Fuel Pump Issues

Bukmaster · January 25, 2008

I have an interesting problem. The truck is a 1997 GMC Jimmy 4x4 4.3 with 190,000 miles on the clock. Last Saturday it was 5 degrees here and my wife tried to start the truck only to have it turn over really slow and no fire. I knew the battery was on borrowed time, so I put the jumper cables on it and get nothing but a cranking engine. After diagnoses I realize that the fuel pump is not kicking on. The wierd thing is that I can only get 5 volts back to the pump. I changed the fuel pump oil pressure switch, and checked that the relays were in working order and I still have only 5 volts to the pump. I do have 12 volts at the realys and the wires look good all of the way back. I don't want to change the pump unless it in fact bad ($300+). Can anyone shed some light on this issue or guild me in the right direction. I almost think that I am missing something obvious and that it has something to do with the cold weather/low battery. What do you think.

Thanks, Bill

minibike132 · January 26, 2008

My first suspect would be connector c107. Unfortunately GM didn't fully switch over to computer based manuals till 1998 and they are way different than a 97 according to the diagrams. There is only a description of where to find it untill I have a paper manual handy. It's described as a cavity (thats holes not necessarely wires) in the engine compartment near the power distribution center. You'll want to check voltage on the grey wire terminal G.

Bukmaster · January 27, 2008

Thanks, I will try to find that tommorrow. I should add that this truck has a remote start that has given us no problems for the past 7 years. Suspecting that it my have caused a possible security problem due to the low battery, the first thing I did was unhook it. So I don't know if this truck has such security measures, but I thought that it might bear mentioning.

Thanks again, Bill

Bukmaster · February 6, 2008

Update: I used a jumper on the fuel pump electrical tap and gave the gas tank a good rap with a baseball bat and the pump began working. It sounds horrible so it will be replaced real soon. But now a new problem has arose. I decided to start the truck since it has been almost 3 weeks When it fired it stuttered and almost died for about 5 seconds then the idle sped up and was surging for another 10 seconds then it smoothed out and idled at 2500 RPM and never slowed down. I started it several times only to have it do the exact same thing every time. On a mechanics advice I looked for a vaccum leak and found none. So my new question is could a lack of fuel cause this surging and high idle, it seems to me that the computer is trying adjust the timing to make it run smooth only to have it idle really high. I am no mechanic so i might be way off base, what do you think?

Thanks, Bill

minibike132 · February 6, 2008

Back up for a second. You are running a jumper so your original problem will still exist if you were testing right. You can have both a bad fuel pump and not enough voltage. When you replace the pump if you only have 5 volts back there you will still have a no start. As far as the idle how long did you let it run each time.

Bukmaster · February 6, 2008

I guess I should have clarified, I used the jumper only until the pump began to run. The pump comes on with the key,build pressure for a couple seconds just like it always has. The only difference is that the pump sounds noticably different and the engine acts as described above. I actually let it run for several minuets while looking for a possible vacuum leak and it showed no sign of slowing. I have not checked voltage since I played with the connectors under the hood. Also there is no difference in the sound of the pump when I have the jumper on or when using the key. You don't think the pump could be running on 5 volts do you? I will check the voltage to the tank again after my son goes to daycare. Oh yeah one more symptom: The gas gauge appears to be pegged way past full and isn't moving, this is new.

Again, I appreciate the help.

Thanks, Bill

Bukmaster · February 6, 2008

I was able to take afew minuets tot est the voltage ging back to the pump again. I don't know why but I now have 12+ volts with and without the jumper. I still have to thump the tank every so often to get the pump to run. I know I have to replace the pump, but I can't explain the hard start, surging and finally very high idle. I hope that this clarifies my current situation a little better.

Thanks, Bill