Question for the good tech guys. Friend and i were discussing ways to get better fuel mileage out of his cobalt SS. Right now he's only getting 25mpg on the highway. I suggested that he just run his bypass valve open all the time and constantly bleed off boost. Would that work? Would the car run if you did that?

If you're confused let me clarify. On the bypass valve there is alittle arm that opens and closes the valve. If you disconnected the arm and constantly had it in the open position would the car run? Would fuel mileage increase?

 

Yes it would run but it would be no different than just not pressing the throttle down as much, keeping the supercharger from creating boost.

When the manifold is in vacuum, that arm is already holding the bypass valve open (via a spring), allowing air to bypass the supercharger's rotors and route directly into the intake manifold.

 

How did i ******* know you would chime in witt. So in your opinion would this substantially increase fuel mileage? Do you think by doing this he could get 30mpg?

I always said someone should come out with a switch that could do this automatically from the drivers seat.

 

Or you couple just couple the boost line to the other side of the valve, and it will keep the bypass open at all times.
It would the the same as if the car went into limp mode and tripped the safety bypass actuator...

Look at my how to for an EBC on an LSJ... it's similar to the switch idea.

 

You could set it up in HPTuners to do it as well. If you can just keep yourself from having a lead foot you would have the exact same effect. You're simply limiting the amount of air pressure/flow into the intake manifold.

 

IMO, I would be very very careful with that bypass valve. Leave it alone unless there's a good reason to change it. The car would run as long as it has enough air for the fuel it's injecting, and it does run with the bypass valve all the way open on any other SS S/C. I believe the bypass valve is integral to controlling boost for the cruise control system on the SS S/C, among other things, and while keeping the car in vacuum all the time is a good way to save gas, playing with the bypass valve to do it is questionable.

Also, have you checked for any other reasons why he might be getting poor gas mileage? I hate to ask, but:

1. Are his tires properly inflated? 32PSI is the rating listed on the driver's side B-pillar. I was getting the same MPG and my tires were 8 pounds under-inflated.
2. When was the last time he had an oil change? Has the oil level and/or color been checked recently (in the last 500 miles)? Good oil will be shiny and nearly clear, bad oil will be very dark.
3. Is he carrying anything heavy in his car all the time? Subs? Random junk? Dead Body? Getting all that stuff out of the trunk will help if he's got anything in there.
4. Are his spark plugs and injectors ok? If the car is relatively up there in miles, has he ever had them checked?
5. Does he speed? Going 90 MPH on the highway doesn't save you any gas. I've found that most MPG issues are because of lead deposits in the driver's right foot.

The above is a bit basic, but it speaks to the same concept, I'd leave the bypass valve alone and focus on the more important stuff first, if all else fails take it in for a diagnostic check. You'll spend a bit of money but you might just find another problem that's sapping mileage, and maybe avoid a costly repair.

 

i dont know how he drive but the coblatss/sc should get 34 miles to the gallon on the highway so either he drives the **** out of it, or it needs a tune up depending on how old it is, i have the lt 5speed coupe and on a flat road, windows down, in 5th gear, at 55 mph i can get 50 mpg. it just depends on how you drive it and what conditions ur driving in, also the tires being under inflated.

 

Good post but I want to clarify a bit the bolded part.

Cruise control is handled solely by the electronic throttle body (ETC). On our model PCM, a P0068 diagnostic is required to pass before the cruise control can enable. The electronic actuator attached to the supercharger bypass valve is to limit calculated output torque when conditions dictated by the PCM are warranted such as a high ECT, IAT, or other DTC fault.

 

And reverse gear.

 

Wow guys thanks. I imagine he gets bad gas mileage on the highway because in rush hour it's stop and go traffic.

Let me ask again just to be sure. Driving with the bypass open 24/7 will not harm the motor correct? The car just won't have power. I just don't want to see him blowing up his engine

 

Shouldn't harm anything....

 

Thank you omega and all.

 

I stand corrected, thanks for the info! I have new questions about this now, but that's another thread

 

leave it be, do some tweaks in hpt with the timing and a couple other things.


i pull 28 mpg in rush hour traffic {40mph and slower}