GMC Western States

 Tech Center Number 31 - December 1, 2000

By Chuck Aulgur

From Under The Shade Tree
At Little Diamond Resort

The last rally of year 2000 ended on a high note with several of our talented members sharing their knowledge and experiences in some excellent technical seminars.
Bill Harvey, Frank Condos and Bob Cook talked about their experiences with installing Electronic Fuel Injection (EFI) on their coaches and how the various systems worked. With our 20-some year old Rochester Quadrajet Carburetors slowly wearing out, and new ones no longer available, EFI is a subject of interest to most everyone as it may be our only option to maintain optimum performance with our Classic GMCs. EFI was developed and refined by the automotive industry over the last couple of decades to help lower engine emissions and improve fuel economy. Several manufacturers are now providing after-market EFI systems that are designed for our GMC engines. The experiences relayed in this seminar indicate that if theses systems are installed properly, our engines can perform better than they did when they came from the factory. EFI can compensate for changes in altitude along with providing a precise metering of fuel over a wide range of operating conditions.

Jim Bounds gave a very informative presentation on how to renovate both the interior and exterior of our coaches. He brought along numerous samples of new materials that are now available and showed how they could be utilized to improve our coaches.

Darren Paget brought samples of the aluminum overhead cabinets that he is manufacturing specifically for our GMCs. He indicated they could save a lot of weight and can also be painted or covered with about any type of material you want to match your interior.

Frank Condos and Egon Elssner showed photos and explained how they constructed their respective systems for removing and installing their engine from the topside. They were both similar in design and function and could easily be made using standard steel angles and beams available from most any metal salvage yard.

Dallas Jensen entertained everyone with his many tales about his travels with his two "Electronic Trip Routing/Navigation" companions that use GPS to guide him on his travels. They don't always agree, but he usually gets to where he is going.

Duane Simmons was not able to attend the rally, so he gave me permission to present his material on "What Every New GMC Owner Should Know". This is a session that is always well attended by both new and seasoned GMC owners, as there is always something new to learn.

Denny Allen brought along an excellent prop that showed the arrangement of the two fuel tanks and how they are interconnected. By moving the prop, he was able to show how fuel travels between the tanks, and the resulting effects it has on the fuel tank gauges when going up or down long hills. He was also able to show how the fuel tank reserve functions.

The last tech session, where a panel of "experts" answered written question from the audience, turned out to be again one of the most informative seminars of the rally. It is a good chance for people to get all their questions answered, and it results in a wide variety of subjects being covered. I've had several people state they would like to see this type of session continued at all future rallies.

This rally ended my tour of duty as your Tech V P and I would like to take this opportunity to thank all of the members that helped me put on the tech seminars and/or supply technical information for the newsletters during this last year. I'm sure you will find Emery Stora our new Tech VP to be an excellent replacement.

From the Newsletter Editors

Special Thanks to Chuck Aulgur
As editors we have especially appreciated the fine submissions from Chuck Aulgur that have made up the Tech Centers of the recent GMC Western States newsletters. His grasp of technical matters as well as his professional writing style, which is easy to read and understand, made our job very easy. We also came to admire him as a person as a result of his contributions to this newsletter. Thank you Chuck for a job well done.

By Emery Stora

A Word From Our New Tech VP

I want to thank all of the Western States members for asking me to serve as your newest Technical Vice President. I hope that I can live up to the high standards set by Chuck Aulgur and his predecessors.
We are now in our ninth year of publishing Tech Center Bulletins. For those of you who have com-puter access you can retrieve past issues from I am working on an index of topics covered in past issues and will publish it in the next Tech Center. I would encourage you to send in your suggestions for topics that you want to see covered in future issues. If you have any questions about your GMC or its operation, send them in, and we will try to get the answers for you.

By Frank Condos

Comparing Brake Modifications
Part 2

In the last issue of the Tech Center, I discussed brake system modifications that have been made to increase the braking force applied to the road. These modifications increase braking force by changes that increase the size of calipers and wheel cylinders or by converting the rear drums to disks. In this issue I will discuss some modifications that can increase braking force by increasing the brake line pressure and hence the force applied to the pads or shoes. Line pressure, measured in pound per square inch (psi), is a function of the force applied to the master cylinder divided by the piston area in square inches. Increasing the hydraulic pressure in the brake line will provide a proportional increase in braking force at all wheels.

With the stock vacuum booster, master cylinder piston force is produced by the difference between atmospheric pressure and engine vacuum acting over the booster dual diaphragms. With the vacuum booster the master cylinder is capable of producing a line pressure on the order of 800-900 psi. A sensitized or handicapped booster does not increase the pressure and resulting braking force. It simply makes the application easier by reducing full application foot force of around 90lbs down to about 25 lbs.

