Avco fuel system repairs & Mods - Fiberglass RV

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Old 06-23-2013, 03:08 PM   #1
Junior Member
Name: Les
Trailer: 1977 Avco 29' Grand Lodge
Posts: 15
Avco fuel system repairs & Mods

Hello all,

I know that this site primarily focuses on fiberglass trailers, but there are few places for owners of similarly constructed RVs to share similar information, so I hope you don't mind us dropping in.

I posted about this time last year concerning our fuel system problems and how we (thought) we fixed them, but problems continued to rear up, so this spring I dove back into the fuel system again.

We discovered that the fuel system wasn't as fixed as we thought it was when the engine started starving for fuel after just a few hundred feet of hill climbing last fall. Since we weren't planning on driving it again until spring, we decided to wait for better weather for repairs.

Last weekend I dropped the main and auxiliary tanks and checked them out. I drained the last couple of gallons of fuel out of the 50g main tank and got way more rust than I was expecting, considering that I cleaned the tank just last year in Boise.

Before we go any farther, let me say that I am a skilled mechanic with several years of heavy equipment repair experience in the Army and another eight years of private-sector heavy-duty truck repair (semis, dump trucks, etc.) which includes stuff like ad-hoc fabrication and field-expedient repairs. So later when you read about me cutting the top off a fuel tank and welding it back on, you need to understand that I know what I'm doing. I know how to minimize risk and not put myself in dangerous situations. There _are_ specific techniques that make these kinds of repairs much safer to do. If you don't know what the are, leave the work to a professional.

I knew that I had most of the rust out of the main tank, so I started with it. I ran a complete POR15 treatment on it. If you're seeing any rust in your fuel, spend the money and do the POR15 treatment. I spent about $200 at Napa for the chemicals to do both tanks. Even if you're paying your local shop to do it, I promise it'll be cheaper than breaking down on the open road.

2 quarts Marine Clean
2 quarts Prep & Ready
1 quart POR15 tank liner

I drained and dropped the tank. There was only a couple of gallons left in it -ran most of it through the generator last fall. I supported the 50gallon tank with a large jack and loosened and unhooked the straps from the frame. I dropped the tank about 6 inches and disconnected the sending unit and fuel lines from the top and dropped it to the ground.

I drilled a hole in the right rear corner of the tank and tapped it for 1/4 in NPT so I could easily drain the chemicals and rinse the tank for the procedure. Later, I screwed in a 1/4 brass plug. I may swap this for a petcock later so I can completely drain the tank when we park long term. This will prevent future bad-gas related issues from happening.

At this point, I just followed the instructions on the POR15 Tank Liner can: Mix the Marine Clean 1:1 with clean, hot water and pour it in the tank, rotating the tank every two hours until the tank walls are clean. Note that this does NOT remove the rust, it removes the petroleum-based crud from the tank walls and baffles. Twelve hours later (six sides) I drained the tank into and oil pan and rinsed the tank. I was suitable shocked to get about four inches of rust in the bottom of the drain pan. Most of it was the consistency of powdered sugar, but there was a lot that was larger, too.

When no more rust was coming out and I could roll the tank around and still get clear water when I drained it, I stuck a 1500 watt heat gun in the fill tube and ran it until the tank was dry.

Quick hint: I set the tank at an angle on the ground, with the drilled hole at the lowest spot. When I couldn't get a q-tip wet at the hole, I was pretty certain it was dry inside. I confirmed this with a small flashlight and looking in through the sending-unit hole.

I followed this up with two quarts of POR15 Ready Prep. I just followed the instructions. I was very concerned part way through this step, as it didn't seem to be removing all the rust. I just kept rotating the tank, and a couple days later it looked good. At this point I repeated the rinse from the previous step.

When the tank was dry, I put aluminum tape over all of the holes except the sending unit hole and poured in the POR15 Tank Sealer. You REALLY have to stir the crap out of this stuff to get all the aluminum powder suspended. I rolled the tank around to coat all the sides. you can see where it's going by looking through the sending unit hole and using a small flashlight. I coated the top of the tank by covering the sending unit hole with tape and rolling the tank so it was upside down.

Everything was pretty easy except for the baffles. It took some creative rolling, but I finally achieved about a 98% coverage of the inside of the tank. I might have hit 100% using two quarts, but I was happy with the results. At this point I pulled off all the aluminum tape and pulled the drain plug so air could circulate.

