Excessive Fuel Vapor in Gas Tank

[Q451990]

So something new!

I was topping off the refrigerant in my a/c today (first time since I bought it in 2001 ) and kept noticing an unburnt fuel smell drifting from the back of the car. As I side note I noticed a weird miss at idle right after a warm start today too... I'm hoping it's all related.

So after a test drive I pulled back into the driveway, and noticed a high pitched squeal or whine coming from near the right rear wheel area. It sounds a lot like some electric leaf blowers, or almost electronic like an alarm of some sort. Anyhow, I opened the gas cap and got a huge rush of fuel vapor. It seemed like a lot more than normal, although I don't normally take the gas cap off with the car running. The noise stopped instantly. I also noticed a new odd sloshing noise in the gas tank after turning the car off.

I've never heard of this on any Q before, but at 26 years old, weird stuff is bound to happen. My theory is that the fuel vapor check valve is stuck closed, allowing the vapor pressure to build up in the tank. I'm guessing the squeal is a vapor leak from the weakest point in the vapor line. I'm not sure that I've ever seen the part, but it looks like there is one somewhere underneath the car.

I'll try to check it out soon and post the results here. If anyone has experienced this before I'd love to know about it too. I did a bit of searching for this on Nissans in general, and didn't see much.

[3Q Jay]

Hey Heath-
Both G50 do this on me.
I thought it was that the cap is properly holding pressure.....
don't smell vapors except when i actually release the cap, but i do get the high pitched 'vent' sound.
FWIW, they did this (maybe not as much,, but they did) when I lived in CA and had to do bi-annual smog, including fuel tank and cap pressure checks.

[Q451990]

Interesting. I did the testing on the vapor purge canister solenoid, as well as blowing air through the canister to check for leaks there, and they checked out OK. I'm using a small hand pump designed for inflating balls. When I first put air into the vapor line going to the back of the car (accessed at the canister) it wouldn't flow backward into the tank, but according to the FSM it says it should. However when I took the check valve off under the car, it seems to flow air from the tank to the canister as it should (air passes with some resistance), and more freely from the canister to the tank... unless I put too much air pressure to it - in which case it locks. I'm not sure if this is normal behavior or not. The FSM just says that air should flow freely in that direction. I'm going to go to the pull-a-part and pull a few of these off of other Nissans and see if they all behave this way. It sort of makes sense that it would lock under higher backward pressure, since the check valve is supposed to lock if the car is ever "wheels up" in an accident, and the valve is marked to be oriented "up." Maybe excessive pressure in that direction pushes the check ball upward as if the valve is upside down?

Here's a picture of a similar one. On the Q it's located just inboard from the right rear wheel well.

[94Q45Beast]

i've never seen that before even though I'm under my Q frequently inspecting everything. let us know what you find, I'm curious since I get the same problem

[Q451990]

Well, I went to the pull-a-part yesterday and tested several older Nissans with a similar design check valve. From what I can tell from reviewing various FSMs, if the car had a carbon canister under the hood like ours, they had this valve. Somewhere around 1996 or 1997 the evap. design changed to the style you see today, with the canister system mounted under the car. I didn't have an actual 1st gen Q45 to test, but all of the early 90s Maximas and Altimas all behaved like my check valve. A moderate amount of air pushed back toward the back of the car with an air pump will flow to the tank, but too much pressure trips the valve, as if it's been turned over. I never actually pulled a valve, because on those cars they're buried under the fuel tank.

I wonder if my valve got tripped somehow, and was locked... then taking it off of the car and rattling it freed up things and it's working again now. I guess I'll just drive it for a while and see if it happens again. All of the other components pass the tests in the FSM.

[94Q45Beast]

can you post pics of the location of the valve?

[Q451990]

I'll try to snap one the next time I'm under there. I need to put the clamps back on the hose...