Low Idle when AC on

Since i bought my tacoma one year ago, I've noticed that the idle drops below 500 rpm when i switch my ac on. Since I've replaced the water pump and the t stat, my engine runs a little bit hotter than before with no overheating issues, but the AC performance dropped and it only blows cold air when i rev it up. Checking the FSM there's one AC idle speed up valve, that increase the rpm around 900 when ac on. Tried to locate the valve as shown in the fsm pictures but didn't found. Where is it located in the 5vzfe?

 

Its definitely a problem with the idle-up system. Your idle rpms should always go up with the A/C compressor on.
Can't help you with the 3.4, since I have a 2.7. good luck

 

i would think it's attached to the throttle body.

 

what year is it? the 03-04 are electronic throttle body so that may be why you don't see it

 

its a 95.5, cable throttle body.

 

I am pretty sure its the ECM that gives the throttle body more gas when the a/c is on

does your throttle body look like this?


if so then the thing on the left there is your warm up idle control based on vacuum supply through the AIC valve and the can on the right im not sure what that one is

you need to see if the ECM is getting the signal that the a/c is on but I wouldn't know which wire it is to test that, but when energized it will (or should) cause the idle speed to increase. it could be as simple as a bad hvac control or maybe a bad wire or plug connection somewhere. wouldn't hurt to unplug and replug the ECM and HVAC control plugs

 

On my '99 truck (cable throttle), the throttle position at cold-idle (around 1200 rpm) and hot-idle (around 750 rpm) is exactly the same. This tells me that the idle is controlled by the computer (by adjusting fuel and/or timing), not by a carburetor-style throttle kick-up.

I would assume the A/C-on idle up is done the same way, likely via the IAC (Idle Air Control) valve. So either the IAC is faulty, or a sensor is out.

 

Its weird because the fsm shows one valve to do this job, and it allows some kind of adjustment. Its definitely not controlled by the TB opening. Some signal is sent to this speed up valve when the compressor kicks on.

 

the IAC functions as the air mixture adjustment for idling when the throttle body is fully closed. this only comes into play after the engine has warmed up and the fast idle solenoid is disengaged and the throttle body is fully closed.

the way the fast idle solenoid idle speed adjustment works is, it functions like a temperature controlled mechanical choke in that, there is no vacuum going to the fast idle diaphram so its fully out keeping the throttle body cracked open slightly to increase idle speed (choked) then the engine coolant warms up and it opens a valve to send the vacuum to the fast idle diaphram to suck it in and let the throttle body flap close all the way on the throttle stop screw (unchoked). if for some reason the diaphragm went bad, then it would stay extended all the time and after a few miles the ECM would compensate for it and readjust the idle speed so, step one is to see if the diaphragm is working, then if it is, reset the ECM to see how it is running and it should idle fast until warmed up, then idle down. but non of this has anything to do with weather the a/c is on or not

the actual idle speed setting as well as increasing idle speed when a/c is turned on is ONLY controlled by the ECM adjusting the fuel air ratio

I learned all about this when I was trying to find a way to raise my idle speed to stay at 750 to solve my rough idle in gear issues, but I found out no matter what you do, the ECM decides what speed it will idle the engine at and nothing you do can change it short of finding the signal wire telling it the a/c is on and sending power to it all the time but I wasn't going to start messing with the ECM wiring (back then I had a 99 with a 2.4)

 

The idle speed variation during the warm up is working properly. From cold start it reaches 1100/1200 and start to drop as the engine warms up. My idle speed with Drive engaged its pretty steady at 650-700 rpm. Just gets messed with the ac on, dropping to 470-500 rpm.

 

yep, your issue will be with the signal to the ECM, I highly doubt its the ECM, more likely a bad wire or HVAC control not sending the signal to it

 

It's in front of the throttle body, should be right there, 3rd drawing down in your post #8. That's assuming you have that system and not the later type. The FSM can be "hazy" in that regard, more so on the early years. All other idle speed actions are done with the IAC/ECM and completely separate. There is no fast idle solenoid or diaphragm and the throttle plate is always at a fixed position at idle.

Later years achieve "AC idle up" with the IAC/ECM just like the warm up cycle. The last years control idle speed thru the Electronic Throttle control.

 

I never thought I would have a reason to disagree with you but ...

this statement is not correct, maybe my terminology is wrong but no matter what you want to call it (see picture below) there is a cold engine warm up diaphragm that holds the throttle stop off its bottomed out position which keeps the valve body barely cracked open until the engine is warmed up, at which time it then fully closes the throttle body plate. I know this because it took me 2 weeks to find out the clearance setting to use on it after I changed the screw settings so there is no doubt here about its function or that it holds the throttle plate open in a normal cold engine condition.



all of this is moot anyway since he said this is all working correctly and only the a/c is the problem

 

Sorry, you are 100% incorrect. That part is called a "throttle opener". The name is a little deceptive. It is a stand alone system with one purpose.
It holds the throttle plate open, just off it's stop when the engine is off, by spring tension. That allows a bit more airflow to help with engine starting and starting only. As soon as vacuum starts to build (in seconds), the vacuum diaphragm overrides the spring and closes the throttle plate to it's normal idle position against it's stop, it's job is done. It has nothing to do with engine temperature, idle speed or anything else.

 

ok, so its not temp controlled but it does hold the throttle plate cracked open when the engine is off. good to know this since I was informed incorrectly as to how and when it deactivates. I thought it was very odd to have contrary info to what you were saying so I figured there might be something I was misunderstanding.

but I can assure you it can and does cause high idle symptoms if you dont get or lose vacuum to it or if that diaphragm fails, at least until the ECM compensates for it, that's just common sense