86 Turbo - High fuel tank pressure
 

My 86 turbo builds up serious pressure with the cap on tight. A big whooooosh when I open it. What could be causing that - and what's the problem with running with a loose cap? Thanks!

Losing fuel to evaporation.

It's not a problem when you have a negative pressure in the tank. Why do you think it's a positive pressure? That's air rushing into the tank.

If you feel it's an issue, buy a new fuel cap.

Thanks for the response. Definitely positive pressure - gas vapors rush out. I checked out other net sites and it looks like a blocked line to the EVAP or a bad canister. I switched the gas cap with my other Z and the same big pressure builds up. Something is blocked. Will start the search and report back. Thanks again

Did you ever resolve this issue, I am having the same problem on my 79. I have just been leaving the cap loose, but there must be a fix.

Tank Pressure Problem not yet solved
 

Thanks for the reminder. My86T with the tank pressure issue has been in storage for 4 months, just took her out yesterday. No pressure noticed but tank was almost empty so... When I bought the Turbo, it had no cap at all - so PO knew something was up but said nothing. I bought an OEM cap.

I switched the new cap with my other Z31 ('86na) in November - both are OEM Nissan caps - and the pressure problem was only intermittent after that. It never went away 100%. Sometimes the opened gas cap whooshed a real blast of fumes, sometimes not. My other Z has no problem with either cap, so no verdict yet

Last fall the mech who knows the car well said there has to be a blocked fuel vapor vent somewhere and the excess pressure is building up in the tank. I believe that kind of pressure can only be built up by the fuel pump - since the volume is way above normal vapor pressure from a tank. Fuel pressure measured at the injectors is in the normal range. So somehow there is back-flow pressure into the tank and it builds up until you open the cap.

Somebody here might know how that could happen. I have never dropped a Z31 fuel tank to check out all the connections and figure out where they go and where the tank actually vents excess pressure - or if maybe the pump could leak positive pressure back into the tank in some way. Maybe a new fuel pump is the solution. Gotta drop the tank for that, I guess. :104:

Hope to get to it this week and maybe get both Zs up on lifts to compare them. Will post for you when I find something. In the meantime, I guess you can leave your cap loose until we figure it out. May affect your mileage, etc.. :confused: This will be solved! I guarantee it. :023:

PS: Forgot I picked up a Z31 canister from a recycler to give that a shot while checking the lines. That is a real possibility here according to the blogs. Would have done it by now but weather has been way too cold.

You can check out your FSM around EC-17 etc to see the possibility of a blocked evap canister or vent line as the cause. Looks like the prob to me at this point.

Carbon canister
 

EVAPORATIVE EMISSION CONTROL SYSTEM

it is the carbon canister and the relief valve, in your FSM, in the Engine Fuel Emission Control, EF& EC 77 for the 84 300zx, may very with other years, there is a step by stop process to check out the system and how it works, basically the same on 280zx's and 300zx's

Normally, the fuel tank is at a slight negative pressure from the gasoline that is slowly removed from it. If the tank was completely sealed, the negative pressure would increase until the car stalled out of fuel tank slowly inplodes. The carbon canister allows the fuel tank to breath. Has that been deleted from your car?

Thanks for the replies, guys. The canister is there, looks original.

So my theory is that just figuring out where that much pressure could come from is most of the battle.

I got a message from a noted Z31 guru who says that so much pressure could only be coming from the air intake/plenum side of things - which means either one or more hoses are hooked up wrong - or it's feeding back through the (non-working) canister into the tank. Check out the '86 FSM diagram on EF&EC 106. Pretty simple system - if everything is working.

Will report back! :icon_super:

As far as I can tell, everything I have is factory as well. And like was stated before, it is a whole lot of pressure built up. Like when a plastic gas tank sits out in the sun.

Yep, dobber. Pretty unlikely that your hoses have been wrong all those years. The amount of pressure that builds up is freaky.

If you can, pick up a canister for your '79 somewhere - either a used '79 or a compatible part - and plug it in. (You can't really complain about yours - a part that worked for 35 years!) Drive for a bit and see if problem disappears. If not, then check the valves in that system. Gotta be canister or valves...

My first go at it will be the canister. Gonna snow here again tomorrow :mad: so it will be later for me. Spring is coming eventually...

I have a guy that lives right by me with another 1979 so I will see if I can try his canister. If that works I will call a placed in Omaha and see if they have one.

Sounds good. Don't forget the Wanted/WTB forums here. Somebody has it. :003:

Do the work
 

The FSM fully covers your issue and how to fix it, have you done the troubleshooting steps on your system ? Endless discussion when a solution is present isn't constructive.

Well I downloaded the FSM and was reading through section on the Fuel Emissions System. I do think I now have direction to go. I also found the pictures real comical, funny how it tells you what to test and tells you to blow in with different amounts of air. So I will start on it tomorrow testing stuff.