fast_rx

Can someone explain the function of the BAC valve?

Mine seems to be opening when it shouldn't be (control solinoid problem I think). I've temporarily plumbed it up to the manifold vacuum to keep it closed. This has fixed the idle problem, but is there any downsides to doing this.

Thanks for the help,
Jason

Rotary-MG

iTrader: (1)

I'm stealing the below from
http://www.nellump.com/peri/epi/firs...gslseIdle.html

My appologies if you've seen it before or if it seems like too much to post here:

BYPASS AIR SUPPLY


As I said in the beginning, the EGI system varies the amount of air entering the engine to control idle. To do so, it needs a supply of filtered air to work with, and a way to bypass the air around the throttle plates since they would otherwise get in the way. This is what the Bypass Air System is all about.


Following the air funnel (black duct) from the Air Flow Meter back to the engine, you come to a small (about 3/4 inch) tube that connects to a short hose which in turn connects to the Dynamic Chamber. What you can't see is that the tube entering the Dynamic Chamber goes all the way through and exits from the other side into another hose. This hose makes a 90 degree turn and connects to the Bypass Air Control (BAC) valve, so called because the air it controls has bypassed the Throttle Chamber. This is the main device for varying the air supply to the engine during idle. What most think of as the BAC valve is really only the forward half of the mechanical assembly. The rearward half is called the Air Supply Valve, and it only comes into play to compensate for the load placed on the engine when the air conditioner compressor clicks in.

PROBLEM #5: Idle speed too low
Car Stalls
Idle speed drops when Air Conditioner Compressor engages
Idle speed drops with additional electrical load on engine


These are symptoms of a malfunctioning Bypass Air Control (BAC) Valve or Air Supply Valve. Since these are both on the same assembly, we will deal with them together. The fix is the same.


The BAC valve is the EGI system's tool for maintaining 800 rpm. With the engine off, the BAC valve is closed. The EGI system opens it when additional air/fuel is needed to compensate for drag on the engine caused when the alternator must work harder to provide electricity for heater fans, headlights, and other devices. If the BAC valve is stuck shut, additional air will not be available, and any of these activities will cause the idle to drop. A properly working BAC valve varies its opening depending on demand.


The aft end of the BAC valve is the Air Supply Valve (ASV), which only responds to the mechanical load when the Air Conditioning compressor engages. It is an on/off device, powered by a small electromagnet, and is less prone to failure. However, if the idle is just fine until you put the air conditioner on, look there for your problem. For simplicity, I will refer to the BAC/ACV valve combination as the BAC Valve assembly.


If you suspect a problem with the BAC valve, the first step is to remove it and see if the valve itself is moving. Disconnect the black hose on top, and the two electrical connectors on the wires that come from each end of the assembly. Remove the small vacuum hose coming off of the front the BAC valve, on the same side as the Dynamic Chamber (it is not easy to see). Remove the two bolts holding the BAC Valve assembly to the Dynamic Chamber. Remove it carefully, noting the condition of the gasket where it attaches to the Dynamic Chamber (they usually come off cleanly, so you probably won't need a new gasket). Note that this is the opening through which "bypass air" enters the Dynamic Chamber to supplement the air coming past the throttle plates.


There are two bolts connecting the BAC valve and the Air Supply Valve together. Remove them and separate the two parts. Look at the BAC valve first. When at rest, the valve itself is closed, and the shaft will be extended outward. You should be able to put your finger on the end and push it in. You should feel some resistance. When released, the valve should smoothly return to it's closed position. If you have it handy, take a piece of vacuum tubing, attach it to the hose barb on the diaphragm and suck on it. This should also open the valve (pull it inward). If it does not move smoothly, or if it is stuck shut, clean it with a suitable cleaner and lube it with WD-40. It does not have to be a high-tech operation, just get it working smoothly. Have a rag handy for the sludgy liquid which will come out as the cleaner drains.


Now work with the Air Supply Valve. To test it, you will have to apply 12 volt power from the battery to the electrical leads. The plunger should snap in and out sharply. You might as well clean it out while you have the part off. Put the BAC/ASV assembly back together and remount it to the Dynamic Chamber. Reconnect the vacuum hose in the front, the two electrical connections, and the larger hose on top. If, in your quest to solve the problem, you have been messing with the TPS or other adjustments, do the Standard Idle Adjustment to bring everything else back in line.


If everything else seems okay, but the idle still falls below 800 rpm when the air conditioner compressor is engaged, you can adjust the Air Supply Valve by removing the black cap that covers the access hole to the adjusting screw (if the cap is still there). Turn the screw counter-clockwise to increase engine speed when the air conditioner compressor is engaged.

batmang

sweet, i needed this info. thanks for posting it!