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Discussion Starter #1
Have a 97 z28 m6. Had replaced the fuel pump sending unit about 15k miles ago. Recently noticed that when I switch the key to the on position, the fuel pump takes about 15-30 seconds to prime. Can also hear a bubbling/gurgling noise coming from tank. The car starts and runs fantastically otherwise.

Not sure if these are related but there is a never ending battle with this car and the EGR. Lately it is throwing P0400 DTC. If I clear it, it comes back in 5 minutes. Replaced the EGR valve with a new one. Checked the EGR valve by pushing on the diaphragm with my fingers while the engine was running, and the Engine boggs down each time. Most of the EGR components are new including: purge solenoid, vacuum switch, EGR valve, evap canister, pcv, and much of the tubing. Any one have any thoughts on what else to check?
 

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I find it interesting that you can turn key on and get 30 seconds of fuel pump run. Factory designed system to prime for 3 or 4 seconds when key turned on. Have you changed the fuel filter?

Or checked for leaking fuel injectors that would make fp take longer to prime fuel rail. Have you put a fuel pressure gauge on fuel rail to see what your fuel pressure is? Should be around 43 psi @ prime.

Another thought that occurs to me; your fuel pickup line inside the tank may have a hole in it, or be coming off of pump. This would keep pressure from building up quickly. Or you could have a fp on it's way out.

Not being an obd 2 expert, I can't give much help on the p0400 dtc. But I can direct you to the 96 service manual which will explain the code fully. Go to the following site to download manual. My Files
 

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Discussion Starter #3
Found the culprit to the fuel pump priming issue. It turns out if I have a OBDII scanner plugged into the port, when I turn the key on it causes the pump to prime for 30 seconds. If I remove the scanner and then turn the key on, the pump primes in 3 seconds like normal. The scanner works fine normally and I am able to get data using the torq app.

This got me thinking that something is up with the OBD2 port. I tried putting the car in field service mode to enable the EGR. By my understanding if I jumper pins 5 and 6 of the DLC and turn the key on, then I should see the dash lights come on and hear the fans come on. I used a paperclip to jumper the pins and turned the key on, and nothing happened. The only dash lights that came on were:
1. The service engine soon light
2. The brake light
3. The check gauges light

The EGR also did not switch on, indicating the car did not enter field service mode. Does anyone know what could cause this? I need some way to enable the EGR to check it.
 
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