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Old 08-02-2008, 11:41 AM
88sunroof 88sunroof is offline
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Join Date: May 23, 2005
Location: North Carolina
Posts: 218
Vapor Lock?!?

My stock '88 GW 360 shut-down on me yesterday. Problem: no fuel to the carb.

I let it cool for an hour and poured cold water over the fuel system; finally it started and gave no further problems.

I don't get it. The day was was hot, but not the hottest of the year. Yes, we were using the A/C, but we were not towing or otherwise "loading" the vehicle.

I checked the gas tank vent and it was working. The timing chain set was replaced four years ago. The fuel pump is apparently the original and I plan to change it (along with whatever rubber hose which I can reach without dropping the fuel tank) soon, but good grief -- one week earlier, we towed a trailer up the side of a mountain with it without so much as a hiccup!

Suggestions?

Jeff Dreibus
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  #2  
Old 08-02-2008, 12:19 PM
Don S's Avatar
Don S Don S is offline
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Join Date: Feb 06, 2002
Location: Burleson TX
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..

True vapor lock is a disease that sometimes slowly grows in a poor old vehicle.

Vapor Lock in a “Nut Shell”.
Basically vapor lock is the fuel temperature being raised to its vaporization point and converting to a vapor in the fuel pump or feed line and the pump not able to pump vapor.
Symptoms are the engine sputters, acting like it is running out of fuel, dies, and will not restart while hot if not fed with a quality electric pump
Major causes are overheating of the fuel, fuel pump and negative pressure in the feed line of the fuel pump.
Best remedies are insulating and/or rerouting fuel pump feed line. Replace mechanical pumps with properly installed quality electric fuel pump. Adding a small return fuel line on the output line of the pump.

Non Vapor Lock in a “Nut Shell”.
Basically something in the fuel supply system becomes stopped up or one of the valves in the fuel pump temperaly locked open or closed.

Heat Soak is Boil Over and in a “Nut Shell
Basically boil over is the fuel converting to a vapor in the bowl of the carburetor when a hot engine is turned off.
Symptoms are the engine will not restart hot or cold until the fuel pump can refill the carburetor. Other symptoms can include poor fuel mileage and poor drivability.
Major cause is overheating of the carburetor also known as heat soak.
Best remedies are insulating the carburetor from the manifold, install/reinstall cool fresh air intake system, checking the two heat rising systems for proper operation.

Most Vapor Lock problems are caused by the following and/or a combination of the following conditions.
(A) Weak valves or ruptured diaphragms, worn cam lobes/wiper arm in the fuel pumps system. Also leaking fuel lines that suck in air while the pump is working may not be a vapor lock but can cause the same symptoms.
(B) A lack of free flow to the fuel pump. These include pinched or clogged lines or filters.
(D) Negative pressure in the fuel tank and supply lines. Can be caused by improper venting of the fuel tank.
(C) The temperature of the gasoline has exceeded its vaporization point. Note. The vaporization point drops to a lower temperature as the pressure drops, they are related. Some blends of gasoline will vaporize at 145 degrees at seas level. Cures include rerouting fuel supply lines, return fuel lines and insulating them, shielding the fuel system from the exhaust pipes and converting to a heavy duty electric fuel pump.
(E) Do not think of ’Boil Over’ as Vapor Lock. Boil over is the loss of fuel, due to vaporization, inside the carburetor caused by ‘Heat ‘Soak. Some cures for Heat Soak include an insulated spacer and or shield under the carburetor and or an electric fuel pump.
(F) The fuel return line is your friend. It can send the vaporized gasoline back to the tank where it condenses back into usable gasoline. The return line should be routed away from heat sources as it can add to the temperature of the fuel in the tank.

Have a Gud'un and see you later.. Don S..
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Sold our 1976 Wagoneer 406, MC4300, TH400, QT, TruTrac, 2" lift, 31x10.50s, duel Optimas,
It’s took us over 161 Colorado Mountain Passes, 3 Jeep Jamboree USAs & 2 Ouray Invasions from 1985 to 2010
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  #3  
Old 08-02-2008, 06:07 PM
88sunroof 88sunroof is offline
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Join Date: May 23, 2005
Location: North Carolina
Posts: 218
Quote:
Originally Posted by Don S
..

