Question about high fuel pressure at the cap

[Ludachris]

At my last track day I noticed something strange and almost forgot to ask about it. When I left the track after running the car all day, I stopped to get some gas for the drive home. I shut off the car and went to pop the gas cap. I noticed a ton of air blowing out as I loosened the cap, moreso than I've ever had before. And then when I took it all the way off, air AND gas were shooting out. I put the cap back on loosely as to allow the air to come out slowly yet keep the gasoline from spitting out.

Now I didn't have a chance to check my fuel pressure gauge at that point to see where the pressure was, but when I got back in and turned the car on everything checked out fine. If the fuel pressure was normal what would cause this to happen? I have an Aeromotive regulator with -6 feedline from the tank and the stock hard return line.

 

[oldman]

Chris, have you check the fuel vapor hose, from the tank to the canister, for kinks?

 

[Ludachris]

I actually pulled the charcoal canister and all the other emissions not too long ago. I'll have to see what I did as far as the vapor line. I don't remember if I capped it or put a filter on it. Must be the source of the problem.

 

[oldman]

Definitely don't cap that, run a longer hose so it points straight down behind the tranny. Excessive tank pressure can also cause poor return capacity which can lead to AFPR overrun, it would be a good idea to keep an eye on the fuel pressure gauge until you find the source of of the excessive tank pressure.

 

[bastarddsm]

My dad's 65 mustang did similar stuff when it was ran hard after he put efi on it, we always thought it was from the gas being swirled around from the return line.

 

[staticbrainwash]

I actually was just experiencing this same thing on my 92. I did an emissions elim, and folded the rubber vapon line over on itself. I experienced some serious hissing issues like you just described.

I unkinked the line, and ran a silicone line down back behind the trans/tc area and now i get no hiss anymore. The car almost seems to run better too. But that could all be in my head.

 

[oldman]

The car almost seems to run better too. But that could all be in my head.

It's very possible.

Excessive tank pressure can also cause poor return capacity which can lead to AFPR overrun

 

[jabinya]

Would it be pertinent to disconnect the vapor line before it goes up to the front of the car? Wouldn't this help to vent quicker? My hard line is kinked underneath the car.

 

[Ludachris]

Yeah, I had it capped. I remember not being able to decide on what to do with it so I capped it and forgot about it. I just put a breather filter on it for now.

 

[staticbrainwash]

What i did is take a piece of silicone line and run it in place of the stock short line, and zip tied it to something on the firewall to help hold it facing down. So now it just vents to ATM behind the TC but away from the DP.

Did the breather fix the problem you were having?

 

[Ludachris]

What i did is take a piece of silicone line and run it in place of the stock short line, and zip tied it to something on the firewall to help hold it facing down. So now it just vents to ATM behind the TC but away from the DP.

Did the breather fix the problem you were having?

Haven't driven the car yet. How much vapor actually comes out of that line? Is there any chance it could ignite being in the engine bay?

 

[FORMONTOYA]

Just a FWIW about the vapor line.....

I had an issue with my fuel pressure not pulling down under vacuum as expected. I would only drop to 39PSI running 20inHg at idle when I expected 34-35PSI. Turned out that the problem was a kinked vapor return line. Although, during this time I didn't have the need to remove the gas cap before it was solved, so I don't know about excessive tank pressure.

http://dsmtuners.com/forums/showthread.php?t=207720 post #48

For me, it was really more of an "anal retentive" issue more than any perceived issue with the cars performance.

 

[staticbrainwash]

From what i understand it takes a while to actually build vapor inside the canister before it releases into the intake.

Now, having that line disconnected and venting i would imagine that it is venting constantly. But, seeing as how there is no canister for vapor to build the ammount released constantly should be very minimal. I've read about more than a few people removing the canister and venting the line without issue, so I imagine it can't be too much vapor to be volatile.

If i'm wrong i apologize. But thats why i vented it under the car, just precautionary. I do know the car runs a lot better with it not plugged though, and i get no more crazy pressure at the cap.

Excellent link btw formontoya.