Fuel Pump Runs Constantly
 

Hello all!

I’ve got a problem with my blazer that’s got me stumped and I can’t figure it out. My fuel pump runs constantly even when the trucks off and the key is removed. The only time I can get it to shut off is when I pull the 30amp HVAC fuse or the fuel pump relay from the under hood fuse box.

I thought I traced it to a bad oil pressure switch and when I replaced it I was good for about a week and it started again. I removed an old crappily installed remote start system and replaced the ignition switch as well thinking maybe it was something with that but it wasn’t. On the fuel pump it’s the gray wire that has constant power for some reason.

If anyone has any ideas to go from I’d be grateful. The truck itself is a 98 Blazer 4x4 4 door with a 5.7L swap tuned by black bear performance. The only know other issue is that the 4x4 does not work and there’s no lights on the 3 button switch indicating the mode.

Thanks!!

Your relay may be stuck, try swapping it with another one to see if your results change

I did do that with a few relays and it made no difference. I've been working on it a bit more while I had time today and when I pull the ECM I and ECM B fuses it shuts off and stays off when I plug them back in. Only problem is if I turn the key to the on position when the pump would normally prime it just stays on until I remove the relay. If I plug the relay back in the pump remains off until the key is turned on again. I tried multiple relays with this as well and I reacts the same with any that get inserted.

You’ll need to start tracing back with a multimeter to all the switching points that can keep it hot until you find the problem

Do you mean the HVAC or HVAC 1 fuse in your first post?

Do you mean ECM I or ECM 1 fuse along with the ECM B fuse?

So you never have the problem if the fuel pump relay is removed regardless of any fuse combinations?

Do you really have to removed more than one fuse to get it to shut off or if you remove and replace each fuse in sequence one of them turns off the fuel pump by itself?

Is the HVAC fuse still involved?

George

So I’m getting 12.5v on pin 85 constantly at the relay even when I unplug the black pcm connector (it should only have power when it’s keyed on and then only for 2 secs). When I pull the 30amp HVAC fuse from the under hood fuse box the voltage at the pin in the fuse box drops to 0.

That Dark Green/White wire from the fuel pump relay to the pcm is a straight shot with no connectors and no additional functions on that wire. Since that wire is now always hot, there is a powered short along its run. There are a couple of choices.

One is to start unwrapping wire looms to see where the short has occurred. Its painful but the upside is that you may find or avoid other issues in the same area in the case of say a bundle of frayed or corroded wires.

The next option is to analyze adjacent HVAC functions to try and be a little more targeted in your wire run analysis. You will have to provide the exact HVAC fuse if you want to go this route. There are more than one.

The next thought is that you talked about a crappy aftermarket install for remote start. If that was a combo security system then there may have been a fuel pump disable function cutting into that Dk Grn/Wht wire so a return to the scene of that crime may be in order.

Lastly since that is a dedicated wire then pulling a new run may be the quickest and safest approach. Its not very elegant but its probably the shortest path to a fix.

George

Alright so as of right now I believe I have solved the problem. Disconnected the wire that feeds the blower motor from the fuse box, that wire runs to the HVAC fuse and the truck runs like it should. Ran a new wire from the fuse box to the blower motor and it’s been fine for now. We’ll see how it goes in the next few weeks haha