ok, here goes.
for a while now, i have noticed my 98 ls400 running rough, particularly on cold mornings. it felt like the car was running on 7 cylinders. it used to always go away once the car warmed up, but now sometimes it doesnt. i was told to start with a tuneup and here is what i did.

changed air filter, spark plugs, fuel filter, pcv

i also cleaned the throttle body.

now, not only has my problem NOT been solved..but i am idling all over the place... started cold, the engine may idle at 1300 rpm!!!!

any suggestions for both problems???

 

IAC* valve and possibly the TPS**

*Idle Control Valve (search)
**Throttle Position Sensor

 

Actually I don’t believe the 98+ models use an IAC valve, it has an electronic throttle motor to control idle speed. It does however use a throttle position sensor and if your experiencing fluctuations at idle then it could possible be the TPS. How many miles do you have and what spark plugs did you use when you did the tune up?? Also 1300 RPM during cold start is normal, it should drop to around 750 once warmed up and in park.

 

i used NGK iridium.
could i have damaged the TPS when i cleaned out the throttle body? I am learning now that i probably shouldn't have sprayed the cleaner on the TB itself, instead use a rag that has cleaner on it...

 

yea VVT-i cars didnt have an IAC, didnt notice the year in the first posting.

sounds like u fried the tps.

 

well, thats an easy replacement then... guess i'll swap off of my buddies 99 ls to see if that is the problem.

my bigger problem still is the rough running, can't figure it out.

 

Thats not the problem. I've done this many times, not something you can mess up in the throttle body. I've even disassembled it and cleaned it before. And even if you did, you would have gotten a check engine light. But it wouldnt hurt to check if there's a car you can swap with.

Have you tried resetting the ECU? Try that.

My car went nuts after i cleaned the MAF without reseting the ECU for a few days.

And it may also be a vacuum leak, check all the hoses.

 

well, i did disconnect the battery for around 10 minutes after i did all of that. Im thinking now that the higher idle may be correct (because it does go down after the car warms), perhaps the clogged air filter i replaced may be allowing it to idle normal now.

still searching for a reason for the rough/misfiring engine though

 

what is ur idle when u start the motor up cold and what does it settle out to? make sure to test w/AC off.

 

cold - between 12 and 1400 in neutral
settles to around 800 when warm, usually, also in neutral.

 

That sounds normal. Now... im not sure about the misfiring... Have you tried getting the codes read?

 

no, but would there be any codes if no 'check engine' light is showing?

 

the idle speeds are a bit on the high side.

for example, on my NON-VVTi 95LS the Factory Correct Idle while warm is 650+/- 50RPM

id adjust your idle down about 100-150 rpm and see if that helps.

 

You cant set your idle on the 98+ if im not mistaken.

 

You live in California what do you know about cold temp start ups.
It is about what mine is for idle as well.
Last timing belt change?