I have searched alot looking to resolve this problem. I have a 94 Lexus SC400 (CA emissions) 200,000 miles. I have a low idle to the point where the car shakes. It looks like it idles at around 200 rpms in drive and 400 in park. And no check engine light.

So far in attempt to fix this problem I have changed air filter, pcv valve, spark plugs, distributor cap and rotors, fuel filter, mass air flow sensor, swapped out throttle bodies, tps sensor, checked for vacuum leaks, cleaned iacv I believe thats it. When I first start the engine (cold): the cold start idle is around 750 rpms then it will gradually decrease down to 400 rpms in park.

Sometimes after driving for thirty minutes and then putting it in park the idle won't even get back up to 400rpms. The car shakes due to the low idle. I also have a misfire. I have just recently tried to fix the misfire by replacing the plugs, caps and rotors. The oem plug wires are only two years old.

I am lost with the low idle. I don't know what is causing it. The only thing I have left to try myself that I know of is to replace the idle air control valve (iacv). And with the misfire the only thing I know of to check would be to check each plug wire to make sure it ohms within specs. Other than the low idle and the misfire, the car seems to drive fine.

So here are my questions:

A. What else could cause an engine to misfire
B. What else could cause this low idle to occur all the time?
C. Could the low idle and the misfire be related?
D. If my spark plug seals are starting to leak would that cause it to misfire?

Sorry for the long post, I was trying to describe it the best I could.

 

I only had it happen once, so far, in stop-and-go traffic. This was before my car had a major tune-up/service/maintenance. I haven't noticed it since but I don't know if the tune-up was what fixed it.

Only issue I have had on a somewhat regular basis is my car sputtering/jerking when just getting into 3rd gear at around 40 MPH.

 

When you replaced the tps sensor did you set it correctly? Rotating it a bit could throw off the idle. I believe cold start should be around 1.5-2k rpms.
Not sure about the misfire. You can check if the wires from the distributor cap are going to the correct cylinders.
Before you experienced this low idle/misfire issue was anything done to the engine.

 

Cam gear eating the spark plug wire??

 

Bad ignition coil? You would likely have some smoke as well.

Bad injector?

Could be the IAC, a really low idle might even cause a misfire.

Good luck.

KC

 

Why don't you do a compression check?

Jonny

 

Thank you for the quick replies!! To answer the following: the tps sensor is set correctly. The only thing that was done was a starter replacement, but I noticed the low idle once when I was reversing into my garage. The plug wire isn't rubbing against the cam gear. I was starting to think it might be an injector, but a mechanic told me injectors rarely fail, but it may be possible. I plan to buy a used working iacv and swap that out to see if that would change anything. I didn't know a low idle could cause I misfire. The compression check is now on the to-do list. The compression check would indicate what cylinder is having a problem. Probably the quickest option. I really appreciate all the input. Keep it coming if anyone could come up with more possibilities.