I have a 99 a4 1.8t. The fuel pump won't prime all the time. I noticed when I turn the key on and all the indicator lights come on and the mil light doesn't come on with them, it will not start, but if the mil light does come on it will start. I'm baffled. New fuel pump new fuel pump relay, new ignition switch. Help please

 

Can you tell if the pump is actually coming on when you turn on the key? You should hear it turn on (do not start the car) for a few seconds and go back off if you do not engage the starter. The ECU will not turn the fuel pump back on until it sees the crank turning over (reference/speed sensor).

But.. First, pull the OBD codes and post them. It should be reporting any other sensor failures if the MIL light came on. Those will help us figure it out.

 

There are no mil codes. When the mile comes on with all the other indicator lights, then yes, I can hear the fuel pump prime

 

If the MIL Light is not coming on when you turn on the ignition, it sounds like it more like the power to the ECU not getting to it. There should be another relay for that. I'd check it.

 

I'll give that a try. I do hear a clicking when it happens. Thanks for your help

 

You may be able to take the relay for the ECU out and pop the cover off. Sometimes some deposits can build up on the primary contacts that passes the high amp power to the unit that can prevent them from making a good connection when engaged. Simply slip a strip of medium (300-400grit) sand paper (folded over so you have the abrasive surfaces on top and bottom) between the contacts, then close the contacts manually with your finger trapping the sand paper gently between them. Pull the sand paper out, again while holding the contacts closed, will sand those contacts taking any deposits that may be interfering with current flow with it. Do this 3 or 4 times, snap the cover back on and put it back in the socket. See if things work normally then.

 

I spent some time last night looking for the ecu relay, you wouldn't happen to know which one it is?

 

I'm really not sure. It seems that some of these cars do not have a separate ECU relay. However I was able to find a thread on another site that may help you find it.

ECU Relay Location ?? HELP - AudiWorld Forums

Read the thread and note the two pictures at the top of the thread.

Also, if your car has an "Immobilizer", it could be causing the issue. I think it will interrupt power to the ECU if there is an issue. I'm not all that familiar with them at the moment.

 

Looks like I don't have a ecu relay, but I never thought to check the immobilizer. I checked all the relays and reseated them and checked all my grounds and everything looks fine. I might pick up a used ecu and immobilizer at the junk yard this weekend just to double check mine. Thanks for your help so far!

 

I fixed the car. I traced it back to a bad solder in the ecu.