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Letter 31

Isuzu Trek Owners Infoletter #31

1994 2400- 119,230 miles, Bruce Matlack , (matlackwindsurfing at Remember to replace “at” with @ in email address.

 Water Heater. I fixed it! I solve more problems on this rig by banging on things! I banged in and around the valve assembly and lo and behold, a bunch of rust flakes started flowing profusely out the air opening at the beginning of the tube. I banged (tapped actually) until no more appeared and then ran a tiny vacuum hose in there. Nothing more came out. It started up on the first try. Rust flakes had interrupted a good flame pattern at the burner, so the burner shut down when the flame was insufficient to heat the thermocouple and keep the gas valve open.

Inverter. AC current from the inverter is back to not working whenever it is damp outside or it has just rained. I have spurts of 15 second use so I can only move the bed up and down in short segments. So far I haven’t had to figure out how to drive the rig from the prone position on the bed! Editor note: I had a similar problem; my fix was written up in Infoletter # 22.

 Tire Install. A tire belt had broken in one of my front tires, so I took it in to get two new Uniroyals installed. After about 2,000 miles on the new tires I noticed a shimmy. I should have checked the pressure! They had only put 50 pounds of air in them. It says right on the side wall full inflation pressure is 80 pounds; a truck /RV place should know it needs more pressure. About 4,000 miles later, I was filling the jack system reservoir from the front tire area and noticed one of the lug nuts was half way off! Checking all lugs nuts on both front wheels, I found another one only finger tight. In the end, four of them were never torqued properly after replacing the tire.

Editor note; to get the ideal pressure for your tires you need to weigh your coach on truck scales and get the front and back axle weights. Then go to the tire manufacturer data and set pressures to get the best ride. (Most people add an extra 5 to 10 psi to get some margin.)

 Alternator Belts. Belts can be cut to pieces if they run too loose and hit sharp metal nearby. Mine had loosened and I figure they thrashed themselves on a protruding piece of metal. Solution: check for proper tension every oil check.

Jack system: My left supply line sprang a leak due to chaffing. I found it was not properly insulated from the chassis in the first place. Then I managed to try to drive off with the front jack down, bending it enough to cause it to start sticking during retraction. Later, I was in an RV park with it fully extended and another party scheduled to take my space in a few hours when I finally managed to bent it to a 30 degree angle. HELP! Park employees mustered up quickly and removed the jack assembly from the Trek so I could get down off my perch and move to another space before the next tenant showed up. (This is why I am not a pilot… driving off in a Isuzu Trek with the “gear down” is like landing an airplane with the “gear up“. I think it is something you only do once in one’s career.)

The RVA company in Escondido, CA. made good on delivery of a new jack assembly for about $600. Another $500 in labor from a local hydraulic shop and I was back in business with three operating jacks again. I was careful to confirm the order when the RVA owner wrote it out.

Bumper Repairs

Keith & Jody, (kjredfern at

We knew this year that we would have to remove & repair both of our bumpers. The rear one had a frame bracket rusted through and broken where it is laminated to the fiberglass bumper. We had 2 “L” brackets made to re-mount it & drilled through the outside of the bumper & attached them to the inside surface. The ends of the both bumpers were loose because the wood blocks were shot. Seems like a poor design that is bound to fail!

We decided to try to improve the situation. Where the screws were run through the body to screw into a block of wood that was screwed to the inside of the bumper, we drilled 4 – 1/4″ vertical holes. Then we put the bumper on & took an artist brush with black paint on it & put it through the holes to mark the inside of the bumper. We numbered each hole & used a depth gauge to measure the depth of each hole & recorded it. Then we removed the bumper & cut pieces of a rubber/poly material that holds screws well to the thicknesses that were recorded. We drilled 1/4″ holes in them & screwed them to the bumper in the marked spot for the correct thickness. We put the bumper back on & ran screws into the blocks through the holes we drilled in the body. Results: Great fit & tight as could be!

Thermostat & overheat, Vacuum leak at controls, Glow plugs.

Ken Harmon, (IsuzuTrek at

 Thermostat & Overheat.

Departing NM in the mid-summer heat I noticed the engine temperature was climbing higher than normal going up hills and then dropping back to normal, normal being just below mid-range on the gauge. Then I noticed while running straight and level the temperature would occasionally climb to near the high mark and then drop back to normal. We had not seen this before. Historically, the coach always had a situation where the temperature would climb quickly to near the hot mark and then return back to normal when we had to climb a long grade right out of camp in the morning.

By the time we got to the Midwest with very hot weather, the straight and level heat problem seemed to be getting worse. I took the coach to an Isuzu dealer in Wisconsin and had the thermostat changed. I kept the old thermostat and did a test by heating it in a pan of water up through 180 degrees. The thermostat opened and seemed to be working okay. The test drive after the thermostat installation seemed to show it worked just like the old thermostat.

Trying to diagnose the problem, I purchased an inexpensive universal water temperature gauge and clamped the temperature bulb to the casting just above the thermostat housing. With this setup I was able to drive the coach and watch the engine water temperature come up on the dash gage and also see the temperature come up on the test gage as the engine got hotter and the thermostat opened, sending coolant to the radiator. What I found was the thermostat was slow to open, allowing the engine (dash) temperature to climb above normal before starting back to mid-range. On some occasions, as the temperature was dropping, it would actually undershoot the normal range and then come back and stabilize at normal mid-range.

