GMC Western States

 Tech Center Number 36 - March 15, 2002

Emery Stora

From our Technical Vice President

Well, we are getting closer to spring. This has been an extremely cold winter in Santa Fe. I am looking forward to a warm spring and also to the next GMC Western States roundup in Prescott, AZ at the end of May.

We again plan to have several vendors at our roundup and will, of course, have some great presentations on a variety of technical topics that will surely be of interest to our members.

For example, we'll have Bebe Pettit tearing down and rebuilding a GMC transmission. Frank Condos will be giving a presentation on the Quadrajet carburetor and its repair and maintenance.

We also plan to have a presentation on the use of "wireless email" -- getting your email through your cell phone while traveling. You can also connect your cell phone to your computer and "surf the net" when you are on the go.

Several other topics are in the planning stages. At this time we still have some unbooked slots, so if any of you has a burning desire to give a presentation on a topic near and dear to you, or if you just have a topic that you would like to see covered by someone else, please let me know as soon as possible. Email me at or telephone me at 505-989-8157 or 505-660-3717 or write to me.

Your input is important if we are going to have a program that will be of value to you, the members. I would also like to start getting some input on what you would like to see at the Cody, WY roundup in September.

We have had a few very good articles submitted to us. Unfortunately we don't have room to include them all at once, but I will try to work them into future newsletters. For this issue I am including a write-up by Denny Allen of Allen's Classic Service, 1745 Pritchard Rd. RR1, Cowichan Bay, BC Canada, 250-746-7381,

Denny explains some of the quirks of the GMC fuel storage system. He makes one point about filling up better if the driver's side is a bit higher. I accidentally discovered this when a few years back I had pulled into a service station, which had a very slanted drive. The passenger's side was about a foot lower than the driver's side. The GMC filled up better that it ever had with no "belching" whatsoever. Ever since then I have lowered my passenger's side air bag a few inches when I pull into a service station to fill up, and I find that it makes filling up much easier and quicker. Apparently it vents much better when the tanks are tilted to the side.

It has really been interesting to see all the GMCs at our roundups. There are a lot of "classic" rigs that have everything as it came from the factory, but there are an increasing number of coaches that have shiny new paint jobs with all sorts of decorative touches in the painting. There are also a lot of technical modifications being done. Many of these are visible but a lot are hidden in the engine box or under the coach. There are many throttle body injector systems (TBI) and even some ported EFI systems (electronic fuel injection), ram air intakes, air dams, higher ratio final drives, electric auxiliary vacuum pumps for the brakes, headers, larger tail pipes, rear mufflers, 6 wheel disc brakes, waste tank macerators, 120 volt inverters, satellite antennas, flat screen TVs, on board computers and GPS units to find your way, sliding screen doors, microwaves, and all sorts of other things. Be sure to look for some of these at the next "open houses" at the roundups.

Denny Allen

Fuel Storage and Delivery System

An explanation of the fuel system from the fill pipe to the carburetor is presented here. I don't intend to discuss carburetors, because I do little more than clean and re-kit a carburetor.

GMC coaches have two fuel tanks: the main tank and the auxiliary tank. The auxiliary tank is located in front of the main tank between the frame rails under the coach. They are cross-connected by the "fuel fill tube" which allows fuel transfer between the tanks. Also the vent system connects the two tanks.

The fuel fill tube has not been a problem to coach owners, except perhaps you have experienced trouble with the gas pump nozzle kicking off way before the tanks are full. This is usually caused by the vent system that we will talk about later.

SO! You pull into the gas station and pour some more dollars into your GMC. The nozzle kicks off (if you read a "Cinnabar Newsletter" it says that the tanks should not be topped off), you pay the bill and drive away. The reason for not topping off the tanks is that there should be an air space left in the top of the tanks to allow for expansion. Fuel is stored underground at the gas stations, and it is about 60 degrees F when pumped into your vehicle. Then you head out onto some hot pavement that is less than one foot from your fuel tanks. The fuel expands and has to go somewhere. This will likely create a fuel leak somewhere.

