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  Chicagoland MG Club:Tech Tips

MG Fuel Smell Diagnosis

There can't be any of us that really like the smell of gasoline, especially inside a closed automobile. I was just about ready to sell my daughter Wendy's '71 MGB GT because I couldn't stand that intermittent smell. I just couldn't find any of the "normal" reasons for gasoline fumes being in the cockpit, not to mention escaping into our atmosphere, instead of staying in the fuel system where they belonged. Yes, I had checked the float level and replaced the needle and seat in the carb float lids, the normal cause of flooding and attendant fuel vapors. That wasn't it. I checked every hose, component, and connection on the fuel system and the emission control systems. All were tight and showed no signs of leaks. Next I checked the gas tank. It had no visible signs of leaks. Wendy's MG had one tight fuel system... except when I could smell gas!

I turned to the shop manual and proceeded to run their "evaporative loss control" test for the closed fuel system that all MGBs have had from 1970 onward.

I still suspected a leak somewhere back in the gas tank/vapor separator area as I could find nothing wrong in the engine bay. The shop manual says to apply no more than 1 psi of air pressure to the vapor pipe at the canister end, to test that there are no leaks in the system. The air gauge should show a drop of no more than .5 psi in 10 seconds, according to the manual for the system to be okay.

My test, using a hand-held vacuum/pressure pump showed no leak down whatsoever! I removed the non-vented gas cap. Still there was no leak down. That was wrong. There should be with the gas cap removed. I had found the problem. After removing the gas cap, any pressure in the system should have vented to the atmosphere. Since it didn't, I now knew that the line was plugged somewhere from the gas tank to the charcoal canister in the engine bay. And if that line is plugged up, the evaporative loss control system cannot work! And if it doesn't draw the excess fumes from the fuel system into the engine and burn them, I get to draw them into my lungs as the expanding vapors escape the "closed" fuel system into my vehicle living space! So much for the "effective" test documented in the shop manual. It didn't leak. But neither did it draw.

Now the challenge was to find where the line was plugged. Here's where this technical article may save you some time and work if your MGB has the same problem. If you look on page 15 of the Moss MGB parts catalog, you will see the vapor line in question (items 83 through 86) together with the rest of the emission components. Notice the union, item 85. It turns out that rust was plugging this union solid. The lines had some of the crud in them too, but only at this union point. Why there? Because the inside diameter of the union is smaller than the line, and if any particles of rust are going to be drawn from the fuel tank, separator tank, and lines, they will lodge here!

Just where is this union? It's just behind the right rear tire. You can see it without removing the wheel, but it's a lot easier to get to if you do. Remember to use a properly placed jack stand under the car for safety if you jack up the car and remove the wheel. Make sure you have correctly identified the vapor line as the fuel line and the brake lines are in the same area. So to be extra sure in your identification of the vapor line, trace it forward to the vapor canister and backward to the vapor separator. You wouldn't want to disconnect the wrong line!

By undoing the hose connection from the vapor canister inside the right rear quarter panel (item 81), removing the union, and undoing the rubber connection between the vapor line and the canister (item 71), I was able to blow compressed air at 100 psi from each end to the disconnected union point successfully. A piece of welding rod or coat hanger can help remove the build-up just at the union connection, then apply more air until there is no impedance to the air flow. For the union itself, I had to use a drill bit to clean it out; the rust and crud was so well packed into it that I couldn't force it out with the welding rod. I guess that evaporative loss control system really applies some vacuum to the lines or that union wouldn't have been so plugged up!

Why was there rust in there in the first place? Probably because the car sat unused for over 4 years before I bought it. I'll also bet the gas tank was not full, either, allowing condensation to build up inside it, and the vapor separator tank. I hope this article helps you to cleaner air, both inside and outside the car! I'm glad the problem was no more difficult to solve than it was. Here's to cleaner air!

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