Hot fuel is a big problem with GMCs. Not only does it cause what I call “the vapors” (vapor lock is a specific type of vapor-caused failure which we don’t really see) but boiling fuel is corrosive and dangerous. It has twice peeled the paint around the fuel-fill opening on my new paint job. In October, on our return trip from our attempt to get to Coos Bay we were driving on the Interstate in Southern Arizona with the outside air temperature at 93 degrees Fahrenheit and we smelled gasoline. I pulled over to see what was happening and heard my gas cap venting – again! I took my laser IR temperature gun and found that the front of the front fuel tank was 155 degrees and the bottoms of both tanks were 139 degrees. In putting together a seminar about “the vapors” a couple of years ago I found that “Winter” gas can boil at about 140 degrees.
The heat that elevated the tank temperatures from the ambient 93 degrees to 155 degrees is coming from the engine compartment, the exhaust system, and the hot asphalt which is only about a foot away. The engine, using the FiTech EFI and Fuel Command Center, was running great but the fuel was producing lots of vapor. The usual story line is that the fuel/vapor separator and the charcoal canister are supposed to take care of that vapor and you should never have pressure in your tanks and definitely not be venting through your gas cap. If you do develop pressure in your tanks you must have a blocked vent line the story goes.
I think that is BS. The opening in the fuel/vapor separator is about 1/16″, maybe slightly more but not much. I opened mine when I got home from that trip because I thought there must be something wrong with it. There is, it is inadequate for today’s fuels and the GMC’s design. There are approximately 20 square feet of fuel in those tanks and that much boiling fuel cannot be adequately vented through a 1/16″ opening – that is obvious to me. The gas cap is designed to vent at 2 PSI (minimum) for safety reasons.
My friend, neighbor and fellow GMCer, Dan Gibb came up with the idea to insulate the tanks with a modern high-tech material called Ceramic Fiber Blanket Insulation and some roofing metal panels. We had heard of some other GMCer who covered his tanks with aluminum diamond plate and claimed it had solved his vapor problems. Dan found and bought all of the materials for the project (both his and my coaches) and we made the panels and installed them on my coach this week. It took two of us about 16 hours to make all 4 panels and to install them on my coach – Dan has a newly discovered fuel leak that has to get fixed first. [Note: long after installing the insulation I developed a leak in my rear air suspension system. Turns out we nicked the air tubing while installing the tin material eventually causing a significant leak. Fixed by replacing all three lines from that point to the driver’s side air bag.]
Below are photos of our project. The photos of the uninstalled panels and the drawing are by Dan. The panels will be installed in his coach after a recently discovered fuel leak is fixed: