Vapor lock questions for the hotter climate guys

[Mark Maras]

@jalexquijano, I'm still gathering heat soak puzzle pieces and trying to fit them together. Nissan says that fuel percolation occurs between the f. pump and the carbs and will cause an over rich condition. Vapor lock, on the other hand, occurs on the suction side of the pump. This got me to thinking about heat sources on the suction side of the pump. Taking into consideration all the work you've done under the hood to stop the problem, is it possible that the fuel is actually getting over-heated, in traffic, in the fuel lines under the car in the tunnel? My 240 tunnel used to get very, very hot, in traffic, in the summer. I can't imagine that a new exhaust with larger pipes would help either. BTW, I don't remember if you still have a mechanical pump along with your electric one (both are supposed to help the problem)

 To anyone, Was the fuel line location under the tunnel ever changed through the years? I'm still gathering puzzle pieces.

[jalexquijano]

PO removed the mechanical fuel pump from my car before i bought it! The Airtex 8012s electric fuel pump i have installed pushes 3psi (i measured it with a gauge last week). So in order to combat this horrible heat soak, i will be installing a 280zx injector cooling blower and using thermo-tec adhesive backed heat barrier: https://www.amazon.com/Thermo-Tec-13575-Adhesive-Backed-Barrier/dp/B00029KC2K to wrap the heat shield extensions. I am just really fed up of trying to solve this fuel starvation during traffic jams.

Anyone has a better solution?? Cant believe i am the only one that has this problem and cant find a way to fix it!

[Tirnipgreen]

On 9/6/2016 at 7:33 PM, Zed Head said:

Three heat sources are there - conductive (what you're talking about above), radiative (anything hotter than the fuel line will radiate heat across space to the colder object), and convective (hot air blowing over colder objects and losing heat energy).  They're all in play and hard to tell which way the heat is going, in or out, or which one is more powerful.  I feel nerdy now...

At the other end of the circuit, the return line in to the tank, the fuel is bringing heat back from the engine bay.  The tank contents warm up over time.  Could be that deadhead systems don't benefit from insulation and return systems do.  Or the size of that orifice is  critical for more than just pressure.  Cool fuel.  Didn't someone mention insulating the mounting points of the fuel rails, somewhere back in this thread or another.  That could remove the conductive path.

Interesting topic.  I've put lots of thought in to the EFI analog.  I have insulation for both convective and radiative heat sources, heat from the exhaust manifold, but left the rail (aluminum) uninsulated so that it could lose heat from the injectors.  The convective effect seemed to be pretty large, lots of heat coming up from the hole in the center of my exhaust heat shields.  Tiny hole, lots of channeled hot air.

How about a radiator for the fuel?  

My radiator has an integral automatic transmission cooler. Could the fuel line be run through it (manual trans)...and would it help or hurt the occurence of vapor lock...? My '73 with round tops starts easily, but won't start easily (if at all) after warming up on a Georgia Summer day unless I jump start it by pushing. Any thoughts...?

[Zed Head]

That would keep the fuel in the line at radiator temperature while the car sat.  Probably around 150 F, guessing.  Not good.  Besides that, if the problem is after the car sits, it's most likely the float bowls boiling dry.

Do you have an electric fuel pump or mechanical?

Edited July 23, 2023 by Zed Head

[Tirnipgreen]

28 minutes ago, Zed Head said:

That would keep the fuel in the line at radiator temperature while the car sat.  Probably around 150 F, guessing.  Not good.  Besides that, if the problem is after the car sits, it's most likely the float bowls boiling dry.

Do you have an electric fuel pump or mechanical?

Electric pusher through the mechanical as retrofitted by Datsun dealer back in the day.

[Zed Head]

I don't know how the relay associated with that retrofit works but, if it was set up likes today's EFI engines it would have a 5 second run timer as soon as you turn the key on, to refill the float bowls.

Other carb guys know more.  Nissans document describes what's happening pretty well.  Percolation, they call it.  They talk about being too rich but if the boiling continues for long eventually you end up at too lean.  Anyway, their descriptions might offer clues.  Good luck.

[siteunseen]

You can push it off and drive it? You may have a failing something in the start up side of business. Starter, solenoid maybe the coil? Have you tried jumping the positive and negative post on the solenoid with the key on, car in neutral with E-brake pulled up tight? I've had efi 280s and carbed 240s but fortunately never had hot restart issues. Good luck!

[Tirnipgreen]

39 minutes ago, siteunseen said:

You can push it off and drive it? You may have a failing something in the start up side of business. Starter, solenoid maybe the coil? Have you tried jumping the positive and negative post on the solenoid with the key on, car in neutral with E-brake pulled up tight? I've had efi 280s and carbed 240s but fortunately never had hot restart issues. Good luck!

Starter spins quickly, battery fully charged. I do believe that the timing may need a slight amount of adjusting, not sure enough to keep it from starting. Maybe things will be different during cooler weather...