My wife's 99 rx300 has recently exhibited a rough idle after a cold start. It did throw a P0446 code, but it hasn't come back since I reset it. The RPMs stutter while in park or at low RPM in D or R.

I changed the oil and filter, and air filter. Those didn't seem to help.

The 02 sensors were replaced 9 mos ago, and prior to that we had a new MAF sensor, too. I also had to have the throttle body (I htink) cleaned b/c it had locked up and wouldn't start.

So - question is - where do I go from here? I will do a PCV valve soon, but I suspect that's not going to be the key. Maybe do an IACV cleaning?

Thanks!

 

Forgot to mention that we have 150K miles.

 

That's what I would do. A moonlighting professional Lexus mechanic I sometimes use says to replace the part rather than clean it, but that's probably factoring in Lexus labor rates.

 

I will give it a shot and let you know.

Question: How much impact on long-term performance does ethanol blend gas have? In MN, we have mandatory 10% Ethanol blend, will it cause parts such as IACV to dirty up faster?

 

I tried the IACV cleaning procedure and didn't notice any improvement.

I was going to do the PCV valve replacement, but I apparently have the metal threaded kind and didn't have a wrench large enough to get on it - so that will have to wait. Is that even the right path?

When first starting, the RPM start at 1100 or so, and then drop towards 600-700 as the engine reaches operating temp. They dip and the engine sputters, as if there's a fuel supply issue. It's a little bit worse when in gear at the low temps.

Any other ideas? The fuel mileage is consistently near 20 mi/gal for daily highway commuting.

Possibly spark plugs? Fuel? I use 89 octane almost exclusively.

I don't enjoy taking this vehicle in, but it's looking more and more like that's what will need to be done. Still - no codes other than the original P0446 that was given earlier, but didn't time out exactly to when the issue started.

 

Well, my wife drove it into work yesterday, but on the way home it started to shake ~50 mph and wouldn't shift she said. And the CEL was flashing - so she pulled over. I gave it a quick look before the tow truck came, and it started right up, CEL was on, but not flashing - drove it around the parking lot and seemed OK at low speed, but still opted for the tow to the shop. We'll find out today what the damage is.

 

sounds like a miss-fire issue. probably a bad coil pack.

 

Lexus114 - you hit it right on the head. Apparently a failing spark plug can damage the coil pack.

Not bad in the grand scheme of things, esp. compared to when they told us a few years ago that we needed a $4K transmission!

 

I used to have an '03 RX. That engine family is designed to run on 87 Octane. I tried 89 a couple of times long enough to know I was pretty much running on it. I had similar issues as you but didn't have your mileage. If your plugs are fresh enough, try going back to 97 Octane (reg). Another thought could be your air filter but I suspect too much octane or dirty fuel filter is your gremlin.

Especially since you mention it's primarily when it's cold.

Good luck

 

my 01 ran the best/best mpg on premium.

 

My RX runs the best and get's the best fuel economy on premium as well. I am pretty sure this engine is NOT designed to run on 87 octane, the manual recommends premium but if that's not available you should use 89. But using regular (87) will cause decreased performance and less mpg's. And in my experience this is true.

 

not to dis-credit jim,as it can be run on 87,but they dont run to their full potential.

 

Mine was a 2003. And the manual recommended 87 octane. It DID run best on that.
This link will give you the recommended octane. https://secure.drivers.lexus.com/lex...html&locale=en
2003 RX300 from Aug. 02’ Prod. (OM48442U)
FUEL
Fuel type
Your vehicle must use only unleaded gasoline.
To help prevent gas station mixups, your Lexus has a smaller
fuel tank opening. The special nozzle on pumps with unleaded
fuel will fit it, but the larger standard nozzle on pumps with
leaded gas will not.
At a minimum, the gasoline you use should meet the
specifications of ASTM D4814 in the U.S.A. and CGSB
3.5–M93 in Canada.
NOTICE
Do not use leaded gasoline. Use of leaded gasoline will
cause the three–way catalytic converter to lose its
effectiveness and the emission control system to
function improperly. Also, this can increase
maintenance costs.
Octane rating
Select Octane Rating 87 (Research Octane Number 91) or
higher. For improved vehicle performance, the use of
premium unleaded gasoline with an Octane Rating of 91
(Research Octane Number 96) or higher is recommended.
Use of unleaded fuel with an Octane Rating or Research
Octane Number lower than stated above will cause persistent
heavy knocking. If it is severe, this will lead to engine damage.
If your engine knocks ...
If you detect heavy knocking even when using the
recommended fuel, or if you hear steady knocking while
holding a steady speed on level roads, consult your Lexus
dealer.
However, occasionally, you may notice light knocking for a
short time while accelerating or driving up hills. This is normal
and there is no need for concern.
Gasoline containing detergent additives
Lexus recommends the use of gasoline that contains
detergent additives to avoid build–up of engine deposits.
However, all gasoline sold in the U.S. contains detergent
additives to keep clean and/or clean intake systems.
Quality gasoline
Automotive manufacturers in the U.S., Europe and Japan
have developed a specification for quality fuel named
World–Wide Fuel Charter (WWFC) that is expected to be
applied world wide. The WWFC consists of three
categories that depend on required emission levels. In the
U.S., category 3 has been adopted. The WWFC improves
air quality by providing for better emissions in vehicle
fleets, and customer satisfaction through better vehicle
performance.

 

Well, to continue the saga . . . got the vehicle back from the shop after having all the plugs replaced and the suspect ignition coil replaced as well. The initial idle seemed high (1800 rpms) but then slowed down to 1200 or so as it warmed. Took it for a drive and CEL pops on. CRAP! Get it home and let 'er sit in the driveway and she almost stalls as the RPM drop so low. A noise from in front of the steering column comes on for a few secs, the RPM shoot up >1000 and the engine sounds fine for a bit, but then drops back.
I'm sure suspecting IACV. Any chance the new plugs blew out some carbon into the line and fouled A/F sensors? I haven't checked the latest code, and I'm fearful of an A/F sensor again, as I just put them in 9 mos. ago! Those bad boys ain't cheap.
As for the great fuel debate - I was under the impression premium (91) was recommened for the early model RX. I use the 89, but we have mand. 10% ethanol here, so not sure how that factors in.

 

did you use OEM sensors?

Salim