Clamped Flapper Valve MPG Decrease

[GMC2015]

Last night I decided to clamp open the exhaust flappy valve on my 4.3 V6 using a bolt to hold it open. The good thing is that the truck sounds better and runs a WHOLE lot smoother. The V6 to V4 shudder is pretty much gone now. Definitely a must if you are having the vibration from AFM kicking in.

 

However, this morning while I was driving I noticed a significant decrease in mileage. I went from 27-28 mpg at 60 mph to around 21-22 mpg. Is this because the truck needs to relearn something? Should I be concerned? I'd like to get my awesome mileage back if all possible.

[2016 Sierra Owner]

I have the 5.3 and clamped mine open about 2 months ago. I have not seen any change in MPG so far. I'm actually thinking of unclamping it as the drone in V4 sucks.

[Waterfowler41]

Placebo effect.

 

 

 

 

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[GMC2015]

I have the 5.3 and clamped mine open about 2 months ago. I have not seen any change in MPG so far. I'm actually thinking of unclamping it as the drone in V4 sucks.

 

Weird. There's only a very slight hint of drone, and that's if you have the radio and the AC off.

[foghorn23]

I had mine clamped on the 14 Silverado. As far as I could determine no ill gas mileage effects. Haven't messed with it on the new truck yet.

[j3st3r]

First thing I would try is unclamping it to see if it goes back to what you were normally getting.

[Black02Silverado]

I would give it a while. Many factors can play on mpg, weather, temp, wind, fuel used. All you did was clamp it open. So if it doesn't go back to better mpg, then unclamp it and see if your mpg returns.

[GMC2015]

So I did the reset thing by disconnecting the battery for about half an hour and then drove around normally to give the truck a chance to relearn the fuel trims and what not.

 

I also forgot to mention that when I went to clamp the valve, I noticed that it was very hard to turn, almost as if it was kinda stuck in the half-way open half-way closed position. Had to give it a slight bit of pressure to get it going, then it became very easy to turn counter clockwise to clamp it. Maybe that had something to do with it?

[seamus2154]

If you were up north with a winter blend gasoline that would be it. But down here I don't think so. I have had 2-4 mpg differences here on the TP just with a head wind. The front of the truck has a big footprint. I have also had 2 mpg differences with bad gas. I don't think the valve has anything to do with it, till you prove it has by same day, same road, same wind, same weather, valve on and valve off.

[GMC2015]

If you were up north with a winter blend gasoline that would be it. But down here I don't think so. I have had 2-4 mpg differences here on the TP just with a head wind. The front of the truck has a big footprint. I have also had 2 mpg differences with bad gas. I don't think the valve has anything to do with it, till you prove it has by same day, same road, same wind, same weather, valve on and valve off.

 

The weather over here in Tampa has been pretty consistent all week so driving conditions haven't been all that different. It most likely has something to do with the valve since it only happened the first time I drove it after clamping it. Hopefully the battery reset fixes it tomorrow.