Does wiring the exhaust bypass valve open cause problems??

I'm new here so I haven't seen any of the other discussions, but here's a rather longwinded description of the secondary air bypass valve.

The secondary air bypass valve as they call it introduces fresh filtered air into the exhaust stream via a rail that is bolted to the heads just outside of the exhaust valves. What it does is it supplies fresh air to the exhaust when the engine is cold which helps reduce the HC and CO levels while warming up.

In other words, when the engine is cold, the ECU richens up the fuel mixture which doesn't burn completely in the combustion chamber. So by providing more air in the exhaust, the rest of the fuel is burned there and in the CATs.

So here's how it's supposed to work. You start your engine in the morning and since the ECU has had to richen the fuel up to keep the engine running, it opens the valve to help burn all of the unburned fuel coming out of the engine (that burbling sound you hear when you first start up is caused by this valve being open). Your CATs start warming up and take care of the rest. Then when the engine warms up, the valve closes (the burbling goes away) and everything operates as programed.

By wiring the valve open, you are adding air to the exhaust all the time, which is not good. While alot of people think the O2 sensor is keeping an eye on the fuel, it really isn't. As the O2 implies, it's watching the air in the exhaust. Two much air means you're too lean, so it tells the ECU to richen up the mixture, even though this isn't what the engine needs. Now you're getting poor fuel mileage, poor performance, fouling plugs and wondering why. Another side affect is backfiring when decelerating because of all of that air in the exhaust.

Leaving the valve closed is better, but then you run the risk of burning up your CATs because now they have to deal with all of that raw gas without any air to help them, causing them to start glowing, and you know the rest of the story.

No, you really should replace them if they're bad, but don't forget to test the check valve which is downstream from the bypass valve. This valve stops exhaust gases from flowing back up the hose and frying your bypass valve, which is probably why the valve went bad in the first place.

To test the check valve, pull the rubber hose off and while the engine is running, hold your hand just above the valve. You should NOT feel any exhaust coming through the valve. If you do, replace it.

 

WOW, great assessment!!
But I thought he was talking about the secondary exhaust vacuum actuated butterfly valve that opens up when one is hard on the throttle. I tried wiring it open on my 1997 F355, it sounded great but I got the CEL soon after. So I put it back to stock. Some have said that the earlier cars won't CEL.

 

Oops, my bad.

I don't have a 355 so this is just a guess on my part. Based on a picture I saw and your description, I believe it's used to modify the engine power curve. When it's closed, it increases the exhaust back pressure which helps with the bottom to mid-range power. It opens under hard throttle to increase flow which helps with top end power. Believe it or not, no back pressure on the exhaust doesn't always mean more power. Of course the average guy probably couldn't tell the difference; you'd have to put it on a Dyno to measure it, or take it to a drag strip and do a bunch of timed runs down the track.

You could probably pull the flapper out of the valve and let the ECU think its working, but then I don't know if the CEL comes on because it doesn't see the back pressure, or because there is a sensor attached to the valve.

Anyway, since it's downstream from the CATs, I can't see how it could possibly do any damage to anything. Just a performance hit.

 

Allgood !! Mine even got a lean code when I wired it open. I guess MOST of the exhaust was not getting to the cats. What part of California are you in?

 

In the high-dessert on the way to Mammoth from Los Angeles. Not far from you.

 

Right on! Hey sometimes people post questions and I assume they have a 355 like mine, then I find out later it was a 348 question. BTW nice 348!!

 

Thanks, I've had it for two years and everytime I drive it (over 12,000 miles since I got it) I still get the same thrill I had the first time I drove it.

You know, I suppose the increased flow could fool the O2 sensor into thinking there is more air in the exhaust, but if that is the case, what happens if you put on a nice set of headers and freer flowing CATs and mufflers?

 

I have hyperflow cats and they flow pretty good. So I think as long as the revs are high enough there is plenty of exhaust the sensors can sample from. but if there are two exhaust paths open at lower rpms the sensors are reading half as much exhaust at best.

I prefer the lower exhaust note at idle and lower RPMs. So when I start the car and let it idle for 5-10 minutes, my neighbors won't freak out

 

Yeah but what's the point of installing all of this stuff if you have a valve that shuts it all down to stock settings? I don't know, it doesn't make sense to me; it shouldn't make any difference to the O2 sensor whether the back pressure is higher or lower since it is upstream and the air mixture in the exhaust doesn't change.

Anyway, I don't have one to play with, so I guess I'll leave it to those that do.