Just what is vapor lock and how do I fix it? I have a 2005 Nissan Altima SL 2.5l and after replacing a leaking radiator, hoses, clamps and thermostat it's still running hot! Someone said it was vapor lock and had something to do with the fuel pump... any idea?

  • 1
    If you have fuel injection ( electric fuel pump ) , you do not have "vapor lock". Commented Apr 9, 2021 at 14:18
  • Welcome to Motor Vehicle Maintenance & Repair! Commented Apr 9, 2021 at 14:36
  • EPA law seems to be 7.8 psi MAXIMUM allowed vapor pressure ( Reid vapor pressure ) in the US. Electric pumps for fuel injection put out much higher pressure than that. . So ,by definition, you do not have "vapor lock". Commented Apr 9, 2021 at 15:56

2 Answers 2


No, not to do with the fuel pump - at least not for the cooling system.

Vapor lock can be for any pipe wehere the flow is stopped due to a pocket of air. Poor bleeding or burping of the cooling system can cause overheating so it is something that has to be done correctly.

Vapor lock can also happen in fuel systems and has often been an issue with carburretors on engines - less so with the pressurised fuel systems common now.


Huh? Not sure I totally agree with Solar Mike here. He's right, a pocket of air in the fuel line will definitely stop flow.

But the term "Vapor Lock" typically involves overheating of fuel in the fuel line to the point where it vaporizes from liquid to gas. And as a gas, you are not going to be able to keep that vehicle running. Typically vapor lock is caused by low fuel pressure in combination with excessive heat.

One key.. the hottest cycle of an engine occurs AFTER the vehicle is shut off. There's typically no fan running. The top of the engine gets hot. In a low fuel pressure situation with that heat, the fuel vaporizes in the fuel lines and you won't be able to start your car, until things cool down.

Clearly this was a huge issue with carburetor systems (with a vacuum type mechanical fuel pump). Modern fuel injection vehicles have a high pressure fuel pump, typically in tank. There is a check valve in the end of the fuel pump that keeps the fuel from flowing back into the tank after the vehicle is shut off. That check valve also keeps the fuel pressure in the lines pretty high. That high pressure prevents the fuel from boiling and causing vapor lock. And yes, that check valve can fail. But all of this has absolutely nothing at all to do with your issue of overheating. Not a single thing. Hint, when a car has a "vapor lock" problem, it won't start.

It would be nice to understand why you are saying your vehicle is "overheating".

  • It that because the temp gauge is getting too hot, and the overheating/check engine lamp is lit?
  • Is that because the car is so hot, the Air Conditioning system shuts off (to protect the engine?)
  • Is that because you are leaking coolant all over the place?

You haven't been all that descriptive. My guess, Nissan, you're seeing coolant leakage. and my guess is bad radiator cap flow back vent. But I can't confirm that until you tell us more...

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