopped the old connector and spliced the new sensor in.

After all that, changing the spark plugs and the oil with mobil 1, and the check engine light popped back on after and hour of driving.

I think I have to give up at this point and take the car to a professional...

L's TLS

iTrader: (1)

Why did you put regular gas in the car? Supposed to be 91(premium) or better unleaded gasoline. Even thought the car can run on 87(regular) it is not recommended or designed to run on that octane. I would think thats the root of your problem.

PlatES300

iTrader: (1)

the manual says regular fuel not premium. I've used regular gas for over 250k miles and no problems.

dom420

It was the bank 1 sensor 2, the o2 sensor after the cat.

Check engine light is off, but there is still a slight hesitation at the 1500-2000 rpm range. Im gonna inspect the intake manifold gasket cuz it looks like it could be leaking vacuum, gonna replace the fuel filter and check fuel pressure too.

L's TLS

iTrader: (1)

I apologize, I never actually had a chnace to read the manual (car didnt come w/ one) But I read it after seeing your post and you are correct. the car can run on 87, manual says for improved performance and MPG to run 91 or better. Sorry for the bad info OP. Hope your problem gets resolved