clancy

per georgeb944:
The fuel pump is not supposed to turn on when the ignition is just turned on and the engine is not running. When you turn the key to start, the fuel pump relay picks (and the fuel pump starts working). The fuel pump continues to run only if there is a good signal from the crank position sensor that the engine is running. If the engine stops (for example, after a crash), the fuel pump stops pumping.
The reason for all of this weirdness is simple, if the pump runs when the ignition is on and the engine is not running, a break in the fuel supply lines or an injector that is stuck open will lead to, at worst, a fire on a hot engine, or, at least, a cylinder filled with fuel, leading to a bent connecting rod, or a ruined catalytic converter.


Makes perfect sense. And I have no reason to doubt this is accurate. What about the people who post about the fuel pump running for a few seconds in order to "charge" the system? Is this wrong, or for another age or series of Audi? Personally, at my advanced age and loss of hearing I have never heard this in any of my Audis.



That's why the fuel pump doesn't turn on when you turn the ignition switch to on and no further.

georgeb944

You can accomplish that by "bumping" the starter. The relay will pick and hold for a second or two. If you do that often enough, the fuel pump will eventually run long enough to raise the fuel pressure. It may take quite a few bumps, though, especially on a warm engine, where the fuel has boiled due to the heat in the engine compartment. The reason for the residual fuel pressure is to help keep the fuel from boiling (I'm old enough to remember when this was call "vapor lock") in the lines when you park a car with a hot engine. In order for the injectors to work properly, they have to inject liquid, not vapor.

I have owned Audis continuously since 1982, and some have had CIS systems, and mostly Motronic systems. Leaking check valves and leaking injectors can both cause "vapor lock"...

clancy

Good to learn this. So the concept of turning the key to "on" but not engaging the starter will not enable the fuel pump to run and charge the system. Those of us who have suggested this in other threads are giving bad info. Thanks for correcting this misinformation.

Living at 8300 feet elevation I am well aware of vapor lock with old carburetors. Used to see people with their hoods raised on mountain passes regularly in the old days. They drove up the pass, parked for photos and then found they could not restart their car. Always thought an electric fuel pump or fuel injection resolved this. But I can understand from your explanation that even injectors or electric fuel pump will not necessarily always fix this. Not if they are not working properly. Rather it is the fuel under pressure that kept it from vapor locking. Guess I was lucky. Or at least my cars were working properly so this was not an issue.

Harleyguy

That is what I suspected re: pump only works when motor is spinning. Kind of like diesel engines (like on large trucks and equipment) that won't supply fuel till at least 30 psi oil pressure. I'm thinking the OP needs a pump. And BTW thanks for jumping in on this. You too Clancy.

AndyHops

Hey guys, Thanks for all the help and information! This is all making a lot more sense now. It sounds like I have a faulty check valve in the fuel pump and I need to actually try cranking over the car to get the pump to switch on and prime the system again, hence why it always wants to start on the second try. I just placed an order for a new pump as well as a new coolant temp sensor (not throwing a code for it but read a lot of threads about how a bad CTS can mess with starting) and will be putting them both in when they arrive. I will reply back and let you guys know what happens in a couple days!