Help! C4 Cold Start Problem... Please Help!

[dhonick]

I can't get her started.
Checked the fuel pressure and get very inconsistant readings. It will go to 38 lbs in spec for a few seconds and then immediately drop right down to zero. Key off, key on - no pressure. key off key on pressure again and so forth. When I engage starter and turn motor, no pressure - zero. If there is pressure it drops off quickly. I have replaced the gas pump OEM, fuel filter, and fuel relay. VATS? Thank you in advance for any help ~ Danny

 

[dhonick]

I have already replaced the fuel injectors as well, if that helps any... Danny

 

[rlg]

I think I read in the FSM about the oil pressure switch. If it doesn't read 4lbs it will disable the fuel pump.

 

[vetteoz]

the oil pressure switch. If it doesn't read 4lbs it will disable the fuel pump.

Wrong ;It powers up the pump at 4psi or greater

There is a check valve on the tank pump assembly that holds pressure This may be bad or your regulator may not be holding pressure or possibly a bad injector bleeding off pressure

Key on ECM runs pump 2 sec, then stops.
Fuel pressure supposed to remain in rails so that when cranking it will fire on this pressure then the oil switch takes over running pump after it sees 4 psi.
There is a test proceedure out there where you clamp supply or return line to determine what is at fault

 

[dhonick]

Thanks VetteOz
The regulator is new, injectors new, Fuel pump new so I will lean towards the back pressure valve in the pump assembly.

I am guessing may be the VATS wires as well in the on/off switch at the key after 96K miles making zero pressure readings at times?

 

[vetteoz]

I am guessing may be the VATS wires as well in the on/off switch at the key after 96K miles making zero pressure readings at times?

VATS takes out the starter and injectors. Has no effect on pump or fuel rail pressure.
If your pump is good then your problem is to do with pressure loss somewhere in the fuel system

 

[TWISTERUP]

The ECM needs to see an ignition pulse BEFORE it will keep the fuel pump running..it will power up the pump for a few seconds when key is turned "on" but will not keep it running while cranking if it doesn't see ignition.
Are you certain you have spark ? Quick check would be...see if it will start by spraying starter fluid in the TB.

 

[Vettefan87]

Or just pull a plug and leave it in the wire boot. You can also see how good of a spark you are getting by what color it is.