On my 1985 351 E150 with a Ford/Holley 4 bbl carb:
For the last few days, I have smelled gasoline coming from the engine compartment when the engine warmed up. I just presumed that a vent hose had come loose so I wasn't worried about it. Then ...
I drove to town about 40 miles away, and the engine started running TERRIBLY, like it was getting WAY too much gasoline. I immediately headed straight home, but I used an entire tank of gas in just 40 miles!
Obviously a sunk float, or so I thought. I restarted the engine and pulled the two float level screws from the float bowls, but no gas ran out! Further, the engine now idles OK!
I suppose that a temporarily stuck float bowl valve could have done this, but that wouldn't explain the smell, and besides, I don't like "bogeyman explanations".
Any ideas what ELSE could have gone wrong to do this?
Oops, I forgot to mention that the air cleaner wasn't very dirty but it WAS pretty much soaked in gasoline! I don't know where ELSE the gas could have come from other than the vents.
However, it now idles OK, and with the air cleaner off I don't see any gas coming from the vents, yet I STILL smell some gas. This doesn't seem to fit ANY of the theories other than multiple problems. Hmmm, I guess multiple problems IS a possibility in a 20 year old vehicle.
Decades ago I ran into a case where the fuel pressure went astronomical, forcing its way into the bowls. This engine has a mechanical pump. I wonder of its internal regulator could sometimes be going berserk?!
Many thanks for everyone who volunteered their opinions, but after tearing the carb apart and finding NOTHING wrong, it has been quite a witch hunt to find the REAL culprit. Here is what apparently happened:
The fuel pump developed a microscopic leak in its diaphragm, which built up fuel behind it (the business side is on the bottom, so the gas collected on the top of the diaphragm). There was a 1/4" hole in the side of the pump to keep any leaking fuel out of the oil, so it leaked out there and THAT is what we smelled for days preceding the semi-disaster. When I removed the fuel pump, it dumped a teaspoon full of gasoline onto my hand from that vent hole. Further, everything below the vent hole was VERY clean.
When, we drove ~50 miles to town on a HOT day, loaded up our trailer with a half a ton of stuff, and generally heated the engine block well above the boiling point of the gasoline. With the PCV system in good working order, the vaporized gasoline from the leak was sucked IN to the crankcase, some went into the oil (that gained about 3 quarts in volume) and the remainder went either through the PCV or the crankcase vent to the air cleaner. I suspect that in the process the leak got a lot worse.
Curiously, once in this mode, we didn't smell any more gasoline - because it was no longer leaking externally, but was all being sucked into the engine!
In total, this hassle ended up including:
Tearing apart a perfectly good carburetor.
Changing the oil and filter.
Replacing the air cleaner.
Replacing the fuel pump.
Replacing the tank full of gasoline that was lost, this being the biggest actual expense of them all!
I had just gotten back from a long trip in this vehicle. I shudder to think how this story would have turned out if this had all happened on the road. By the time some repair shop had replaced the carb, etc., I could have been out a LOT of money.
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