Let this cure for the FULL FOUR DAYS. Out of curiosity, I stuck the stick I stirred the POR15 with in a bit of gas on the third day and it softened the POR15 up a bit.

I don't have any pictures of this, but since it all happened on the inside of a dark gas tank, I don't think it matters much.

While I was waiting for the tank liner to cure, I crawled under the Avco and removed the factory fuel lines, along with the sixty feet of rubber hose that some previous owner had installed at some point. The rubber line had been there long enough to dry rot, so out it went.

I went through a similar procedure with the auxiliary tank, but the rust was so bad that I elected to cut the top off the tank with a grinder to remove most of the rust by hand.

With the loose rust and most of the scale removed, I also elected to modify the tank a bit. In its original configuration, the tank was 'L' shaped - there was a square chunk missing from one corner, like the tank had been designed to fit around something. Oddly, there wasn't anything on the frame that required the clearance notch on the tank, so I grabbed some scrap steel and re-worked the tank.

This brought it's capacity up from about 30 gallons to 35. Not as big of an improvement as I'd hoped, but it'll do for now. It could hold as much as 40 gallons, but the fill tube is attached in a less-than-optimal manner. In fact, it's not even vented and if I fill it 'full', fuel sloshes out the tube when driving down the road. I will fix this in the future.

Since I had removed the fuel lines to eliminate any chance of more rust problems, we obviously had to replace them with something.

This is where the 'modification' part comes in. The original fuel lines consisted of a 5/16" supply line with a slightly smaller return line. Given that a Dodge 440 is a fuel-hungry beast (or any other big-block, for that matter), I upped the ante and installed a 3/8' zinc coated steel fuel supply line. I picked it up for about $20 from JEGS.COM. Our RV has a mechanical fuel pump installed on the engine, as most do; but it also has an electric pump about halfway between the engine and the tank, just forward of the tank selector valve. To make sure that we don't cause an over-fueling problem by forcing too much fuel into the carburetor, I installed a Holley 12-803 non-bypass fuel regulator. Since it's a non-bypass, it doesn't return fuel to the tank, so no return line was needed.

If you do a similar modification, you CAN use the -bp model of the regulator and connect it to the factory return line. It shouldn't affect how the system works at all.

I installed an inline fuel pressure gauge at the carb so I could accurately set the regulator pressure and check it if the engine seems to be short on power. I may install an 'isolator' later so I can mount the pressure gauge on the dash. The isolator allows the pressure gauge to be safely mounted in the passenger compartment without the risk of leaking fuel inside the vehicle.

Between these repairs and some general tune-up work, we increased our mileage from 6mpg without towing, 3mpg towing to 6mpg towing the same load. We haven't driven it without towing since the repair, but I'm hoping for some good numbers.

Have a great day!

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Old 06-23-2013, 03:41 PM   #2
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Steve Outlaw's Avatar
Trailer: Oliver Legacy Elite II
Posts: 590
Very impressive work. I find it very hard to weld up a tank so there is not a tiny pinhole leak somewhere. I repaired an old air tank once and never got one place to quit leaking. It will loose about 80-100 lb over a couple of weeks.

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Old 06-23-2013, 05:54 PM   #3
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cpaharley2008's Avatar
Name: jim
Trailer: 2019 2ndG Escape21 DeJa View pulled by 2014 Dodge Ram Hemi Sport
British Columbia
Posts: 6,510
Glad it worked out for you. Interesting reading...
Never in doubt, often wrong
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Old 06-24-2013, 11:00 AM   #4
Junior Member
Name: Les
Trailer: 1977 Avco 29' Grand Lodge
Posts: 15
Originally Posted by Steve Outlaw View Post
Very impressive work. I find it very hard to weld up a tank so there is not a tiny pinhole leak somewhere. I repaired an old air tank once and never got one place to quit leaking. It will loose about 80-100 lb over a couple of weeks.
thanks! my welds aren't the nicest looking, but they work.

Absolutely correct. I had three pinholes, but they were all in the weld at the top of the modified section. I leak tested the tank and touched up the welds. it still leaked slightly in two spots, but they were sealed up by the POR15 tank liner.
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