At first glance one means of increasing line pressure might be to reduce the size of the master cylinder piston from 1.25 in to 1.125 in. While this size reduction increases the line pressure by 20%, the fluid volume for a normal stroke is reduced by the same 20%. A longer stroke would be required to provide the same fluid volume available to move the calipers and shoes. Remember, smaller is the key. Larger master cylinders such as the P-30 chassis units will decrease line pressure in our application. The fluid volume selected by vehicle designers includes a margin of safety to account for worn shoes not adjusted to full drum contact. Reducing the piston size at best reduces this margin. Increasing the volume could require a change to the brake pedal linkage and a selection of a long stroke master cylinder or possibly one of the GM quick fill units.

Other possible modifications have been offered that increase line pressure. These include replacing the vacuum booster with the Hydro-Boost or Powermaster boosters used by General Motors on past or current production vehicles. A Hydro-Boost uses power steering pump pressure to activate a piston that operates the stock master cylinder. The Hydro-Boost has been designed to operate with the larger trucks and is compatible with bigger master cylinders and therefore capable of generating higher pressure within the motorhome.

The Hydro-Boost installation replaces the vacuum booster but requires minor modifications. Parts for late model trucks can be installed to fit within the engine compartment cover. This system has been used on some 1978 and later Cadillac's but more importantly is currently used on ¾ ton and up trucks. When applied to the 1.25 in. master cylinder, it should be capable of producing around 1400 psi.

The other conversion uses the Powermaster power brake system. The Powermaster unit is a complete system with an integral master cylinder, electrical motor driven hydraulic pump, accumulator and booster fluid reservoir. The Powermaster unit was originally used on some four-cylinder GM cars for a few years during the eighties but was plagued with electrical failures. It is reported that aftermarket units can be modified to eliminate early failure modes. The unit is reported to be capable of generating higher line pressures, up to 1700 psi, with the smaller diameter master cylinder piston. The volume reserve is not known.

Are there any safety considerations with these conversions? All production based booster systems are required by the US DOT to provide at least two reserve applications without power. This reserve is built in as an accumulator in the hydraulic based systems or excess capacity in the case of the vacuum booster. In that sense, all systems are fail-safe and meet current standards. The Hydro-Boost requires the engine to be running and the steering pump to be operating. Note that there are two belts engaging the pump. The Powermaster requires electrical power and is susceptible to blown fuses or, if a circuit breaker is used, to electrical shorts. The vacuum booster system won't work after the residual vacuum is used up after the engine stalls. To remedy this situation some owners have provided an effective back up to the vacuum booster by installing the electrical vacuum booster pump found on early GM J cars (mostly four cylinder cars from the eighties).

In summary, greater braking force can be achieved by:

* Increasing force at the wheels with larger calipers, wheel cylinders, or substituting disk brakes in the rear for the stock drums.

* Increasing brake line pressure by master cylinder changes or replacement of the vacuum booster with one of the hydraulic units.

* Increasing the coefficient of friction by using carbon-metallic pads or shoes or emergency vehicle asbestos shoes.

When considering any brake system modification, remember that braking force front to rear balance is important. Shifting more braking to the front wheels by only installing larger calipers will increase heating of the front knuckles and wheel bearings under heavy braking. Shifting more to the rear can result in rear wheel lock-up under severe braking or slick roads with possible loss of control. One other consideration, if you do not do your own work, is that you may have problems finding shops willing to make safety related modifications or work on modified systems.

There are a number of maintenance related items that apply regardless of system modifications to assure optimum performance. These include:

* Periodic adjustment of the drum brakes manually, not relying on the auto adjustment.

* Use at least DOT3 high boiling temperature or DOT 4 brake fluid to minimize fading. Some owners prefer DOT 5 silicone fluid but only after a complete flush and rebuild.

* Flush the brake system every two years when using DOT 3 or DOT 4.

* Check and maintaining the rear parking brake cables so that they are free.

By Emery Stora

Some Topics of Interest

Winterizing the Water System
Winter is here and for those of you living in the northern climates, you are again faced with winterizing your water system. (Those living where it never freezes can ignore this section). The owner's manual shows you where the water line drains are and how to drain the water tank and the water heater. Many pull out drawers and panels and open these valves and then attempt to blow out the lines with compressed air. Sometimes this is not successful and lines still freeze and break.

There is a better way to do it. First let me suggest a couple of products that should help greatly. Get a hot water heater bypass valve kit. Camping World has one, part number 15717 for $19.99. They call it a Quick Turn Permanent Water Heater By-Pass Kit Featuring a Single Valve Operation. This kit fits on the face of the water heater in the sink cabinet. I have a side wet bath model. I drilled a hole in the handle of the bypass valve and fastened a push/pull wire of about 1/8" diameter so that I could open the right hand sliding door and turn the bypass on and off without taking out the cabinet. This kit comes with a check valve that is installed into the heater output (top). This check valve also will prevent you from getting hot water out of your cold water faucets that some GMCers have complained about.

The second part you should buy is a Permanent Pump Converter that lets your pump draw RV antifreeze directly from the bottle (Camping World, 6279 $13.98). Both of the items I have listed are also available at J.C. Whitney and at several RV parts shops.