The amount of fuel you got onboard before the nozzle kicked off depends on how level the coach was while filling. You will get the most fuel if the driver's side front corner is higher than the passenger's side. If the driver's side front corner is lower, then the nozzle will kick off early.

At any rate the first hill you head down will transfer fuel from the rear tank (the main tank) to the front tank (the auxiliary tank). On the next uphill, only some of this fuel finds its way back to the main/rear tank. This occurs because of the "fuel fill tube" between the tanks. This transferring of fuel back and forth between the tanks explains why the fuel gauge readings are sometimes all over the place.

Assume you got the tanks full with approximately 50 gallons on board, and you are traveling along the interstate that is about level and fairly straight. The tank selector switch is on "MAIN", so you are pumping fuel from the main/rear tank. However because some fuel is transferred from the auxiliary tank to the main tank through the fuel fill tube, the first 300-350 miles will actually be using fuel from BOTH tanks. Then the fuel gauge drops like a rock. This is because you are now using only the main/rear tank since the fuel level in the tanks is too low to allow fuel transfer. At this point the auxiliary/front tank is retaining 7 to 9 gallons in reserve.

The following is assuming 10 miles per gallon for ease in explaining what is happening:

Switch to "AUX", 7-9 gallons in front tank 75 miles to go

 Full  50 total gallons in tanks  
 3/4 full  38 total gallons in tanks  gone 125 miles
 1/2 full  25 total gallons in tanks  gone 250 miles
 1/4 full  12 total gallons in tanks  gone 380 miles
 Empty  0 in main/rear tank  gone 425 miles

Does that also explain why the top half goes a lot farther than the bottom half?

The original tank selector valve is located on the inside of the left frame rail in front of the front/auxiliary fuel tank. When the dashboard switch is in "MAIN" position, there is no 12-volt power going to the valve, and it is directing fuel from the rear/main tank. The switch also causes the fuel gauge to show the rear/main tank level.

Switch the dashboard switch to "AUX" and 12-volt power is directed to the selector valve causing it to shift and draw fuel from the front/auxiliary tank. The fuel gauge also reads the front/auxiliary tank level when in this position.

The original selector valve was a 3/8" valve. The replacement valves available now are 5/16" size, but I don't see that as a problem.

One of the common mistakes found on our GMC coaches is the addition of an inline fuel filter in the steel fuel line between the fuel pump and the carburetor. This is not the place to put a filter. The reason being that these filters are usually installed using short pieces of hose and hose clamps. If a leak develops at this connection, there is liquid gasoline squirting all over the top of a hot engine.

The fuel line from the fuel pump to the carburetor should be all steel.

Originally there is a filter sock inside each fuel tank and a filter in the carburetor fuel inlet. If you want to install another filter it should be installed on the suction side of the fuel pump. This could be in the rubber hose that feeds the fuel pump at the front right corner of the engine. Another place could be at the tank selector valve outlet.

Fuel filters should not be pressurized.
(Remember this if you install an electric fuel pump)

If you experience what seems like vapor lock once in a while, and not necessarily when it is hot, a check of the filter that is in the carburetor fuel inlet is suggested. Some of these filters had a check valve inside them. I removed this little valve and my symptoms of vapor lock went away.

Lots of owners have installed electric fuel pumps in their coaches. The primary reason is to cure the problem of "vapor lock". The original equipment mechanical fuel pump is a diaphragm pump and works fine most of the time. But if the temperature in and around the mechanical fuel pump gets too high, the fuel within can vaporize to the gaseous state. Liquid cannot be expanded or compressed, so diaphragm pumps work well, but gases can be expanded or compressed, and diaphragm pumps become useless.

Electric pumps are installed close to the fuel tanks away from most of the heat sources, so vaporized fuel doesn't become a problem to their pumping ability. Electric fuel pumps are not diaphragm pumps.

