i have a 1990 sr5 truck 4x4 5 speed, 3.0 sometimes it idles at 400 to 500 rpms then sometimes it idles at 800 rpms? coolant system is full and no air pockets. tps is correct. I ohmed it out and checked against tsm. new air filter and a cleaned throttle body. it has 150,000 miles. I have check for vacuum leaks .I have a code 52 but I heard at idle it the computer doesn't "listen" to knock sensor. could it be one of the temp sensors at the back of intake ?

 

It could potentially be related to the code as, from what I remember, that is the knock sensor code which would relate to the ignition timing and therefore idle. However, I do know that since the ECT (Engine Coolant Tempature) sensor is located at the back of the engine, (the hottest part of the engine), these engines are susceptible to "heat soaking". In other words, that sensor will heat up when you go into the store or let it sit for five or so minutes since that sensor is heating up while it's sitting and tell the ECU that the whole engine is hotter than it really is. Therefor the ecu will dump a bunch of fuel into the engine and cause a rough idle. This is completely normal on the 3vzes and simply a frustration us 4runner owner must deal with.

Describe in detail the circumstances of your rough idle.

 

my problem is while driving????? at one light the idle may be 400 and two lights down the road it will be 800 ???
the heat soak makes sence also to me.

 

Sounds like a dirty throttle body issue.

Sticky Air Flow Meter air door...?

Possible idle speed out of adjustment.

Possible ignition timing out of adjustment.

Intake air leaks between Air Flow Meter and Throttle body.

Engine maintenance.

Sticky throttle cable/linkage..?

 

I always tell people to do a full tune up on their cars before starting any diagnosis. This problem sounds like some type of sensor issue however if, for example, your air filter is really dirty perhaps it could throw off your VAFM. You'd be chasing down sensors when it was really something as simple as a dirty air filter all along. That example is a little far fetched but I'm sure you see my point that you should try all the simple things first. Once you've done a tune up and if the problem persists then I would address your one known issue which is that code 52.

One thing that I just thought of is that there is an O ring on the throttle adjustment screw that hardens and causes a tiny vacuum leak. It never caused your specific idle issue for any of my 4runners but it did cause a rough idle on one. You may stand to try changing it out. Spray some throttle body cleaner into the hole that it goes into to clean it out just a bit then put the crew back in with the new O ring and adjust your idle. It's worth a shot.

 

wow that sounds like plan. ill check back.