Lexus082

Well I took my car to advanced auto and had the charging system checked and everything came out fine. The machine he used to check it also displayed the rpms. It showed my car at 713 rpms at idle. The machine he used just connected to the battery. So its gotta be pretty accurate. Which means my tachometer is not receiving the correct tach signal. (Which is what Toyota told me also. That would be the answer to one of my questions concerning the low idle. But I'm still stumped on why my car shakes. Can the engine mounts just be worn out that much? I also have a random misfire. Could that cause the engine to shake? I have replaced the plugs, caps and rotors and I have pulled each plug wire one by one to see if anything was abnormal. What else could cause a random misfire??

So basically to sum it up:

--The idle speed is within spec. approx. 700 rpms in park and 583 in gear.
--Tachometer is not getting the proper tach. signal.
--Have a random misfire.
--Car shakes slightly after it warms up when the idle comes down to 700 rpms.
--Engine mounts have never been replaced.

Any thoughts after reading those symptoms?

Lexus082

Anyone have any suggestions to my last post?

C-Stone

Some questions: Does the car idle normally if you press the gas pedal slightly to stabilize it at say 800-1000 rpm (when warm)? If so, then it may be air starvation at idle which points to the IACV, IACV passage into the Throttle body, or the Air Assist Hose dirty.

How thoroughly did you clean the IACV and IACV passage?

There are some excellent posts on how to do this (although RX300). On mine recently, I had to remove the IACV solenoid and spray Napa Carb cleaner down the IACV passage through the Throttle Body while twisting the IACV shaft. It was either gunked up or something had lodged preventing idle air to get through with the TB butterfly valves closed.

Here are a few RX300 posts. Someone else can direct you to pics of the SC400 IACV and TB, etc. Hope it helps.

http://us.lexusownersclub.com/forums...ded&pid=199368

https://www.clublexus.com/forums/sho...d.php?t=295546

C-Stone

Another thought, if the above doesn't work: How old are your spark plug wires? Sometimes, they simply need to replaced over time. (versus miles)

...I just noticed from another post that they are only 2 years old. One thing you can check to see if they are still good is observe the engine compartment at night with lights out. If you see some light sparks jumping around, a plug wire/wires may be bad. How is your gas mileage? This may also be an indicator.

I have a '96 Vette which only had 59,000 miles with original plug wires. After replacing them, mpg went from 22 hwy to 25-28 mpg hwy.

Good luck.

Lexus082

The car/engine doesn't skake when given a little gas (in park). I cleaned the idle air control valve. Not sure if the RX is the same but there's a bypass hose that connects from the intake to the iacv. I'm not sure what the air assist hose is or the iacv passage in the throttle body.

I took the valve off the car and took it apart and cleaned it. As far as the wires I tuned off the lights after removing all the covers and I didn't see any sparks. I just wish I could find this misfire thats causing it to shake at idle. My idle speed is ok, its the tach on the dash which I have recently learned is either receiving a bad tach signal or the needle itself is displaying something different than what its receiving... I haven't paid too much attention to the mpg. I will pay closer attention to that the next time I fill up.

C-Stone

If the car/engine doesn't shake when given a little gas (in park), it seems to me that the engine still isn't getting it's idle air at idle. (I am assuming that when giving it a little gas, you are holding it at a steady low RPM.)

I tried looking up SC400 throttle bodies. But couldn't determine from any pictures where that Idle Air comes into the intake. But it has to be in the throttle body itself, adjacent to the IACV somewhere.

I did find these links to thoroughly cleaning the TB (Throttle body) itself. Cleaning it with the Valvoline Carb cleaner, helped solve alot of symptoms like you've described. Cleaning the TB as thoroughly as possible may clean out that idle port we are trying to find as well. I hope these help.

http://www.lextreme.com/tb.htm

https://www.clublexus.com/forums/sho...d.php?t=224751

Lexus082

The engine is misfiring. I swapped throttle bodies and that did not resolve the problem. Before I though it was idling low since that was what my tach was displaying. I think somehow whatever is wrong is possibly causing the misfire and that particular part is causing the tach signal to the guage cluster to show something different that what its actually idling at.

So I plan to buy a new ignition coil and then place the new one on the left side and if that doesn't work I will put that old coil back on and then try the one on the right side of the engine.

Right now I am trying to figure out where the tach signal comes from. I was told two different things. One person said the tach signal comes from the igniters and another said usually the tach signal comes from the ignition coils. So I am not sure who is right...

C-Stone

I'm sorry. Somehow, I missed that you replaced the TB already.

A random misfire... You've probably already checked this, but are any spark plug wires crossed? (All running to the correct cylinder?) Are all the plug wires in good shape still? (None burned through or touching ground.)

Lexus082

Found out the tach signal comes from the #1 igniter. Its at 700 rpms coming off the igniter so unless the wiring is messed up to the guage cluster's tach, I think the tachometer is displaying the wrong tach speed. I will try to swap out clusters and see what happens with a different tach.

I have checked one side to make sure no wires were crossed. I am going to check the other side today to make sure everything is ok.

Lexus082

The tach in my cluster is bad. Thank God! I thought it might be the wiring or something else.

C-Stone

It sounds like you found a fix!

Lexus082

Yes, that problem with the cluster has been. But I still am in the process of finding out the cause of my misfire... I am going to do a compression check right now.

p3lon

any updates.. i am the same problem