slow96z

iTrader: (12)

I have a MAP sensor code and evap code showing right now and I was wondering if that may be why I'm getting such bad mileage. I removed the solenoids for the evap canister, but I don't think those are the reason for the mileage. Any help would be appreciated.

dhdenney

iTrader: (53)

I think it is understood that the evap canister is important and should not be messed with. I don't understand it but I haven't touched it. The MAP sensor would cause bad mileage yes.

HBHRacing

the cause of the MAP sensor code would rationally cause the problem more so then the MAP sensor itself.

thesoundandthefury

iTrader: (3)

The entire fuel system in the 4th gens is pressurized. The reason for this is that one of the detriments of gasoline is that it evaporates. There are 3 lines that run from the gas tank: one feed line, one return line, and one line that feeds evaporated gas fumes back into the intake through the purge solenoid to aid in combustion. When the EVAP purge solenoid is removed from the equation, this essentially creates a "leak" in the system. Since after the solenoid is removed the system is no longer pressurized, this allows gas fumes to continuously escape from the tank, whereas before the gas could only evaporate until it had completely displaced all the air in the tank. The gas being allowed to evaporate constantly by depressurizing the system is the reason for your decreased mileage.

On a side note, did you ever notice how sometimes when you'd stop to refuel, and you removed your gas cap, you'd hear that weird sucking sound? This sound was caused by air rushing into the tank to displace the vaccuum that had been created by all the gas fumes being burned up by the engine. Your mileage has decreased simply because your gas is now able to evaporate on a continuing basis instead of only until the fumes displace the air in the tank.

slow96z

iTrader: (12)

That sounds great in theory, but the mileage was bad before I removed the purge solenoid, plus I still get the hissing sound from the fill neck. I did cap off both openings that were left by removing the solenoid so I would assume there is no leak. Also, I was getting the MAP sensor code before I removed the solenoid so I don't think that's the cause for the code. The reason I removed the solenoid was to clean up and make room for my Delteq wires, but I may put it back if it can't be programmed out. I do know that a lot of the guys that have relocated their fuel lines have also removed the purge line without issue so I'm pretty sure it can be programmed out.

dhdenney

iTrader: (53)

I know you can turn it off in LT1 Edit but you can't program out the mechanical function that was mentioned above. I would leave it alone but if you think can swing it then go for it. I would check that MAP out and see if it's doing ok. Does it totally run like hell? When you say decreased mileage, like what is it?

thesoundandthefury

iTrader: (3)

There's no theory involved in gas evaporating. It's a proven scientific fact. Did you install a vented gas cap after you removed the purge solenoid? Since you mentioned that you capped everything off, if you haven't switched to a vented cap then the hissing sound you hear now is gas fumes escaping the tank whenever the gas cap is removed. Since the gas fumes are no longer being filtered into the intake, without a vent in the cap they have nowhere to go until the cap is removed.
You have to understand, that EVEN IF you didn't have the MAP sensor code, by removing the EVAP system you're going to have worse gas mileage anyway. Don't be shocked when after the source of the MAP code is figured out, that you don't get the same mileage as you were before the code ever happened.