Bypass Valve always open...doing damage?

No, leave it as is

 

its fine.

 

It will cause incorrect feedback in the o2 sensors at part throttle and cause the computer to adjust the mixture incorrectly, but if you have no cats it's probably not a big issue - though remember the secondary system also has cats, so it could potentially damage those. I wouldn't run like that on the street, but track it doesn't much matter as the computer doesn't use o2 sensor feedback.

 

Are you sure the valve is open, the oem valve defaults closed.

 

If your cats are empty as in no ceramic inside them they can't overheat!

 

Pete,

Wouldn't the exhaust stream be split with part still going past the O2 sensors? If there were no cats, wouldn't the front (first) O2 sensor still read the mixture F/A ratio and adjust the injector flow accordingly? Why would this differ when the valve opens?

By the way, when racing, how does the system work? If the O2 sensor doesn't adjust the mixture when racing, what does?

 

Problem is the o2 sensors are in the cats, after the "Y" in the headers. It's said at part throttle there can be some recirculation of exhaust gases by a vacuum created by the y-pipe/bypass flow. If that condition does exist, it could effect o2 readings.

The o2 sensors only work under lower load situations. At WOT/racing you are in open loop/no o2 feedback and nothing adjusts the mixture. It relies on the fuel map and the assumption that everything is up to spec and functioning properly. (fuel pressure/sensors).

 

Depending on the muffler, it could take in varying degrees of ambient air across the o2, which would cause a false lean condition. The ecu would compensate by adding fuel, attempting to drive the o2 mv up.

Again, I have not done any testing to confirm this condition exists but it seems like a possibility at part throttle based on what Capristo has said.

Easiest thing to do would be to lock open the valve on a 5.2 car and drive it while monitoring the fuel trims. If everything stays the same as before then there shouldn't be any issues with leaving it open.

If the trims are driven upward by say, more than 5%, that is not a situation I would want to be running the car in.

That being said, my 95B had the Challenge bypass valve delete in it when I bought it. Made good power on the dyno so that's at least some indication that nothing was damaged by running it that way.

Edit- I should note that the primary cats were starting to fail at 11k miles. Metal particles from the cores were showing up on the back of the car so I replaced them. Whether or not their failure had anything to do with the bypass being removed I don't know. Either way, no worries on the cats if they are gutted. Nothing to overheat as Phrogs said.

 

The ecu has a base fuel map that ideally, would require zero fueling corrections to achieve target air fuel ratio.

So, a vehicle with no issues will be running in closed loop with close to zero fuel trims. (little deviation from base fuel map).

So as an example, lets say ambient air is being pulled into the exhaust due to a leak pre o2 (or possibly from the exhaust tips as we are discussing here?), the sensor is now sampling partially atmosphere rather than only exhaust gases and this would drive o2 MV low, thus being recognized by the ecu(s) as a lean condition. In order to get the sensor output back into the 450mv switching range and what the ecu believes to be stoich it would add fuel via short term fuel trims until o2 sensor voltage pulled back up into the proper range.

If within the boundaries of the trims and the condition exists long enough, stft values get applied to long term fuel trims. If enough ambient air is introduced it can drive both short and long term trims to max values. The engine is actually running very rich at this point but the ecu can rely on it's only form of measurement, the sensors.

Note that this can happen for many other reasons. Vacuum leak ahead of the maf sensors (not too probably with ITB's), low fuel pressure, low MAF output/etc.

You could also have negative fuel trims if for instance a failed fuel pressure regulator caused excessively high pressure or maf output was high, though this is not as common an issue with the way things normally fail.

So basically, with any obd2 scanner you would want to get the car up to operating temp and monitor long and short term fuel trims. Any large deviation from zero is an indication things are not right.

Your exhaust with no cats will certainly smell like fuel as they are no longer handling the unburned hydrocarbons.

 

So what didi you learn from this?

 

Bypass valve always open would be a bad thing.

I have posted numerous times about having the bypass valve always open on the 5.2 Motronic. Ferrari of Seattle did extensive testing on my car to put in a remote switch to open the valve and leave it open.. They were easily able to spoof the Motronic not to throw a CE light for the valve always being open (the Motronic monitors bypass valve operation through a thermoprobe just past it). But they found the fuel trims were going crazy, as very little exhaust goes through the catalysts (and past the O2 sensors), when the bypass valve is constantly open. The only solution to this would be to move the pre catalyst O2 sensors to the merge collector. I asked them to stop, and return everything to stock.

 

mine has quit operating i am going to wire it open until i can trouble shoot it....its better than being closed all the time

 

That can be debated. I certainly would not want to drive it for extended periods in either condition. Remember it is only open for short bursts at wide open throttle. There is mention on the Capristo website of issues in Europe (presumably Germany) of stuck closed bypass valves causing engine damage with high speed driving...but it is not normally open at cruising speed. Perhaps it opens partially at some throttle positions? I don't know...

I would check the hose first. Then check the condition of the plugs to the vacuum solenoid valves. The boots to the connectors crack and fail, allowing moisture into the connections, which can make them intermittent.

 

Doesn't it open over a certain throttle % when over the rpm min for the gear (2,700 in all gears except 3rd) vs requiring WOT? I think mine opens with abrupt or somewhat aggressive throttle vs only opening with very aggressive or WOT

 

Yes, you are correct....you have to dip at least half way into the throttle, as well as have the revs up, to get it to open. I forget what the WSM and owners manual states, but it is about 3500rpm in my experience..

 

I have a '96 OBD2. My valve definitely operates in a continuously variable condition. When I get the RPM's up above the gear threshold value so the valve is then free to operate, with part throttle I begin to hear the exhaust note get progressively stronger and stronger as I continue to open the throttle more and more (like when you are trying to get in position to pass someone on the freeway). Then, if I open it up all the way, it goes to full scream indicating a wide open valve condition.

 

My '95 2.7 was wired open. No cats. XOST muffler.
It sounded delightful.
But coasting down any hill with the accelerator negating any motor drag and I'd throw cells religiously. Cels would pop up every now & then at other times too.

I unwired it & the cells stopped. Of course it started rattling like heck. I swapped the diaphragm out for the (Porsche?) one & it's much better.

 

No harm.

I just got my car back with the stock exhaust and the next day I wired it so it stays open. It generates a CEL, but whatever. This exhaust design with the valve is the most retarded thing I've ever seen in automobiles.

 

I disagree.