What is vapor lock? I think I have it.

[DNA DOA]

Just picked up a 89 Fox black coupe and working the bugs out of it. Last week it was warm out, been driving the car a bit and car shuttered and slowed down for about 1 second. I thought it was in the drive train like a bad U joint or the fuel inj were dirty. Untill I was sitting at a drive through and it did it while I was at an idle in gear (auto). Car died that time. Then it took some time to get it started again.

Someone said it might be vapor lock. What is that and what can cause it? How do I stop that from happening again? I do have a gas cap that when tight it still turns like it's loose. I believe this type of cap is suppose to let gas tank pressure escape. Maybe I'm not even on the right track here.

Thanks for the help.
Ben Semper Fi

 

[jrichker]

Vapor lock is when the fuel boils inside the fuel lines. It is common in carb cars, but not in EFI with a properly working fuel system.

Here’s why: when you pressurize a liquid, you raise the boiling point. It doesn’t matter what the liquid is: water, oil, gasoline, brake fluid all react the same to pressure. That’s why your car has a pressurized radiator: instead of water boiling at 212 F, with 14-15 PSI pressure, it boils at 250 F. Gasoline pressurized to 40 PSI would react similarly, and its boiling point would go up beyond the 180F-220F commonly found in engine compartments.

The fuel in the return line might boil, but since it goes back to the fuel tank, it doesn't matter. Over time it will condense back to a liquid if your carbon canister & the plumbing associated with it are working properly. If they aren't you boil off the gasoline and fuel economy and air quality suffers.

 

[mustang03ally]

I've actually had that happen with one of the vehicles I own. It was difficult to start, and once it did it seemed like I had to keep my foot steady on the gas, just slightly to slowly keep moving. As soon as I let off the gas it would completely stall. After I pulled over and had someone check it out, they just turned the gas cap off and turned it back shut to see if it would help. Turns out I haven't had that happen since.
I'm not sure if it's true or not, but someone told me that if you top off your gas tank when filling up sometimes it can cause it to stall out like that.
Good luck!

 

[DNA DOA]

Thanks guys. I might look at the carbon canister & the plumbing to make sure everything is ok.

Thanks again.

 

[BlackGT89]

As stated above, the days of vapor lock are pretty much over thanks to EFI. I would more suspect you might just need to clean the idle air control / bypass valve. If it continues or gets worse after you clean that it might be your TFI ignition module on the distributor, TPS or 10 pin connections. Just a few suggestions.