So its been a good year or so with my SC300 little to no issues, man I love this car lol.....well up until today anyways.

I noticed when I put my car in park there is a slight clunk and the RPM's go from slightly under 1k (normal idle) to 1800-2100 and stay there. I also noticed while driving when I hit 2k rpm's the the car has slight hum noise and it have to press the gas pedal fairly hard to get the rpms to increase even slightly. I'm not sure if these 2 issues are related or not.

Now I know this could be a few different issues....tps, distributor, timing etc.....any thoughts on where I should start first?

 

Sounds like the TPS or IACV. Start by cleaning the throttle and IACV chamber and see if that works.

Do you have HID's? I found that my idle would spike if I turned on my HID's. I just swapped them for regular halogens and everything went back to normal. These cars get a little strange the older they get. I have a feeling my older alternator and IACV can't keep proper idle with the HID's.

 

Yes sir I do have HID's I have had them for close to 3 years now without a single issue, but I will give that a shot and see what happens.

I'm praying its not the IACV....those are crazy expensive no matter if you go after market or oem.

I wonder if it could be an injector issue.

Whats the safest solution to use for cleaning the Throttle body while on the car? 3 years ago when I changed the VCG, plug and wires I cleaned the throttle body with a wire brush blood sweat and beer...there was so much build up in there it was amazing the car even ran. After I was done it look brand new...but I have never cleaned it with the throttle body on the car

 

cleaned mine with it on the car, just took a lot of different brushes, carb cleaner, q-tips, etc. The only way to really clean the IACV chambers is with q-tips. I probably went through an entire box before it was clean.

 

UGH sounds fun lol.......I think I might just remove the throttle body and do it by hand again lol.

Well I removed the hid's and still the samething the idle shoots up once i put it in park

 

You may not need to clean the throttle body. no need to take it off just take off the intake pipe, crack the throttle open and check it out with a light. you only really need to clean the area right around the throttle plate.
Since your idle is high, the problem is you are getting too much air not too little.
Also check/change your rotor and cap if you didn't do that with the plugs.

My guess is you may have a vacuum leak, which would explain both the high idle and why it runs bad @ 2k.
A common one is the valve cover gaskets leaking. not only do they leak oil they will let in unmetered air.
Since you did yours a while ago double check them make sure they are still good and not pinched or anything.
Check the PCV hoses and the vac lines like the power steering ones, throttle body gasket, etc.

Your IACV could be sticking, a quick test it to have it idling and turn on the AC. If the needle settles alot lower than before, then your idle valve may have an issue, if it stays constant then its doing its job. your idle should be around 650-750 in drive, not that close to 1k.
I dont see why you would have the roughness at 2k though from a bad IACV, because that air would be metered air still. So I would check all the hoses and any work you have had done recently for leaks.

your problem sounds alot like this guys, he even wrote that he changed the VCG out the same way you abbreviated it, wierd right?
http://www.clubna-t.com/forums/showthread.php?t=8991

 

Yo Ali, my idle is definitely x10 better than last summer. But I noticed that my idle drops when I push on the brake pedal? What could cause that? Keep in mind that mine is manual. In "neutral", the idle is at like 900, but If I push on the brake it drops down to 750...sup with that? It doesn't stall anymore or anything crazy like it use too. It's been driving quite well during the Fall and winter months.

 

Its normal for it to drop a 100 rpm or so for a second, basically the brake booster works off of vacuum, so every time you apply the brake, the brake booster has to also refill to be able to do it again (which is a good thing).
When you do that at idle, it has to refill, which sort of take away your metered idle air and can cause a slight dro in idle. The IACV normally compensates for this, but sometimes its noticeable. lets say if you press the brake pedal many times in a row rapidly, it should detect the low rpm situation and kick up the rpm's back to normal even though you are still pressing the brake pedal over and over again. This is normal operation.
150 rpm's might be too much though, your brakebooster could have an issue, or maybe your brake system needs a bleed if it requires excessive pressure to actuate.
Do you have you power steering idle up switch attatched? Im putting mine back in I have had some funny steering and idle issues since i pulled those 2 vac hoses that come off the PS reservoir.

 

I'm having a similar problem with my 92 SC 400. From a cold start the car either revs at a very high RPM, like around 2000 or so, or it revs and falls, like you are revving the gas except the car is doing it all by itself. I changed the plugs, caps, and wires, and cleaned the throttle, but still no good. I was wondering if anyone had any luck with the IACV before I go down that road. Oh yeah, I checked for vacuum leaks too....nothing. This only happened after I had the transmission fluid changed. I can't imagine the link but an odd coincidence still. Any help or advice is much appreciated.

 

The IACV can make the car rev all by itself. next time it is fluctuating, unplug the IACV and see if it stops.
Generally, when you unplug it it will stay in the last position it was in.