As time went on the thermostat seemed to “break in” and capture and hold temperatures better. Now the cooling system seems to work like it did before this problem started. I think my old thermostat was degrading after 18 years of service and it is just the nature of the Isuzu thermostats to be a little slow reacting to heat changes.

 Vacuum Leaks at engine controls.

On my Trek the small vacuum control hoses have failed three separate times during our 250,000 miles of travel. I have had very poor luck with standard vacuum hoses purchased from auto parts stores. Some of the replacement hoses seem to be made of poor quality materials that will start showing signs of deterioration in less than one year when exposed to engine compartment temperatures. I have had better luck with NAPA Vacuum Tubing H452 made by Gates in Denver, but I still had a problems on our last trip. Since it is so hard to replace the hoses on the road I decided to replace mine one more time when we got home. The best hose I could find was the more expensive NAPA Silicone Vacuum Hose, 5/32″ (4mm), part number H467. I hope they will give me a longer service life.

Troubleshooting information: The first thing you may notice is it will take longer and longer to get the vacuum warning buzzer to go off as the engine is warming up. Also, if the vacuum buzzer and light come on quickly as you turn the engine off by moving the key from the run position to the accessory position you may have a leak. Another clue your system has a serious leak is when the vacuum warning light and buzzer come on when you apply the brakes.

A quick way to detect a vacuum system with a significant leak is to run the engine with the engine access cover removed. Shut the engine off and immediately put your ear down near the right side of the engine. If you are in an area with very little background noise you may be able to hear the hissing sound of a vacuum leak. If you hear a leak the next step is to visually inspect the small vacuum hoses associated with the engine controls.

The vacuum components referred to here are located on the right side of the engine and are outboard of the injector pump. The components are mounted on a plate that is attached to the inboard side of the frame rail. The four components will be described based on viewing them from the top and in sequence starting from the front and going aft. The components on my Trek can be identified by their color. All of the tube connections are made below the components except the last (red) one. This document will attempt to describe how the vacuum hoses are connected to the various components (as seen on my coach).

-The top of the first component has a square green block on top and is called the Fast Idle Vacuum Switching Valve in my service manual. On the bottom of this device there are two small horizontal tubes (nipples) designed to slide the small vacuum hose over to make the connection. One connection is located above the other with the open end of both tubes facing inboard (toward the engine).

-The second component moving aft has a white square block visible from the top and is described as the VSV (Vacuum Switching Valve). On the bottom of this unit there are two small horizontal tubes, one above the other, with the open ends facing inboard.

-The third unit is round and looks black from the top and is called the VRV (Vacuum Regulating Valve). Down below, it has two small tubes that face straight down; one tube is inboard (closer to the engine) while the other is outboard (closer to the frame).

-The fourth component is called the Delay Valve and is round with a red top. This device has one tube connection on the top and one tube connection on the bottom.

Now the vacuum hose routing: The vacuum source for all of the components comes from a single hose that crosses ahead of the radiator and turns aft near the lower right side of the radiator. In the area of the vacuum valves there is a “T” in the hose that allows the vacuum supply to split to two short hose. One hose goes to the lower tube connection on the green Fast Idle valve and the other hose to the lower tube connection on the white VSV valve.

The next hose connects the remaining (upper) connection on the green Fast Idle unit to the idle positioner near the top of the injector pump. The remaining tube connection on the white VSV unit has a hose that connects it to the inboard side of the round black VRV unit. The next hose connects the outboard side of the black VRV unit to the bottom of the round red Delay Valve. Another hose connects from the top of the red unit to the aft end of the injector pump. This is the maximum fuel flow limiter that reduces fuel flow 5 % when the engine is hot.

Before starting to replace hoses on your Trek it would be a good idea to check and record your hose routing (it may not be the same as described above).

 Glow plugs

At lower altitudes and with around 250,000 miles on the engine, the start-up and warm-up was getting rougher than I thought it should be. A few years ago I tried to remove the glow plugs but they were in so tight I was afraid I would damage them. After this summer’s trip I was determined to get the glow plugs out so I applied Liquid Wrench, let it soak for two days, and the plugs came right out. Initial inspection seemed to show the stainless steel tips were distorted and seemed to be covered with a foreign substance. A search for replacement plugs by part number on the web indicated they are obsolete and I could not find a substitute number.

Examining the glow plugs more closely I discovered there was a very hard “rock”- like tan deposit that made the tips look distorted. Using a stainless steel scraper along with a lot of concentrated pressure I was able to gradually chip the “rock” material off. After sanding the last of the material off the tips they looked straight and serviceable.

I tested each glow plug using test leads connected to the individual glow plugs. One of the leads had a 0-20 ammeter so I could see the amp draw. By touching the wires momentarily to a 12 volt car battery I could record the amps – all the plug tested okay with a 10 amp draw. I reinstalled the plugs using anti-seize compound and now I think the engine starts better and has a smoother idle during warm-up.

While I had the glow plugs out I decided it was time for a compression test. Harbor Freight had a diesel compression tester on sale for under $30 that would do the job (check to see the little valves in the end of each glow plug adapters are in tight). Compression range was 440 to 460 psi, good according to the service manual.