Rough idle

Hey guys I recently just got all new spark plugs and replaced a broken coil on my 02 tacoma, it's always had a rough idle but just this week it started to drop rpms and shake only with the ac on. Any ideas? Maybe a dirty throttle body?

 

Run a compression test first. Confirm all the cylinders are even.

 

Your idle should increase when the AC is on. The AC has an idle up circuit that should increase your idle by about 100rpm.

 

there is a little bellows on the throttle body that works like a choke to add gas for the truck to warm up. it runs on vacuum so if the hose or that little diaphragm could be leaking so its not giving it extra gas to idle right when cold

the ECM is supposed to raise rpms when the compressor is on so that might be your problem if the ecm isn't getting a signal telling it the a/c is on. but im not really sure where or what sensor tells it that, maybe its in the hvac control in the truck?

 

I believe that was for 96-97. I think everything newer is electronic.

 

I mis spoke, I was referring to the fast idle for choke controlled by the iac

not sure when it was but yes the hvac control tells the ecm to raise idle when a/c cycles on

afaik the 03-04 were the only ones with the new electronic throttle controls, the 96-02 are all the older style throttle setups

 

I didn't read all the way thru the symptoms. Yes it should kick up the idle with AC on. I started with its always idled rough.

 

I know it should idle up with ac on but once the truck is sitting the rpms will drop to about 500-600 and shake, then it will adjust and smooth out to normal rpm

 

It also does it when I come to a stop like at a red light lol

 

With or without AC on it hunts for idle speed?

 

If the ac is on it will do it, but if it's off it will constantly idle at about 800 and stay there

 

something is wrong then, if its warmed up it should idle at no more then 650-700, if its at 800 then its in a/c mode running with a higher idle rpm

if it is "searching" as you say, that could be anything from a vacuum hose leak, throttle body gasket leak, bad/dirty MAF sensor, IAC valve, fuel filter restricted, if none of those try removing and properly cleaning the throttle body.

 

It would be normal for it to shake at such a low rpm though right?

 

normal, I would say no, it should idle smooth, any multi cylindered engine should, but well, in reality, all these Tacoma engines shake at idle, the I4 shakes pretty noticeably and the steering wheel vibrates, the v6 isnt that bad and it just kinda always has a rough idle like it needs a tune up.

the key is they idle at a steady rpm the whole time, they don't fluctuate unless something is wrong. the problem is, there are so many damned different things that talk to the ECM to tell it how to set idle speed and mixtures that sometimes until it throws a CEL code its hard to know for sure whats not working right.

 

Yup my truck has always had that rough idle but I don't remember it fluctuating so much, thanks for the info hopefully I can figure out the problem

 

just a thought but try unplugging the hvac control, if it has gone haywire it might be turning the a/c fast idle on and off causing it? after that it gets complicated to find I think

 

Thanks, today it didn't do it at all lol it stayed at 700rpm and didn't drop down at all.

 

you scared the hell out of it just by talking about fixing it lol

the randomness makes me lean towards a bad/dirty sensor but which one?

I suggest you clean the maf and iac "if you have one" then clean throttle body and clean and/or replace the o2 sensor, doing that pretty much covers all the bases. worst case is it didn't help but improves fuel economy so its still not a wasted effort

if it was the tps it would be about a certain spot giving it gas and not an idle issue, at least as far as I know a bad tps wouldn't cause idle issues

 

Ok thanks it's had a bad o2 sensor and cat for a while now...

 

a bad cat could be clogged up and that can definitely mess with it bad