Here you go.

 

I had the same intermittent rough idle. All I did was after taking off the intake tube, turn the ignition on and have someone hold the gas pedal to the floor and sprayed wd40 on a paper towel and cleaned the butterfly and housing with it and now it idles smooth. I didn't touch the Mass Airflow Sensor in the intake tube.

 

with the car turned off (kind of an important note)

 

Sure thing.......

 

Here is another PDF showing sensors that is slightly more detailed than Bob's.

 

Installed a new ECT Sensor this morning (couldn't have done it without ya'll!) and threw another P0456 code again on Thursday. I replaced the gas cap and found another split hose when I was preparing to install the ECT Sensor. The second picture is of my original ECT Sensor. I'll give it a week and see if the rough idle still exists.

 

After the Sea Foam, the TB Cleaning, changing the ECT Sensor, she threw a code, P0456 small evap leak. Changed the gas cap and replaced some hoses. Cleared the code, then 2 days later another CEL, I don't know what that code was, Advanced Auto cleared it. Took it to my mechanic this morning and the new code is P0442. He said they need to hook it up to a smoke machine to find the small leak. So I have to go back another time since he doesn't own that particular machine. Came home and started checking more hoses that I though looked good and found another hose with a split in it and replaced that hose. At least I can report that my rough idle seems to be going away! Lets see what this weekend brings with any new possible codes.

Had Advanced check my charging system and everything checked our perfect, even the battery was indicating 901 cold cranking amps and everything he checked was in spec (nice to have some good news!). The guy who works at Advanced Auto surprising lives in the building next to me and recognized my car. He has always loved the SC430!

 

Perhaps an intake manifold gasket leak? If I remember one of your first systems was the rougher idle. I should have mentioned this earlier (sorry), but my 2005 had an intake manifold gasket leak that had to be replaced under warranty last year that was causing a rough idle as well. No codes, though Might be worth checking around that area...I know you said your mechanic was going to look at the exhaust. Just a thought...

 

Thanks for the info. I replaced some more cracked hoses this past weekend (so many dang hoses!), cleared the code again on Saturday. Monday afternoon the code came back. I took some carb cleaner last evening and lightly sprayed around all the vacuum hoses that I could find and found one of them was not fitting very tight at all (the idle dropped significantly), so I tightened it up, the idle picked up and stayed steady, and will see what happens. I also cleaned the PCV valve which was filthy as well. I can say though the rough idle is slowly getting better.

If the CEL comes back again this week, I did find a local shop (thanks to Advance Auto parts) who will do a smoke diagnostic test for $95.00. I will get that done and have my mechanic check the intake manifold gasket as well.

 

Been reading through TSB's and I am sure people have seen this on the rough idle, but maybe worth it:

Some customers may experience a rough idle when the vehicle is at a standstill and in gear (Dposition). This vibration is not noticed when the engine is under load above idle.
S2002—2004 model year SC430 vehicles produced BEFORE the Production Change Effective VINshownbelow.

MODELPRODUCTIONCHANGEEFFECTIVE VINSC430 JTHFN4#Y#40056830 PREVIOUS PART NUMBER CURRENT PART NUMBER PART NAME QTY
12361--5015012361—50151 Insulator,
Engine Mounting, Front 112381--5007012381--5007112381—50140
Stabilizer, Engine Mounting 1

1.Verify the rough idle with vehicle in gear (Dposition).
2.Inspect the rubber on the front engine mount insulators for cracking or tearing.
.If a front engine mount insulator shows damage, replace the insulator and engine mounting stabilizer.
4.For insulatorandstabilizertorquespecifications,refertotheTechnicalInformationSystem(TIS)

 

Thanks Mandyfig, I had my mechanic check out the motor mounts and they are fine, I wish it would have been that easy of a find.

Its been a week now and no CEL! I guess I finally found that one small leak that stopped the CEL from glowing on my dash. The idle is still not as "butter" smooth as I know she can run, but it is about 90% better than what it was sometimes.

 

We just recently did a 90k service on the car and before it kind of had a rough idle. It was a day and night difference on the idle after the timing belt, water pump, thermostat, drive belt tensioner, idlers, and pulleys were changed out.

 

Well after 2 years and taking my SC to multiple repair shops for the rough idle (and none of them confirmed it was the motor mounts even after I told them to check them out), I finally took my car to the Lexus dealer. Originally I took her to the dealer for a leak and found out it was the power steering pump. I told the tech that I had a rough idle when I was stopped at a light and had the A/C on and/or the headlights on (and I never mentioned anything about the motor mounts or tranny mount to the Lexus tech). After looking over my SC, he found the following:

Ball joints and control arms were shot
Leaking power steering pump
Water pump leaking (had replaced in 2010 with an aftermarket pump)
Bad motor mounts and tranny mount (the cause of the rough idle)
Leaking valve cover gaskets (had replaced in 2010 as well)

She drives like a new car, no more vibrations, the steering is tight as hell, and I couldn't be happier with how she is running and driving. My 430 now has 89,000 miles on her and I purchased her with 69,900 miles on her in November 2010.

Lexus gave me a loaner vehicle for the entire time they had my SC (1.5 weeks). I have an extended warranty from Mercury Insurance (thank god). Mercury Insurance sent a rep down to verify all the repairs and authorized all the repairs under warranty to the tune of $2,880 (My warranty only cost $2,000). I had the dealer replace the timing belt at the same time as the water pump, which I originally had the timing belt replaced with the aftermarket water pump back in 2010. My cost for the timing belt and serpentine belt plus my warranty deductible was $322.

So, after 2 years of telling mechanics to check out the motor mounts for the rough idle and all of them telling me my motor mounts were fine, I am now going to take my 430 back to the dealer from this point forward, it may be more expensive but I think it is worth it in the long run.

 

Mark,

Which delaer did you end up using?

My SC has been at Wilde Lexus for 3 days now for a slight miss at moderate throttle.

Actually tonight it is in North Port at the tech's house. He drove it home tonight with a scan tool connected to try to figure it out.

No codes. They tested the gas they said and think it's bad gas. Gas tested fair. I doubt it's bad gas as the last 3 tankfulls, all purchased at least 100+ miles miles apart. Tech also said he does not like Shell Gas. ???

I also have an extended warranty but from Lexus. zero deductable 8 y 100k purchased for $2400.It's nice to have this.

I asked if they thought it may need a Injection service. The reply was, they didn't want to sell me a service unless they were sure it would be helpful.

JR

 

I highly doubt it's bad fuel, very unlikely. Perhaps a lose vacuum hose, MAF, bad spark plug or wire(s), or some sort of sensor, but kind of doubt it's a sensor since it's not throwing a code. The moderate throttle makes this problem a bit tough to diagnose.