Roadrunner777
Explorer Addict
- Joined
- February 5, 2011
- Messages
- 2,044
- Reaction score
- 346
- City, State
- Bemidji, MN
- Year, Model & Trim Level
- 94 4x4 Sport 88k
Over the years, I have seen quite a few posts about a problem with warm starts, where the truck is parked on hot pavement for a while and then it runs awful for a minute or two. Well, yesterday I was doing some reading (I'm working on a reference thread for the fuel system), and found this in the factory manual:
"The fuel pump is mounted on the fuel sender assembly inside the fuel tank. This assembly includes a check valve which is in the outlet of the fuel pump. The function of this valve is to maintain pressure in the system after the vehicle is shut down. The pressure retention helps prevent hot starting problems." (second paragraph of the scanned page below)
How would lack of pressure cause hot starting problems? Simple, once you see it. Pressure increases boiling point, like our cooling systems. Without it, gasoline vaporizes during hot conditions in the fuel lines. If you have dealt with carbureted vehicles in hot weather, this is called 'Vapor Lock'. With pressure, it stays a liquid (for the most part, it's all relative).
The test for this is simple. Hook up a fuel pressure gauge, run the engine and shut it down, and note how long it takes for the pressure to drop. Try and do this when the temperature is going to be steady outside... not drastically warming up or cooling down. It turns out temperature has a great impact on pressure as well. Here are the pressure measurements from my system, which has a new fuel pump and starts/runs properly:
At engine shutoff, 37 PSI
After 1 hour, 18 PSI
After 2 hours, 11 PSI
After a much longer period of time, like overnight, it will drop to zero.
A bad fuel pump check valve will drop pressure much more rapidly and may even allow fuel in the lines to drain back into the tank. Incidentally, it takes a surprisingly small volume of gas to cause a major pressure drop, like a teaspoon. If you have a pressure gauge with a relief valve, you can see this for yourself.
There are some suggestions on the internet that this problem can be minimized by running a higher tier or higher octane gasoline, or adding an additive such as Techron.
Anyway, if you have starting issues in hot weather, measure the leakdown of your system. It may be the check valve inside the pump.
(Ford Factory Service Manual, 1994 Ford Explorer) Sorry for the crooked scan, I need to set up some way to hold the book better.
"The fuel pump is mounted on the fuel sender assembly inside the fuel tank. This assembly includes a check valve which is in the outlet of the fuel pump. The function of this valve is to maintain pressure in the system after the vehicle is shut down. The pressure retention helps prevent hot starting problems." (second paragraph of the scanned page below)
How would lack of pressure cause hot starting problems? Simple, once you see it. Pressure increases boiling point, like our cooling systems. Without it, gasoline vaporizes during hot conditions in the fuel lines. If you have dealt with carbureted vehicles in hot weather, this is called 'Vapor Lock'. With pressure, it stays a liquid (for the most part, it's all relative).
The test for this is simple. Hook up a fuel pressure gauge, run the engine and shut it down, and note how long it takes for the pressure to drop. Try and do this when the temperature is going to be steady outside... not drastically warming up or cooling down. It turns out temperature has a great impact on pressure as well. Here are the pressure measurements from my system, which has a new fuel pump and starts/runs properly:
At engine shutoff, 37 PSI
After 1 hour, 18 PSI
After 2 hours, 11 PSI
After a much longer period of time, like overnight, it will drop to zero.
A bad fuel pump check valve will drop pressure much more rapidly and may even allow fuel in the lines to drain back into the tank. Incidentally, it takes a surprisingly small volume of gas to cause a major pressure drop, like a teaspoon. If you have a pressure gauge with a relief valve, you can see this for yourself.
There are some suggestions on the internet that this problem can be minimized by running a higher tier or higher octane gasoline, or adding an additive such as Techron.
Anyway, if you have starting issues in hot weather, measure the leakdown of your system. It may be the check valve inside the pump.
(Ford Factory Service Manual, 1994 Ford Explorer) Sorry for the crooked scan, I need to set up some way to hold the book better.
