Excess fuel pressure: tank vent or pump

[David F]

'72 240.  Been having a periodic problem of flooding while at idle.  Symptom is fuel bowl overflowing usually while sitting in traffic or after returning from short engine off period.  My initial thought was that my 3psi electric pump was overloading the needle valves.  But, this does not explain why it runs fine normally.  So, this past weekend, I filled the tank (probably more full than usual and after the car sat in the sun for a few hours I smelled fuel and noticed fuel spilling out from the filler (with cap in place).

So, I disconnect the vapor valve under the hood and blew through the vent line back to the tank...it was free flowing.  So, my theory is hot fuel create vapor pressure that is overwhelming the carb needle valve during idling (low fuel demand).  I am in the process of testing my theory by disconnecting the vent line from the valve (i.e. tank open to atmosphere).

Main question is do you think I am on the right track and second, how is that valve suppose to work (can it be tested)?

Edited February 4, 2020 by David F

[Mark Maras]

 I think you're on the right track. Is the stock heat shield in place under the carbs? I'm curious about your float levels. A higher than normal float level could exacerbate the flooding. How many turns down are the mixture screws, what color are the plugs and how well does it run when it's not flooding?

[David F]

Heat shield in place, float level set via 12 turn down method, plugs are good color, and runs fabulously when not trying to flood out.  Problem seems to occur when temp creeps above half-way on gauge

[Mark Maras]

14 hours ago, David F said:

Heat shield in place, float level set via 12 turn down method, plugs are good color, and runs fabulously when not trying to flood out.  Problem seems to occur when temp creeps above half-way on gauge

  I'm not familiar with the 12 turn down method but assuming the float levels are OK, the problem appears to be heat soak and or vapor lock. Common in warm climates and a problem that Nissan tried many things to resolve. If your carbs are 3 screw round tops you may have engine coolant circulating thru the carbs. Many have eliminated that feature to try to improve the fuel overheating. @siteunseen Other attempts have been an FI fan that came on later models, 280 Z style hood vents and in extreme heat , pulling the hood release to allow hot air to escape from the engine compartment. I wish I had a definitive answer for you hopefully others in hot climates will chime in. We don't get a lot of these problems living in the rainy NW.

[Zed Head]

17 hours ago, David F said:

 So, this past weekend, I filled the tank (probably more full than usual and after the car sat in the sun for a few hours I smelled fuel and noticed fuel spilling out from the filler (with cap in place).

Looks like you have a separate problem of a leaking filler neck.  Unless the fuel is all the way up to the top of the neck and is leaking from the gas cap.  Take the cap off and see where the fuel level is.  Fuel should never leak from the filler area.

Here is the document that describes the heat soak problem.  

 

[siteunseen]

For me the reason was the thermostat on the coolant line entering the intake closest to the windshield was stuck open, allowing hot coolant to constantly flow through my carbs.  I capped off all the parts associated with it, intakes and the main thermostat housing on the side of the cylinder head.  I had to leave that pipe that connects the two intakes, that SOB wouldn't budge!

Anyhow my carbs are cold to the touch now and it had absolutely no effect on the driveability of my car.  It's 70F outside right now.  

[siteunseen]

And uh... get you a fire extinguisher if you haven't already!  I found a refillable, all metal on at Lowes for around $20.  Fits perfect behind the passenger's seat.

[David F]

On 2/4/2020 at 10:39 AM, Zed Head said:

Looks like you have a separate problem of a leaking filler neck.  Unless the fuel is all the way up to the top of the neck and is leaking from the gas cap.  Take the cap off and see where the fuel level is.  Fuel should never leak from the filler area.

Here is the document that describes the heat soak problem.  

 

I did just that, the fuel is expanding to the point it is at the top of the filler neck and escaping past the cap.

[David F]

On 2/4/2020 at 11:14 AM, siteunseen said:

For me the reason was the thermostat on the coolant line entering the intake closest to the windshield was stuck open, allowing hot coolant to constantly flow through my carbs.  I capped off all the parts associated with it, intakes and the main thermostat housing on the side of the cylinder head.  I had to leave that pipe that connects the two intakes, that SOB wouldn't budge!

Anyhow my carbs are cold to the touch now and it had absolutely no effect on the driveability of my car.  It's 70F outside right now.  

When I purchased the car and went through it get it running, I cleaned the carb heat thermostat and got it working.  Since then, I have completely forgotten about it.  Incidentally, it is usually the rear carb that acts up.  I will look into bypassing the carb heat and see if things are improved.

Yesterday, I pulled the tach signal from my electric pump controller (this way, it still primes on key on, but does not run while engine is running) and relied on my mechanical pump.  I drove the car 200 miles without the flooding issue.  But, I was not struck in traffic.  Outside temp was 80 degrees.  I was reminded that the engine misses under heavy acceleration when running on mechanical pump only.  When electric pump is on, no missing.  So, it seems I need something in between.

[Zed Head]

That's a bummer then.  The caps are super-expensive.  I tried to find one for my 76 and couldn't do it.

https://zcardepot.com/products/gas-cap-fuel-lid-oem-240z-260z-280z-70-76?_pos=6&_sid=0d0ab3a6a&_ss=r

[siteunseen]

You may just need a new mechanical pump???

Mine would sputter on hard acceleration and going up steep inclines until I got my floats adjusted properly.  Now I try to get it to stumble but it will not.  Life is good!

[Captain Obvious]

There is a device called the "flow guide valve" that lives up in your engine compartment who's job is to prevent exactly what is happening to you. I'm assuming that's what you referred to as the "vapor valve"?

The flow guide valve has three connections to it. One to the vent line that leads to the gas tank, one to the crankcase, and one to the air cleaner.

If the pressure in the tank goes above four inches of mercury (about 2psi), the flow guide valve is supposed to crack open and allow the excess pressure to bleed off into the engine crankcase.

I suspect there's something wrong with your flow guide valve (like it's sticky gummed shut). There's a good description of the system in the EC (Emissions Control) section of the FSM. There's also some info on how to test it.