Blowing fuses in 99 Sierra Z71

Reyn · March 15, 2015

I have a 99 Z71 with 302k miles on it. It went dead driving down the road yesterday. Checked everything and replaced the 20A fuse next to the fuel pump relay. It was blown. I even switched the relay with another to test it.

It cranks right up but if I drive it more than a couple miles then the fuse blows again. My first thought is maybe the fuel pump is going out and drawing too much causing the blown fuse? The relay switch gets hot also. Just it though none of the others in the fuse box.

I decided to just let it idle for 20 minutes in the driveway. It never went dead. I would think if it was the fuel pump then why is it only blowing the fuse when I drive?

Opinions on what to change or test next?

&CDFiXER% · March 19, 2015

Could the locking 'ears' on the collar have failed/become fatigued?

Doug_Scott · March 16, 2015

I do not have any direct experience with your particular model vehicle, but, my 2000 Grand Prix GTP had a two stage fuel pump, that once the supercharger started creating boost the fuel pump would increase output. The original pump blew a fuse while I was driving home one day. Car was less than a year old, but, I was more interested in getting home than waiting for Roadside Assist to show up, so I did some road side checking, found a blown fuse, and I changed it. The car started and idled fine. I drove the car home and had no issues at all. Traffic was somewhat heavy at the time so it was a light throttle drive home. Next day I drove the car around all day, no issues again, but, also no hard throttle that day as well. It was not until the next day that I drove the car hard enough to get boost from supercharger, and the fuse blew again. Dealer came and got it. They replaced the fuel pump and fuses (they replaced the spare fuse that came in the panel I had used up earlier). Pretty sure they also changed the relay. There was a service bulletin on this I think at the time.

No idea if your pump is a similar type or not, but, your issue is more than likely the pump. Also change fuel filter. You may want to take a look at the filter first actually, see if you can blow through it. A plugged filter could cause the pump to blow fuses due to higher current draw to push fuel through the plugged filter. If it is plugged, change the pump as well, it will not be long for this world.

Reyn · March 19, 2015

Thanks for the replies. Pulled the bed off and the dang pump was floating. Wasn't even locked in. Looked like it had been that way a while but I've never smelled gas. Replaced it and everything is good. Sock was loaded with crap.

gmtech4 · March 19, 2015

Thanks for the replies. Pulled the bed off and the dang pump was floating. Wasn't even locked in. Looked like it had been that way a while but I've never smelled gas. Replaced it and everything is good. Sock was loaded with crap.

Wow , never saw that before ., crazy

oldscsc · March 15, 2015

Test amperage draw of the fuel pump and compare it to either a spec or known good pump.

Idling requires very little fuel. A pump in it's way out may be able to produce a small amount of fuel with relatively lower effort, but have to work far harder to make the pump the increased amount of fuel required to move the pickup down the road.

Pump working harder -> higher amperage draw -> more heat produced. Heat and resistance changes are proportional, so higher heat = higher resistance = possibly exceeding the fused capacity of your circuit.

Reyn · March 20, 2015

My buddy fixed it at his shop. He said everything looked fine. He even took the new pump in and out and tested to make sure it locked in. The new pump I can't hear but the old one was always noticeable.