I have kind of a strange problem in my 97 Ranger. 4.0 v6 5 speed auto 2 wheel drive. 131,000 miles.

When I start the truck in the morning with a low fuel tank, the engine starts kinda weak, runs rough for about 3 seconds, then stabilizes. When the tank is full it seems to start much better and idles much for smoothly right off the bat. Any idea what could be causing this? Or is this just a symptom of a high mileage engine?


EDIT: I should add that I haven't done much work to the truck since buying it a few years ago. I changed the fuel filter a few months back with no change in start performance. I do change the oil on time and replaced the air filter when needed. But apart from that I haven't done any other work to it.

 

Welcome to the forum

Try this next time fuel is low
Turn on the key, count to 3, then turn it off, repeat 2 more times, then try to start engine

The Ford computers only run the fuel pump for 2 seconds when key is turned on, this is to Prime the system.
So by turning key on and off a few times you are building up any lost fuel pressure.

Fuel injection systems should hold fuel pressure for MONTHS, but fuel pressure regulator can leak and fuel pump backflow preventer valve can leak as well, both leak fuel back to the tank so none is lost but there is no fuel pressure to start engine quickly.

1997 and earlier Rangers should have 30 to 40psi fuel pressure
1998 and up Rangers got a newer system, 60 to 70psi fuel pressure


The fuel pump is at the bottom of the tank, so any gasoline above it will have a weight so adds pressure, about 8lbs per gallon of gasoline, to the fuel pumps inlet, this would hold some pressure in the system if fuel pumps valve was leaking, the fuller the tank the more the pressure.
so low fuel level might allow pressure at injectors to drop to 0psi and one 2 second pump might not get it up high enough for quick start
Once engine starts, fuel pump comes on full time so instant full pressure

The back flow preventer valve is part of the pump not a separate piece

 

Thanks for the reply. So it sounds like overall a new fuel pump would solve the problem. Not sure If I want to go through the trouble of tearing open the fuel tank. I'll try to cycle the ignition a few times next time and see what that does.

 

Yes, first ID the problem, then look at the options