The replacement fuel selector valve has a caution on the box and in the instructions warning that they should not be subjected to any pressure. Therefore an electric fuel pump should be installed downstream of the tank selector valve. Any electric fuel pump installation that I have seen on a GMC coach was not installed that way.

Regardless of where you install an electric fuel pump, it should include a safety cut-off switch that is actuated by engine oil pressure. This feature will shut down the electric fuel pump if there was a crash. As soon as the engine stops, the electric pump stops preventing fuel from being pumped and avoiding a fire hazard.

Schematic sketch and pressure switch number are available from me if you want a copy.

Again, rubber hoses should not be used on the pressure side of the electric fuel pump. Use steel as much as possible. A short section of flexible line should be used between the frame and engine to allow for movement of the engine. The best type is a hose rated for gasoline and having crimped threaded fittings on each end. Avoid using hose clamps on pressure lines.

Like the tank selector valve, the main-auxiliary switch on the dashboard controls the fuel gauge on the dashboard. If the switch is on "MAIN", the gauge reads the level in the rear/main tank. If the switch is on "AUX", the gauge reads the level in the front/auxiliary tank.

If the fuel gauge reads full all the time, there is a break in the circuit somewhere from the gauge to the sender unit or from the sender unit to ground.

If it reads empty all the time, the wire from the gauge to the sending unit is grounded somewhere.
Gauges or sending units are not usually the problem in the fuel system, unlike the domestic water or holding tank systems.

The fuel filler cap is an important part of the complete fuel system. The cap is supposed to relieve any pressure over about 2 PSI and relieve any vacuum that is created as fuel is pumped from the tanks. So, just because the cap goes on the filler neck and turns does not mean it is correct.

The GMC parts book lists three different caps: 73-74 coaches, 75-78 coaches and the third for coaches with fuel evaporative emission system. GMC International lists Napa #7031049, Stant #BG807 and Stant #11807 as correct.

There are two vent hoses attached to the top of each tank. One of these is part of the gauge-sending unit. They are teed together and connect to the filler tube up by the gas cap. This is how the air in the tank escapes as you fill with fuel. When you are nearly full, some liquid comes up the line and kicks off the pump nozzle. If the nozzle is kicking off earlier than you think it should, check this vent system. It will help a little if the vent hoses and steel lines are routed as level as possible. If there are sags in the hoses, this leaves pockets for liquid to pool up and get pushed up to the filler tube and kick off the nozzle. It would be a good idea to slide under the coach and look at this vent system piping and hoses. It is common to find the sections of steel tubing rubbing on the steel coach frame especially where it passes between the coach floor and the steel coach frame. If the rubber body-mounting pads have moved out of place, the steel tube may even be squashed. These rubber body-mounting pads becoming displaced can also cause leaks in the air suspension plastic tubing.

The other vents are directly on the top of the tanks. They are also teed together in the left rear wheel well. The third connection on the tee goes to the liquid/vapor separator that is also in the wheel well. As you are filling up, vapors and liquid can transfer from one tank to the other through these hoses. The vapors can find their way to the carbon canister and be absorbed in it. As soon as liquid gets to the liquid/vapor separator it shuts off the flow to the carbon canister. Therefore if you leak liquid fuel at the carbon canister when filling up, the separator is the cause. When the engine is running, the vapors absorbed into the carbon canister are pulled into the carburetor and burned.
California certified coaches have two carbon canisters and all others have only one.

I have come across two different fuel supply systems for the generator.

The first is in the GMC built coaches and is a dip tube near the rear left corner of the rear/main tank. The dip tube does not reach to the bottom of the tank, so running the generator will not empty the tank. The generator will run out of fuel and shut down as soon as the fuel level drops to the bottom of the dip tube. If you are below 1/4 level in the rear/main tank, the generator might not run because the fuel level is too low.

The second system was on coaches outfitted by other builders such as "COACHMEN". This system simply put a tee in the fuel line between the rear/main tank and the tank selector valve. The generator will run until the rear/main tank is empty. Then you only have left the 7 to 9 gallons that are in the front/auxiliary tank.

