[Note 5/12/2016 – even though they were “virtually” gone they were still around once in a while so I just converted to FiTech EFI]

Every year, when the outside temperatures across the country begin to rise, the subject of “vapor lock” rears its ugly head. I live in Arizona and, if gas is going to vaporize, it will do it here. I have had my GMC for almost 7 years and have gone from being stuck by the side of the road, at a gas pump, or at intersections many times but, so far, that is not happening any more. I say “so far” because there are variables like gasoline formulations which could affect us at any time. I said “virtually gone” because there have been slight hesitations on two occasions (on a coast-to-coast trip last summer) when it would have died before. Some quick pumping of the gas pedal fixed the problem immediately both times. I remember that happening occasionally when I was a kid when we had “real” gas.

So, how did I get to this point? Of course I don’t know for sure but I think the accumulation of several things has done it. Most of the things result in reducing the heat in the engine compartment and at the top of the engine. I have done nothing to insulate any fuel lines or tanks. The biggest things were: blocking the exhaust crossover passage on the intake manifold, fixing the exhaust system with good flow-through mufflers, and improving the air flow in the engine compartment (see the blog post HERE). Other things I did were to use an electric fuel pump full time and fix the fuel vapor recovery system. The mechanical fuel pump is gone and the electric one is installed on the outside of the frame just in front of the tank selector valve.

I think that fixing the fuel vapor recovery system helps the vapor lock problem a lot. I know of at least one very knowledgeable GMCer who would argue with me but I have evidence and some logic to help my argument. I “fixed” the system early in 2012 and, by that, I mean I connected all of the hoses the way they were meant to be connected and I replaced the fuel/vapor separator and the charcoal cannister. The reason I did this was that my navigator is very sensitive to odors and could smell gasoline in both the back bedroom and the flight deck.

Some people think that the fuel vapor system is allowed to vent fuel vapor to the atmosphere as rapidly as internal tank pressure demands and I think that, logically, cannot be true. The pollution rules, even back then, did not allow fuel vapor to just vent from open tanks. The charcoal cannister has a valve in it that limits the ventilation flow rate and that limit results in pressure in the tanks if fuel vapor is being produced faster than it can filter. That is my logical view – and now for some evidence:on April 27th of 2012, after the fuel vapor system, mufflers, and intake crossover were all done we left for California and the GMCWS rally.

I remember the date of April 27th because it set records for heat. We crossed the desert on our way to Palm Desert at well over 100 degrees. At our first stop for fuel in Gila Bend it was right around 100 degrees and when I went to remove the gas cap there was significant pressure to relieve before I could just take it off. The first time I experienced that was a couple of years ago and the vapor was accompanied by liquid gas when I just let it gush out. It took a couple of real clock minutes to bleed the pressure off completely. When I was re-entering the highway I had a “vapor lockish” hesitation but it went away quickly and did not recur. Our next stop for fuel was at Yuma where it was still over 100 degrees. This time the gas cap was playing a tune – the pressure in the tank had overcome the cap’s pressure limit (there is one). Gas caps are not allowed to be vented for pollution reasons. Since then I have found that the EPA requirements for gas say that April is outside of the season when summer formulations are required (to limit fuel vapor). We were burning “winter gas” which “boils” at lower temperatures.

Having that much pressure in the tanks seemed a little scary so I decided to allow the system to just vent by leaving the gas cap loose. The coach would barely run like that. We went just down the road to a shopping center lot to turn around and I could hardly keep it running. Finally, after about 5 minutes of deja vous with vapor lock I tightened the gas cap and within a minute it ran great and never did it again even through Brawley, California where the outside temperatures got to 110 degrees.

My theory is that the pressure in the tank keeps the fuel from boiling. In both cases I described above, I could hear the fuel boiling in the tank once I got the gas cap off. That is why I include fixing the fuel vapor recovery system as part of the problem/solution. Keeping the gas from boiling in the tanks helps a lot but eliminating it completely is probably not necessary because the fuel pump only has to keep the carburetor bowl full. A few bubbles getting that far are not a big deal and pressure in the tank minimizes it. Keeping the gas from boiling in the carburetor bowl is a bigger problem since it is essentially a small reservoir of fuel and there is not much room for bubbles. That is why keeping the temperature in the engine compartment, especially under the carburetor (the intake manifold), lower is the most important thing.

Comments are welcome.