I have done hours of searching about the rough idle on the SC430. What I haven't been able to find is any information about an intermittent rough idle. I have cleaned my MAF sensor twice.

I am having my mechanic replace the radiator tomorrow because of a leak and he is also replacing the idler pulley and the belt tensioner (one of them is making a noise and thank god I purchased an extended warranty!) At the same time he will be doing a throttle body cleaning. I had my timing belt replaced as well as the water pump and had my mechanic check all the spark plugs and all were fine.

I am going to have him check the motor mounts as well. Could bad motor mounts and/or transmission mounts cause an intermittent rough idle? The RPM's are fine with the AC on or off and I have used Lucas fuel treatment on this current tank of gas without any results and my next tank I will try Seafoam as suggested by my mechanic.

Anyone have any other suggestions?

 

A good throttle boby cleaning should take care of this.... Worked for me!

 

Just came back from my mechanic. Replaced the radiator, the belt tensioner and the idler pulley. He cleaned the throttle body and said it was dirty but it wasn't that bad. Checked the motor mounts and he said they look a little worn but were fine and intact. It didn't idle rough on the way home (about 20 miles from my mechanic). I just added the Seafoam this morning when I got home. I am off today, so it's time to drop the top and see how she does. I am hoping it was the TB and the problem will be solved. Will let you know.

 

Do we have TB cleaning DIY guys?

 

Yeah, there is one out there. Azkaty's husband who used to be a SCer did it when he did his injen intake. I will search and post if I find it.

Lexus dealer offers a thorough service called "fuel injector cleaning" where they put a machine up to your exhaust and run chemicals through your system and clean the TB manually as well as the carbon buildup removal, etc etc. I think it's like $150 or so.

 

Will periodic use of seafoam help?

 

Like it when you are not as busy. We miss you around here.

 

It's been a one week and half tank of gas with Seafoam. The intermittent rough idle still exists. The TB was cleaned and I did notice my mpg average is at 20+ mpg up from 16 mpg.

 

An educated guess based on past experience with other cars, if the idle is rough when the car is cold, but smoother when it is warm, I would try replacing the Engine Coolant Temperature Sensor (ECT). Way back when when trying to solve a rough idle problem on another car after replacing almost everything on the engine, the culprit was the ECT and it did not throw a code. I also know an auto mechanic that had a very difficult time diagnosing his own car with this same problem and it ended being the ECT. Other sensors that you may want to check after replacing the ECT, if it does not solve the problem, which may not throw a code if they are operating intermittently, but I'm much less sure of are: Throttle Position Sensor;Crankshaft Position Sensor; and Camshaft Position Sensor.

Also, when replacing sensors, I would highly recommend using OEM parts/sensors only. I've tried after market sensors on other vehicles and it's not worth the small savings for the trouble it may cause you.

 

The rough idle happens on cold or at warm temperatures. Its nearly impossible to pinpoint and exact time when this rough idle happens. I usually let her warm up for a couple of minutes before I take off. I will be looking at the all the sensors you listed above, but now another problem has crept up and I am not sure if it is related or not to the rough idle.

Yesterday on my way to work she threw a CEL (P0456), checked all the hoses yesterday and they seem somewhat loose fitting even with the hose clamps, re-tightened the gas cap and cleared the code and will see what happens on my way to work today.

 

Didn't you have this same rough idle problem before? Thought you discovered a loose vacuum line (https://www.clublexus.com/forums/sc-...ht-are-on.html post #12 ) P0456 indicates the same or another small leak or loose fitting vacuum hose. I'd suggest going over EVAP system and carefully inspecting the clamps and vacuum hoses for small cracks or loose fits.

 

Let us know the resolution on this interesting phenomenah.

 

You have a great memory Bob! You are correct.

I didn't have a code this morning on the way to work, but stopped at Autozone this evening and bought some replacement hoses and clamps.

I replaced the the hose that decided to pop off last time and a couple of others at the same time and they were or course the original hoses and had cracks in them.

My last CEL episode was 3 codes and I found one of the hoses had come completely off. Searched on CL and found my answer and found the lose hose and put it back on last time. This time it was only 1 code P0456. Hopefully the new hoses and clamps has fixed this problem.

Back to the idle issue, I am going to replace the ECT Sensor hopefully this weekend and see if that helps.

Can anyone tell me where the ECT sensor is on our engine?

 

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.