Once you have these parts installed, when you are ready to winterize, it is a 10-minute operation to prepare your GMC for storage in cold winter climates.

1. Open the water tank drain.
2. Rotate antifreeze winterizing valve at pump, remove cap and attach hose.
3. Put hose into the antifreeze jug (use RV waterline antifreeze available from Wal-mart, K-Mart or almost any RV store). The bottle should show that it is safe for drinking water contact and that it is made of propylene glycol. NEVER use automotive antifreeze, ethylene glycol. It is toxic.
4. Kitchen - if you have a filter, remove it and close bypass valve.
5. Bathroom - close water heater bypass valve. Open drain on bottom of water heater. Open lever of temperature/pressure release valve to allow air to enter hot water tank.
6. Turn on water pump.
7. Open bathroom cold water faucet until you get pink antifreeze out of it. Close faucet.
8. Open bathroom hot water faucet until you get pink antifreeze. Close it.
9. Flush toilet until you see pink.
10. Turn on shower while running water into toilet. Run till pink and close it.
11. Open kitchen cold water faucet until it runs pink. Close it.
12. Open kitchen hot water faucet until it runs pink. Close it.
13. Take hose out of antifreeze jug. Remove hose from valve and put cap on valve.
14. Pour a little bit of antifreeze into the shower drain to freeze proof the trap.
15. Disconnect electrical wire from pump to prevent it from running when tank is empty.

You are done. I often do this several times each winter season as we use our GMC year around. I use less than one gallon of RV antifreeze each time. When ready to use your GMC, just fill your water tank, hook up the electrical wire to the pump, open the water heater bypass, close the water tank and hot water drain valves, close the temp/pressure release valve, and then pump water into your lines until they no longer show any pink color.

Radiator Antifreeze and Cap
While on the topic of winterizing it is also a good time to check your radiator coolant. You should drain and replace it every two years with a good quality antifreeze/coolant. Even if you live in a warm weather climate, it is important to service this every two years. The coolant contains not only antifreeze but also corrosion inhibitors, antifoam agents, rubber swell additives for seals and hoses and water pump lubricant. The antifreeze itself will not wear out, but these additives will. Refill with 50% antifreeze and 50% water. This will protect you for freezing down to -34 degrees F, but just as important, and it will raise the boiling point for summer boil out protection.

Plan to replace the radiator cap at least every two years. Some don't last that long. At the Albuquerque Balloon Fiesta Rally in October 1999, I had a pressure tester and tested approximately twenty caps for GMC members. Eighteen of them were bad and no longer held pressure. Some were only a year old. By pressurizing the system it will raise the boiling point even more and prevent loss of coolant by overflow. The GMC uses a 9 lb cap. Most auto parts stores do not stock it. It is an AC/Delco type RC32, part number 6410619. If you can't find this one you can usually find a 7 lb or 10 lb Stant cap at auto parts stores. Do not use a higher pressure one as the tube design of the GMC radiator will cause the tubes to swell more with higher pressure, and the constant flexing of the tubes with heating and cooling at higher pressures will result in a leaky radiator core.

Larger Radiator Core
Gene Fisher reports that he needed to recore his radiator. He had gotten some information from Chuck Aulgur that there was a core available that would fit the standard side tanks of the GMC radiator. This core has 53 rows of tubes instead of the stock 42 rows. It has the same 14 fins per inch and the same tube size as the OEM core. The tubes are just a little closer together. Chuck says it will give around 20% more cooling capacity then the OEM core. He said that with this larger core, he could not force engine temperature above 195 degrees F (when the fan clutch kicks in) even when towing his Tracker up a 7 % grade under full throttle with an outside temperature of 110 degrees F and with the A/C running. Gene spoke with a radiator shop and they told him that 14 fins per inch would restrict the flow to the rear tube. He told them that 14 fins per inch was the GMC stock configuration. They argued a bit and then looked it up and found that 14 fins was indeed the stock configuration. Then they said the 53 tubes would restrict the airflow and Gene should go with 10 fins per inch. He insisted on 14 fins per inch and just wanted to know what the cost difference would be. The cost was more than he thought, and he feels that others might get a better deal if they shop around but this is what he came up with:
10 fins per inch 42 tubes (what Gene had) $360
14 fins per inch 42 tubes (stock) $380
14 fins per inch 53 tubes $440
Gene had his done at Cat's Radiator in Northern California.

Wall To Ceiling Molding
With age several motorhomes are experiencing cracking and discoloring of this strip piece that covers the gap between the ceiling and wall panels. Replacement trim is no longer available. Some have fashioned wood molding trim to replace it. GMCer Dick Missett has reported that he purchased 4" rubber cove base cabinet molding at a flooring store. It is available in several colors. He indicates that the backside looks just like the GMC molding but it only has one radius (top or bottom - depending on how you decide to install it. You glue it down over your old molding pieces. It comes in 4' and 20' pieces.

 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



 Tech Info