Another Fuel Pump Question

My 94 RMW just started having fuel pump related problems. When you turn on the key, you do not hear the fuel pump run. But, after cranking it for about 3 seconds, she starts. "JaySS" told me when the oil pressure builds up, the fuel pump will start. Which it does. I changed the fuel pump relay, but it did not make a difference.

The fuel pump pressure hits 42 pounds, but levels off at about 37 pounds, which I think is where it should be.

I very well could be wrong, but the fuel pump seems to work correctly. It is just not running when the ignition is turned on.

Anyone have any thoughts?

thanks

Tom C.

Fuel pressure spec should be 41-47 psi, regulated by the fuel pressure regulator, 37 psi is too low. Here is the fuel pump circuit in the form of a wiring diagram and hopefully I can answer your questions here.


The PCM controls the 3 sec fuel pump prime when the key is first turned on. The F/P relay is powered up from the PCM as shown in the diagram above from CKT 465, DK GRN/WHT wire (read that as DarK GReeN with WHiTe tracer) it is the highlighted circuit. This circuit sends power (12V) to the F/P Relay. Relay is grounded at G110, a good clean ground is required, I have seen a lot of issues with this ground. High resistance here will affect relay operation/function.

As seen in the diagram the Oil Pressure Switch effectively by-passes the F/P Relay in the event of a fault and this is why you could have an extended crank before starting.

An open or high resistance in CKT 465 will prevent the F/P Relay from working. Power must be checked at the PCM pin location 7 and at the F/P Relay, typically terminal 86 location. Also, check circuit 465 itself from these 2 points, must be checked for continuity. Less than .5 ohms of resistance.

If you have any questions do not hesitate to ask.

Thanks techie!

That's a lot of info, it will take me a while to digest that. Luckily, I am an electrician, that will be some help, I think.

I think I will start with the simple things. First will be checking the ground, as you said they are a cause of many problems. Heading to WF the end of next week, so may not be able to figure it out till after that.

thanks

tom

Techie,

May have found the problem. First thing I did was check the grounds, as you suggested. There were 3 ring terminals on the bolt. Two of them were completely broken and the the 3rd barely holding. That definitely was causing a problem someplace. May not get a chance to repair till Monday, I will keep you informed.

Tom

I can almost guarantee that fixing those grounds will fix your problem. They are a trouble area in the 94-96 cars, probably because of the location. I keep mine squirted with any spray grease or oil I have handy.

Several years ago, my kid's 95 Cavalier took forever to start. I traced out the problem and it turned out to be that Green/white wire to the PCM. On Cavaliers, GM put the PCM ahead of the right front wheel, where it could get frequently splattered with road crap. The wire was corroded right off the terminal.

Techie,

You were right on......two of the three grounds were broken. The third was barely connected. Fixed them and the fuel pump worked correctly. I imagine other things started to work correctly because of the broken grounds.

Thanks a million for the good advice. You saved me a lot of time and money.

Tom

Thats exactly what I thought it was.Good job Paul and if you ever have any questions just
ask me.









Yea right!