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Old 07-27-2013, 04:13 PM
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Rzeppa Rzeppa is offline
Cruise Moab Chairman
Join Date: Aug 2005
Location: Kittredge CO, USA
Posts: 6,667

Originally Posted by JadeRunner View Post
Nice trip ad write up. Dan's not afraid to wheel that stock 3rd gen.

Jeff, help me understand this. It could have been dangerous for a novice in a large group on that hill. I have seen this before on older rigs and always thought it was a carb setup issue or maintenance issue causing that problem. Some older rigs don't have that issue on high altitude hills. I know that you have a properly built carb and maintain your stuff. So just help me understand what else you could do to prevent this in the future.
Use actual gasoline that doesn't have ethanol in it so it has a lower vapor pressure, or maybe some additive as has been suggested, which would lower the vapor pressure of the fuel. The other thing that could be done is to install an electric pusher fuel pump back at the fuel tank. That would increase the ambient pressure in the fuel line.

You see, mechanical fuel pumps are located on the engine, which is convenient from a mechanical engineering point of view.

But they have two disadvantages in these particular situations. One is that they get hot because of being attached to the block instead of being immersed in cool fuel inside the fuel tank as is found in newer fuel injected vehicles. The huge disadvantage of the latter arrangement is that it is usually a huge PITA to replace a faulty pump inside the tank, and on many vehicles the tank must be drained and removed from the vehicle to get at the access port on the top.

The other is that they must use suction (AKA vacuum or negative pressure) to pull fuel from the tank, and a good bit of it when on a steep incline. The definition of boiling in chemistry is when the vapor pressure of the liquid equals or exceeds the ambient pressure. When using negative pressure (vacuum) to pull fuel from the tank, we have just lowered the ambient pressure and thus lowered the boiling point of the liquid. The liquid turns to gas (not gasoline, but gaseous state) and the fuel pump cavitates and is pretty ineffective at pumping. "Vapor Lock" is an incorrect term but is widely understood - it isn't really locked, it is just that we don't have liquid fuel to pump.

60s are more prone to this than 40s partly because the tank is lower relative to the pump, and farther back, which causes issues when on a steep incline like this. Once I was on level ground at the top I had no issues whatsoever. I didn't have any further problems later during the day because we didn't have any uphills that were as long as Radical Hill was. And it was cooler outside, which helps a bit.

It was reassuring to know what was going on so as not to worry or waste battery trying in vain to crank it over. I started pulling the fuel line downstream from the pump to verify my suspicions, but stopped when it wasn't super easy, and we were pretty confident we knew what the problem was and what the remedy was. In a situation like this, the mantra is "Patience Padiwan".
Jeff Zepp
Kittredge CO USA
1971 & 1976 FJ40s, 1978 FJ45, 1987 FJ60
Rising Sun 4WD Club, TLCA #4063
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