Finally got my car on a dyno today. The dyno was a Mustang dyno, ambient temp was about 70 degrees.

Started out at 7.3 psi, then 10.9, and finally 14.5 psi.

Everything was fine on the 10.9 psi runs, added some timing and picked up some power, although the dyno was giving some weird readings (big dips/spikes) at the top end so I don't fully trust the numbers. IIRC it was about 496 hp at 10.9 psi. The spikes were higher but I don't believe them.

Then we turned it up to 14.5 psi and did 3-4 pulls. At first the numbers were low, but I started adding timing and they improved, I think we saw about 510 hp and I was ready to keep adding timing when the dyno operator noticed that fuel was pouring out of my filler cap and the end of the run! I had a gopro with me and you can clearly see a large amount of fuel spray out from the fuel door area about 1-2 seconds after I let off. After a couple minutes I tried to remove the filler cap and it there was a massive amount of vapor being released as soon as I cracked it loose - so much that it started to gurgle and spew more fuel so I had to open it just a tiny amount and let the pressure out slowly, which took like 5 minutes! I can't believe I didn't pop the gas tank.

I think what happened was that my EVAP system valve is being forced open by the boost pressure and I was pressurizing the gas tank to 15 psi!!! The other issue is that my Haltech harness has a broken pin, so the EVAP system is not functioning, and the valve just never opens. So as far as I can tell the tank got pressurized and then the pressure had nowhere to go, until the filler cap finally vented it out. Obviously fuel spewing out onto the floor is very dangerous so we ended the dyno session.

Has this ever happened to anyone???? Even if my EVAP valve was functioning, the tank would still have been pressurized by the boost. I can see it causing some weird fuel pressure issues and also seems like it would be bad when the ECU decided to open the valve and vent 15psi worth of fuel vapor into the intake manifold!

I am thinking a check valve, along with a safety vent valve is in order. Or I can just vent the tank to atmosphere and remove the system altogether but I would rather not smell like gas every time I drive it.

 

Video!!!

http://s922.photobucket.com/user/mx5...08d9b.mp4.html

 

Found some evidence on the interwebs of this happening to others:

http://www.8thcivic.com/forums/force...ap-system.html

They also mention that the HKS supercharger kit for the 350Z includes a check valve for the EVAP line. I believe I have found out why!

I can't believe more people with boosted Z's haven't had this problem!

 

Do you have a fuel return setup?

 

If the evap was properly working it wouldn't pressurize the gas tank otherwise thousands of boosted z's would be having this issue.

Don't modify anything just fix the evap problem and everything will work out. There isn't a design flaw there is a part failure.

It wouldn't vent 15psi of fuel into the intake since the evap solenoid is CLOSED most of the time and in properly functioning systems it will not be open during boost. That prevents the pressurization of the tank. Fix the pin on your haltech. It's there for a reason.

 

Also, you may not be pressurizing the tank via boost.
The fuel heats up, from the consecutive runs and creates vapor. If your evap system is not venting correctly, this will pressurize the fuel tank.

 

Yeah, just like thousands of boosted z's dont have problems with fuel resonance when they added return systems

Others have tested the purge valve and found it to leak at as little as 6 psi. So even when it is "closed" it can bleed pressure backwards through the system. Its designed to seal against vacuum on the intake side, not boost. There is probably a poppet with a spring inside the valve and 15 psi of boost is enough to lift the valve off it's seat. HKS includes a check valve for the EVAP system in their supercharger kit.

Your logic is flawed. My purge valve is ALWAYS closed. So how could I have pressurized the tank unless the valve leaked?

I don't disagree with you that I need to fix it. But I don't think a functioning EVAP would have prevented the tank from seeing pressure.

My G hasn't had a functioning EVAP for close to two years, and I have never had excess pressure in the tank - I would have noticed it when stopping to get gas if it were caused by fuel vapors alone! This is also the first time I have boosted more than 11 psi, which tells me that the valve probably started leaking badly at 12-13 psi. I intend to pressure test the valve and report my findings.

 

You answered your own question: "So how could I have pressurized the tank unless the valve leaked?"

The valve is bad. Put new one in and it should hold the 20+ psi that all of us build engine guys run without any issues. I definitely wouldn't band aid it by putting a check valve until you fix the original problem of leaking valve.

 

Do you have a hand pump or another way to generate a regulated pressure? I would be very interested to have you test your valve, and I will test mine and we can compare results. A new valve is going to cost a lot more than a check valve, and I hate to buy a new one only to find out that it leaks as well.

 

Why do you feel the need to immediately discredit or argue against everything I say? I have done scientific testing, posted data, and confirmed hypotheses on various subjects multiple times on this forum. And yet you seem to poo poo every idea that isn't your own and you think you know more than everyone else.

If the EVAP solenoid never leaks, why does HKS go out of their way to include this check valve with their supercharger kits? Is HKS the type of company you associate with products that are "band-aid" solutions? Not to mention the HKS supercharger doesn't produce anywhere near the boost you suggest that the EVAP solenoid is able to hold.

 

Mine did the same, I got rid of all of the emissions related lines. I have feed and return as the only lines to my tank and I have never had an issue since.

 

ya good thing my evap lines arent hooked up either .. no way to pressurize the tank

 

Check valve is easy insurance. Problem solved.

 

Does anyone know what pressure the fuel tank sits at under normal conditions on a boosted car?

 

It changes constantly based on environmental conditions (temperature, time of day, etc), fuel type, fuel level, etc. I suspect it is not more than a couple psi at most, ever, boosted or non-boosted. Keep in mind that the fuel cap allows air in but won't allow it out. So you will never see vacuum in the fuel tank.

 

I setup a regulator and a pressure gauage and tested my valve. It starts leaking at exactly 6 psi, and by 15 psi there is a healthy stream of air running through it. This data agrees perfectly with the results from the other gentleman who boost tested his Z and found the valve to leak at 6 psi as well. I took a video that I might upload. I also tested the valve backwards, basically simulating pressure in the tank. When connected backwards it will hold 20+ psi and doesn't leak at all. Obviously it is a poppet style valve and the spring that holds it closed can only handle 6 psi, while pressure on the opposite side forces it to seat.

I considered connecting it backwards as a solution. The problem with that though is that under high vacuum there would be more than a 6psi differential and you would constantly be sucking in air from the tank, which would not be good. So it looks like a check valve is mandatory.

 

So, I guess with a 'working' EVAP system any leaked boost will dump back into the plenum once you're in vacuum? Maybe that's why most people don't notice any issue?

 

I would say yes, that is correct

 

You might want to fix your EVAP anyways, because wouldn't pressure build up between the EVAP valve and the check valve and stay there? Over time, I think the check valve would be a bit tired and might let go and you're back to where you were... if you can't fix the EVAP electronics, perhaps just cap it all off...

 

I am going to fix the electronics, its just going to be a pain in the *** because I either have to spend like $600 on a new Haltech harness (no way) or splice into the factory harness AND the Haltech harness to bypass the Haltech connector.