True vapor lock is a disease that sometimes slowly grows in a poor old vehicle.

Vapor Lock in a “Nut Shell”.
Basically vapor lock is the fuel temperature being raised to its vaporization point and converting to a vapor in the fuel pump or feed line and the pump not able to pump vapor.
Symptoms are the engine sputters, acting like it is running out of fuel, dies, and will not restart while hot if not fed with a quality electric pump
Major causes are overheating of the fuel, fuel pump and negative pressure in the feed line of the fuel pump.
Best remedies are insulating and/or rerouting fuel pump feed line. Replace mechanical pumps with properly installed quality electric fuel pump. Adding a small return fuel line on the output line of the pump.

Non Vapor Lock in a “Nut Shell”.
Basically something in the fuel supply system becomes stopped up or one of the valves in the fuel pump temperaly locked open or closed.

Heat Soak is Boil Over and in a “Nut Shell
Basically boil over is the fuel converting to a vapor in the bowl of the carburetor when a hot engine is turned off.
Symptoms are the engine will not restart hot or cold until the fuel pump can refill the carburetor. Other symptoms can include poor fuel mileage and poor drivability.
Major cause is overheating of the carburetor also known as heat soak.
Best remedies are insulating the carburetor from the manifold, install/reinstall cool fresh air intake system, checking the two heat rising systems for proper operation.

Most Vapor Lock problems are caused by the following and/or a combination of the following conditions.
(A) Weak valves or ruptured diaphragms, worn cam lobes/wiper arm in the fuel pumps system. Also leaking fuel lines that suck in air while the pump is working may not be a vapor lock but can cause the same symptoms.
(B) A lack of free flow to the fuel pump. These include pinched or clogged lines or filters.
(D) Negative pressure in the fuel tank and supply lines. Can be caused by improper venting of the fuel tank.
(C) The temperature of the gasoline has exceeded its vaporization point. Note. The vaporization point drops to a lower temperature as the pressure drops, they are related. Some blends of gasoline will vaporize at 145 degrees at seas level. Cures include rerouting fuel supply lines, return fuel lines and insulating them, shielding the fuel system from the exhaust pipes and converting to a heavy duty electric fuel pump.
(E) Do not think of ’Boil Over’ as Vapor Lock. Boil over is the loss of fuel, due to vaporization, inside the carburetor caused by ‘Heat ‘Soak. Some cures for Heat Soak include an insulated spacer and or shield under the carburetor and or an electric fuel pump.
(F) The fuel return line is your friend. It can send the vaporized gasoline back to the tank where it condenses back into usable gasoline. The return line should be routed away from heat sources as it can add to the temperature of the fuel in the tank.

Have a Gud'un and see you later.. Don S..

Doctor Don,

Thanks. Since I work in fuel systems, I am aware of what you are discussing and I have eliminated some of it.

The fuel tank vent is indeed open.

The carburetor itself is not heat-soaking. I know this because the Jeep started up and ran (after a "hot stop") for about 1/8 mile before shutting down, so the carb bowl must have been full when I got onto the highway -- yet no liquid fuel in the delivery system.

I am aware of the value of the fuel return system and mine, which is integrated into the fuel filter, is in fine working order.

I replaced the timing set four years ago; I know (the hard way) that serious wear in that system can mimic the symptoms of vapor lock.

That leaves the pump and its drive system, the filter and the hose. I will be replacing the fuel pump, filter and hose (I always use SAE 30R9 fuel injection hose on everything) shortly.

But I still find it odd that, if the pump is the culprit, it hasn't caused a complete shut-down but has rather become heat-sensitive. Do they often go bad this way on Jeeps? I am more familiar with Chrysler and GM which fail completely and permanently. Following cool-down, we drove our Jeep forty+ miles after this happened yesterday -- leaving the A/C off, of course. No further problems.

Jeff Dreibus
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