I have been having a problem with my stock '98 SC300 RPM at idle (never stalls; Idles @ or near 300).

Understanding that it could be the IAC, i took it to my mechanic and we found that the IAC in this car is different from the IAC in the <'97 SC's. Apparently this IAC is mechanized (?) and gears are used to turn a plate instead of the "plunger" style of the earlier models. I wanted to clean the IAC, but he said that it wouldn't do much good since it wasn't the generic plunger IAC. I wanted to call around to some salvage yards in the area, but before I did, I replace the MAF sensor, the PCV valve, vacuum hose, and a few other old hoses. I still have my RPM problem after the minor fixes, although the car does seem to idle smoother...

Is there anything else that can cause this idle problem other than the above replacements? Perhaps the IAC *can* be cleaned even though its different from the older models? Or should I just bite the bullet and shell out the 300 bones for a new one? I realize that it could be a hose leak, but I am unsure which hoses to check, if that is the case.

Thank you very much in advance.

 

Could someone move this to the performance/ maintenance area, please. Thank you.

 

I'm sure this is of little help to you, but I experienced similar low idles on my 97 SC300 after replacing the battery a couple of months ago. I'm sure you've already ruled it out, but to get my idle back up to ~500 RPM I had to turn on all the accessories - A/C on max, radio, headlights, heated seats, etc. - and drive the car around for a bit so the ECU could re-learn my driving patterns, fuel mixtures, etc. After around 20 minutes of driving, my idle was back to normal.

 

You can try cleaning the maf sensor and throttle body.

 

I think you may have a ignitor on its way out soon. Just wait another 6months to a year, if that's the case you will know (bucking when the car is cold etc etc)

 

I had the exact issue with mine, its been documented in a few other threads throughout the forum.
Turns out nothing was wrong with the car, simply the rpm needle was off. I plugged an OBD2 scanner into it to see if it gave any codes, and the scanner read rmps at about 700 even thought he needle showed 2-300.
Would serve you well to plug a scanner into it to see what your rpms are actually at before you spend money on possible repairs.

 

Thank you, but since it has been doing this for the last few weeks, I'm not sure that it would be the ECU relearning anything. Unless perhaps the ECU is wonky... what are the symptoms of a bad ECU?

I replaced the MAF sensor, but cleaning the throttle body may be a good idea even if it doesn't help. Thanks

When you say igniter, do you mean the ignition control module or the coils? Thanks.

 

Definitely not bad ECU or bad ignitor. It would be much severe.

 

+1 ^^^

I doubt your ECU is bad. As GS4_Fiend says, a bad ECU would probably have other symptoms that were more severe - inability to accelerate past a particular RPM, severe drops in power, misfires, running on less than 6 cylinders, etc. One way to tell straight away if your RPM needle is off is to let the car idle and see if you feel any shudder through the wheel, seats, etc. Also turn on your interior and driving lights, see if you notice any dimming. If you do, give it some light throttle and bring it up to ~500 RPM to see if the shudder disappears. If it is noticeably smoother while the RPM gauge reads 500, then the needle is likely accurate.

Also, did your mechanic check for bad tensioner/pulley bearings, as well as any accessories such as power steering pump, A/C compressor, basically anything that runs off the serpentine belt? Seems like I recall my friends SC300 running strange at idle and it was just a bad tensioner bearing.

 

Sorry I meant ignition coil. Mine was cylinder number 7 that went bad, low idle, then after a while, bucking when cold and that would resolve itself etc etc then the slight misses and hesitation during driving started