AAC valve or ECU???

Hey guys, I need an opinion on a lil problem...

I've been having a problem with my idle... I finally set the TPS correctly, which wasn't correct before. BUT now my idle jumps up over 1200-1400 RPM. I set the idle properly with the TPS disconnected and timing is good. When I reconnect the tps, the idle jumps up and is erratic.

I disconnected the AAC valve connector and the idle is stable at 750 rpm

since the AAC is controlled by ECU, which is bad? the ECU or the AAC?

I removed the whole AAI assembly and it looks good, clean, and functional.

also, what input sensors does the ECU use to detect the proper idle and signal to control the AAC?

with the AAC disconnected, the car runs great and idle is where it should be. :screwy:

OK guys, heres some new revisions of my problem...

When I start my car when its cold, it idles about 900-1000 rpm which is normal for the 'cold idle' when the air regulator is open. OK... once my engine warms up, the idle goes to normal for a few seconds then jumps up to about 1400 rpm. With the IAA adjusted and the TPS harness disconnected, it idles stable at 750 rpm. When I reconnect the TPS harness the idle slowly jumps back up. If I disconnect the AAC solenoid connector, the idle drops back down to the base setting. So basically I'm trying to figure out if its the AAC thats bad or the ECU sending a bad signal, or some other input sensor.


I also previously removed and cleaned out the whole IAA assembly and disassembled everything including the FICD and the AAC. Is it possible I reinstalled something wrong in the AAC? There was a small metal pintle and a spring, but I dont know if I positioned them back wrong or something... I really dont remember if it was in right. Is it possible or would it just not work at all?

Other than my idle problem, the car runs great, accellerates great, but just wont idle right. I doubt its a vac. leak because like I said earlier, it idles fine when I disconnect the AAC.
Help me please!!! :screwy: :banghead:

Don't feel alone. I'm having the same problem. I've tried turning the screw on the side of the AAC and now it's all screwed up. If I let off the gas from 2000rpm or greater, pushing in the clutch, the car spits and sputters and sometimes dies on me. So I am hoping someone can answer this question.

did u get an aftermarket ECU? did u replace the ECU? DId u replace the AAC valve? does ur OBD1 sensor kick anything out? do u have the right equipment (ecu an aac valve) for your specific model and year?

hope this helps:

The idle control mechanism is the most complex of all the tune-up items, and getting the idle to be correct and stable under all conditions (cold, hot, while turning, coasting down or braking, etc) can be a hair-tearing nightmare. There are FIVE parallel subsystems for controlling the amount of airflow at idle, and it's very difficult to diagnose which of the five is responsible for any problems. The first thing to do is to do a cursory check of connectors, resistances, etc., looking for obvious problems. Failing that, the only reliable way to find the problem is to check EVERTHING in detail, one by one, skipping nothing. This will involve a good deal of disassembly and cleaning of parts off the car. Unfortunately, there is no easy way to get around this. You can always set the idle artificially high to prevent stalling around corners, but the car will just feel better when you do it right, so be prepared to get greasy and do some work. All of it is stuff that might seem confusing and hard at first, but after a day you'll be telling people "it's really easy once you know how."
The five parallel systems, in order of how much of the time they are used:

Air flow around the throttle plate. There is a tiny gap between the throttle plate and the walls of the throttle body. This gap tends to become clogged with nasty particles (from the EGR system piping exhaust into the intake). I don't know how much air is intended to bypass through here - some small percentage of the air handled by the others. As this clogs up, other systems (AAC) have to take up the slack, which may cause some confusion in the ECU. The ECU may be able to deal with it, but I think clean is good so clean it. Another thing to check (somewhat unlikely, but check anyway) is the throttle set screw - a little screw that sets the stop limit of the throttle plate. If this gets turned somehow, it might not let the throttle close fully, causing high idle problems.
Idle Adjust Screw. This is a small plastic screw in the IAA unit (pointing up) that adjusts how much air bypasses the throttle at all times. Clockwise is less air, counterclockwise is more air. This is used to set the base idle. If the base idle isn't right, the ECU has to start doing feedback control of the idle, which will be less stable and more prone to idle "divebombs" (where the idle speed drops suddenly for no reason, kicks up to high, and takes a while to settle).
Auxiliary Air Control (AAC) valve. This is the ECU's main idle control mechanism. It's a solenoid valve that opens or blocks airflow around the throttle plate. It's located on the Idle Air Adjusting (IAA) unit (along with the idle adjust screw and the FICD valve) on the passenger side of the engine block just behind the oil filter. The AAC valve points back toward the firewall and its wiring harness points toward the passenger-side wheelwell. The AAC valve operates a metal plunger on a metal valve seat, both of which could conceivably get dirty and fail to seal properly (or perhaps they need some sludge to seal at all. No data on this). The AAC valve itself seems robust, although it seems to get replaced quite often by dealers with undiagnosable idle/hesitation problems. I'd guess that replacing the valve is rarely necessary.
Idle Air Regulator (IAR). This valve lives on the bottom of the intake manifold, above the passenger-side axle. It's temperature sensitive - open when cold, closed when hot. That makes the car idle higher when it's warming up, which is good. It also has an electric heater, so it will heat up and close a couple of minutes after starting no matter what temperature the intake manifold is (which can take a while to heat up, even if the rest of the engine is warm). This seems like a likely culprit for cold-idle problems, but my little experience with this guy tells me it's very robust and not prone to collecting sludge, so it goes at the bottom of my list of stuff to check.
Fast Idle Control Device (FICD). This hex-sided solenoid screws into the side of the IAA unit, and opens when you turn on the air conditioning, to compensate for the added drag of the compressor. I don't know if this valve is missing if you don't have AC. Again, I think it's pretty robust so it may be safe to ignore.Now, there are lots of other things that can have an effect on the idle speed (EGR, fuel problems, idle leaks, etc) but these are all of the direct control pathways.

I reset the computer which seemed to have eliminated this quirk.
The best way to see if you have this problem is to warm the car up, turn the engine off, disconnect the TPS, and start the car back up. Hopefully, the idle will be at about 750rpm. If so, disconnect the AAC valve while the car is still running. If the idle stays constant, your idle is set correctly. If the engine stalls or the rpm's drop, your idle may be set incorrectly.
Another cause of poor idle is exhaust gas recirculating back into your intake. A faulty EGR is the most widely known culprit but there is another source that can potentially choke your engine, the PAIR(AIV). This is that EGR looking device mounted on the front of the engine close to the radiator. It's whole purpose is to allow fresh air into the exhaust stream at idle. On a stock set-up it takes this air from the intake just past the air filter. The PAIR(AIV) has reed valves in it that are supposed to allow air to be sucked into the exhaust stream and prevent exhaust from flowing back into the intake. Unfortunately, the reed valves on my unit are warped due to the heat of the engine and don't prevent backflow of exhaust gas. If you want to check the PAIR(AIV), open your filter box while the car is idling. There should be a port on the left hand side of the upper half of the box. You should be able to feel a vacuum on that port if it's functioning right. If not, you can feel puffs of air coming out of the port, and it will sound just like an exhaust leak.