Fuel Tank & Evap Design

[NylonAdmiral]

Hi guys, I'm trying to figure out a few fuel tank/evap requirements.

 

I know this has been discussed in a few different places on HybridZ but I haven't got a conclusive answer because most of the posts seem to discuss the pros/cons of deleting the evap system.

 

Basically I have a US spec '71 240z.  I've got my RB26 installed and am now just plumbing in the fuel system.

 

I'm fabricating a custom aluminium fuel tank to sit in the OEM position and am working out the final requirements for fittings on the tank.

 

As this gives me somewhat of a blank slate, I had considered designing out the evap tank and using a vent valve to vent the tank but a number of posts seem to advocate actually keeping and using the evap tank.

 

Anyway, my question is, those of you that have installed RB's or other fuel injected engines, have you maintained the original evap tank? And if so, what did you do with tube 13 in the attached diagram? I believe in totally original trim, tube 13 goes to a small valve in the engine bay which is like a check valve that vents the vapour tank to either the air intake or the crank case?

 

Any help would be appreciated. Thanks!

[KevvinG]

I'm not quite sure how the '71 gas tank differs from my '77 tank but I believe that tube goes to the charcoal canister on mine.  In my case after removing the charcoal canister I've just run a hose to the bottom of the front valence just to expel those fumes.

[Tony D]

On later EFI Engines, the EVAP system is a critical part of keeping the fuel pump primed when the fuel is hot.
It maintains a positive pressure up to a couple of PSI to give an anti-cavitation NPSH to the pump inlet.

 

VW has a closed fuel tank system on a 68 Type III, there were no EVAP Canister requirements before 1974. 

 

Think about that...since you are using an EFI setup.

 

To your question on Line 13, it goes  to a  "Diverter Valve" which allows the fumes from a hot tank to go to engine crankcase for accumulation, where they are consumed on startup. This valve has a pressure rating, and it's a few inches of water positive pressure (much lower than EFI systems) so it will also hold positive pressure in the tank if suctioning of fuel out does not surpass fuel vapor pressure from hot fuel expansion. When it needs air to make up the  suctioning of fuel from the tank running down the road, it allows the filtered air from the air filter into the fuel tank. It works both ways. If it's plugged, and your fuel tank is well maintained, you will suck the fill hose flat as a pancake driving down the road with a good vane-style electric pump like an old Holley Red. Fuel vapors need a place to go as it gets hot, and air needs a way to get in as a vacuum breaker as  fuel is consumed going down the road.

Edited August 7, 2016 by Tony D

[Zetsaz]

I've been looking into what to do about this as well. I replaced my tank with an '02 f-body tank and I'm still thinking of how to redo that section as well as what to do about the charcoal canister up front vs. the charcoal canister mounted on the new tank. 

 

Not completely the same, so I hope I'm not hijacking the thread. 

[NewZed]

Nissan did essentially the same thing from 76 to 95.  Lost the liquid/vapor separator or incorporated it in to the tank by 95.  Some hose, a check valve and a carbon canister, plus the correct fuel filler cap, and you should be able to rig something up.

 

There was a thread a while back about how the newest cars have a pump to the tank to pressurize it, the tank itself not the fuel line, to a certain low level.  Thought it was crazy, but it's true.  Could be the for anti-cavitation as Tony D described.  Edit - actually it appears to be just for system-checking.  But, who knows for sure.  http://www.obd-codes.com/p0453

 

Diagrams from 76 and 95, 280Z and 300ZX.

Edited August 7, 2016 by NewZed

[NylonAdmiral]

Thanks for the input.  Tony D, it was actually your posts on other threads that had made me lean more towards keeping the evap system and incorporating the relevant fittings into the new tank.

I've been mulling it over but was interested to see how other people have done it on here.  At present, I'm thinking of keeping the evap system etc and making the new tank with the required fittings.  I'm just working out what to do with "tube 13" now really and had thought about a couple of options.

1.  I guess I could fit it with a remote vent valve in order to allow the tank to breath.

 

2.  Would there be any issue in plumbing tube 13 into the inlet system?  I guess the connection to the the crankcase is for environmental reasons so I could live with that?  If I connected the hose straight to the inlet then the tank would still get the filtered air during fuel consumption, but the fumes from the hot tank would still be able to vent back into the inlet if they had to?

