I have a 1999 Mercury Mountaineer, 5.0 V8 with approx. 147k miles. Recently it’s been difficult to start when cold (e.g., sitting overnight in the garage). It’ll crank fine, but doesn’t want to start. Not wanting to stress out the battery, I don’t let it crank long — a second or two — then turn the key off, let it sit a second, then try again. Usually on the 3rd or 4th try it’ll fire right up. Once running, there are no issues.
After it’s been running for as little as 2 or 3 minutes or longer (for example, running errands to different locations), it will fire right up almost instantly, no problem. Plugs and wires have been replaced within the last year, as has the fuel filter.
I’m guessing fuel pump or starter?
Not the starter or it wouldn’t crank over. Possible fuel pump issue.
Lets perform a quick test to confirm it is a fuel related issue. The next time it has been setting over night, cycle the ignition key 4 to 5 times before trying to start it. Turn the key to “ON”(NOT START) and hold for 5 seconds and then turn the key to “OFF” . Repeat this 4 to 5 times. What this is doing is priming the fuel system. Once you have cycled the key several times go ahead and start the engine. It should start right up like normal. If it does then you have successful confirmed the problem is fuel related.
The fuel pressure is bleeding off when it sets overnight. Cycling the ignition key several times activates the fuel pump relay(which only runs for a few seconds) and builds the fuel pressure back up to where it needs to be in order to start.
What do I need to Replace?
The most common failure part that will cause this issue would be a faulty fuel pressure regulator. It calls for about .07 of an hour to replace. Two screws and one vacuum hose and can be purchased for around $40.
Fuel Pressure Regulator
The fuel pressure regulator is attached to the fuel rail downstream of the fuel injectors. It regulates fuel pressure supplied to the fuel injectors. The regulator is a diaphragm-operated relief valve. One side of the diaphragm senses fuel pressure and the other side is connected to the intake manifold vacuum. Fuel pressure is established by a spring preload applied to the diaphragm. Balancing one side of the diaphragm with manifold vacuum maintains a constant fuel pressure drop across the fuel injectors. Fuel pressure is high when engine vacuum is low. Excess fuel is bypassed through the fuel pressure regulator and returned through the fuel return line to the fuel tank.