The original ONAN generator used a fuel shut off solenoid valve and electric fuel pump. There is a fuel filter inside the bottom of the electric fuel pump. This is something to check if the generator will not run properly but has a fuel supply.

Gasoline leaks from the gas tank area are usually a hose problem. There is about 50 feet of various sizes of hose on the complete system. Both fuel tanks have to be removed to do this work. If you are going through all the trouble of removing the tanks to repair a leak, REPLACE ALL THE HOSES. Replacing only those that appear to need replacement is false economy. If you decide to tackle this job yourself, there are some very important safety issues to stress.

Do not use any extension light cord.
(A single drop of gasoline can break the light bulb and cause an explosion and/or fire. Have you ever experienced a drop of water hitting a hot light bulb?)

Try to have the fuel tanks as empty as possible before starting the job. Then disconnect the hoses from the selector valve one at a time, and allow them to drain the tanks into a suitable container. There will still be some fuel in the tanks when the flow stops. You will not get the remaining fuel out until the tanks are removed, so be cautious because of the weight of the tanks and remaining fuel. A floor jack can hold the tanks up while fasteners are removed. The floor jack will also give you some control while lowering the tanks.

All the time this draining procedure is underway make sure there is no source of ignition.
Gasoline vapors explode very violently.
You cannot be too careful with gasoline!!!

FUEL TANK HOSES (coaches built after serial # 5v100529)

 Rear tank liquid to selector valve top connection  3/8 x 76"
 Front tank liquid to selector valve bottom connection  3/8 x 44"
 Rear tank vent to steel tube  3/8 x 28"
 Front tank vent to steel tube  3/8 x 28"
 Rear tank fuel to generator  1/4 x 96"
 Liquid separator top connection to 1/4 x 5/16 adapter  1/4 x 28"
 1/4 x 5/16 adapter to steel line to carbon canister near front wheel  5/16 x 120"
 Rear tank vent to tee by the liquid separator  5/16 x 32"
 Front tank vent to tee by the liquid separator  5/16 x 108"
 Tee to liquid separator bottom connection  5/16 x 15"
 Fuel selector valve to steel line to fuel pump by the front wheel  3/8 x 64"
 Steel line to the fuel pump  3/8 x 18"
 Filter socks GM P/N 5651711  $13 US
 Sender "O" ring GM p/n 22515965  $4 US
 Hose-- be sure to specify gasoline service  $45 US

Checking Ride Height

If you have not checked your front and rear ride height in a couple of years, you should do so. I checked mine last year and found that the front was riding about 2" low. After adjusting it back to specs, the GMC handled much better on the highway. These adjustments can change by themselves over time. The front torsion bar adjusters can move or the torsion bars can weaken. The rear height adjustment is done by adjusting the air bag leveler links. If either the front or the rear adjustment changes, it will also affect the height of other end of the motorhome.

If you want to do this yourself, there is an excellent write-up on the GMC Western States web site, Just click on Tech Info on the main page and then look on the left side of the Tech page. This was written by our President Claude Brousson and presented at our Palm Desert Roundup in March 2001. It is called Ride Height Adjustment / Rear Wheel Alignment.

If you need any instructions as to how to download this paper from our web page, let me or our Webmaster Billy Massey know, and we will guide you.

If you don't have access to the Internet, write or call me, and I will send you a copy of this very informative booklet. Even if you don't do your own work, this would be an excellent guide for your mechanic.

 This material is based on my personal experience and the personal experience of other club members. It is our viewpoint and does not represent authorized data pertaining to the GMC Motorhome. It is the responsibility of the readers to make their own judgment as to the validity of this material in relation to any repairs and/or modifications to their own vehicles.

 3128 Vista Sandia, Santa Fe, NM  87501-8526

 (505) 989-8157

 Please send your comments and ideas for the Tech Center to:
Emery Stora, Technical Vice President,
3128 Vista Sandia, Santa Fe, NM  87501-8526
Phone: (505) 989-8157



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