I still have an almost complete donor R33 GTR sat in my garden so if there are any relevant parts that could be reappropriated from that to suit the newly installed RB26 then I should have them.

Thanks again for your thoughts.

[madkaw]

Glad this thread came up since I am getting ready to go EFI with my 71 . I have a hybrid fuel tank also(CJ-5) , and I have been using the stock vent box, but I want to just get rid of it. Fuel Tank inc just uses 2 vents on their universal tanks. One for the filler neck and one other to vent tank to atmosphere. My problem is tank is right up against floor and the vent needs to stay vertical and there isn't much room to go anywhere. I could use the original vent line and run the vent up front and grab a canister off of some other car.

So doing all this emissions cause me to get check engine lights now?

[wrcbonk]

I had the same concerns. I traded a few emails with Tony D and came up with the following. I kept 13 plumbed from the tank to the two way valve in the engine bay. The blow part of the valve (the end where excess fuel vapors are expelled from the tank) is plumbed to my intake. The Suck end of the valve is plumbed to a small air filter (a low pressure fuel filter I had laying around, whatever it does the job) ensuring no particulates get into my fuel system.

 

In the past I had issues with fuel tank vapor pressure forcing fuel out my filler causing havoc. I have had no issues with the present plumbing and I live in HOT Sacramento.

 

Bonk 2 cents 

[NylonAdmiral]

Thanks Bonk

 

I think it would still work without the diverter valve if tube 13 were plumbed into the inlet?  When the pressure is high, excess vapours would be vented into the inlet, when the pressure is low, the tank would draw fresh air down the same line?  As far as I can see, the diverter valve is only actually necessary if you want to vent the fuel vapour to a different location, such as the crank case?

[Tony D]

A straight 'breather' off the top would do both things, vent and break vacuum. The diverter valve is the device that separates the common line 13 into doing two things at a specific pressure or vacuum level.

 

I don't know exactly how 13 goes to the intake... in my mind this is a 'must' through the crankcase as you can't just have crankcase vacuum sucking on the tank...hence the diverter valve popping over to the intake filter when the vacuum is sensed. A straight tube to the crankcase would be exposed to vacuum and likely exacerbate fuel feed issues when hot. The diverter valve provides the vacuum-breaker action that allows the line 13 to then vent the tank directly to the intake filter or little remote filter in the back)...

 

Worst case, putting the diverter valve up above the tank, and letting BOTH of the lines dangle outside the car with little filters on them would allow the tank t o pressure up slightly, and once the car sucked out enough fuel admit air when necessary to prevent a properly-sealed system from sucking the filler neck flat from highway driving.

Keep in mind with the later EFI cars the EVAP canister valves on the top of the can provided these functions and as I recall they permitted quite a bit more pressure in the tank to promote a 'flooded inlet' situation with the EFI pump and it's hard draw. There is an FSM test for those as well, and the pressures are called out. Worth investigating closer if so inclined.

[Randall]

Wow its been a while since I have been on here. Still have the car and have done a lot to it since. I am currently making close to 400 plus hp with a flex fuel sensor. I removed the charcoal canister probably over 5 years ago to make room for A/C and intercooler piping when this thread was started and noticed that fuel would come out of the line that was left behind after the removal. I blocked it off shortly after and never had a problem since then. 

 

I have the tank out again just refreshing lines and was thinking of blocking the two lines on top of the tank off (one large one about 5/8 and the other is 5/16). I personally don't see the issue after going through the manual. It looks like the fuel cap is vented to allow fresh air in which would prevent a vacuum situation and at the same time I don't see an issue with building excess pressure in the tank. Although I suppose the real issue is when the tank is full and its hot outside causing fuel to work its way out of the filler cap. I'm guessing I have never noticed this since I typically only put about 5-7 gallons in and never fill the tank all the way. Mainly due to an incorrect hose being used for the large 5/8 hose (heater hose) detoriorating and causing partial clogs in my pre pump 100 micron filter. 

 

Still trying to decide what to do...

[Tony D]

I literally had to cut  guys line in back when his car wouldn't make fuel pressure and the filler neck was sucked down FLAT. If the system is fully sealed without some sort of vacuum break or vent, you won't fill it fully, and as fuel sucks out  of the  tank running down the highway...you get lower and lower pressure blanketing the tank, to the point if you run long enough, that the tank goes into vacuum and you burn up the fuel pump or lose fuel pressure till you pop the gas cap.