We went on a trip to Utah this last weekend, round trip about 1400 miles. With about 60 miles to go my '99 2wd 4dr blazer started chugging a little when going up hills, no big deal I though, it was probabally time to replace spark plugs, wires, rotor and cap. So we made it home and parked outside our apartment to unload the kids and luggage and when I went to move the blazer to our car port the darn thing wouldn't start. I started checking things out and determined that there was a spark, the fueses and relays for the fuel pump were good but for some reason fuel barely drizzled out of the schrader valve in the fuel line just behind the throttle body when the valve was pressed open. I couldn't hear the fuel pump running after turning the key off at all at this point.

I was starting to think it was a fuel pump problem but the sun had already gone down and twilight was just about gone so I got a neighbor to help me push the truck into the car port. Because we were pushing the truck and because the carport is narrow I ended up parked so close to the car next to me that I couldn't open my drivers side door, and because he had backed in he couldn't open his driver's side door either, so I left a note of apology and promised to have it moved the next day. Monday at lunch I came home to push the truck back into the visitor space directly behind our carport but before I started sweating I told my wife to turn the key just for kicks and the stupid thing fired right up. I mentioned the problem to a couple of co workers and we decided that it was probabally just a dirty fuel filter and over night the pressure behind the filter had equalized with the pressure in front of the filter providing enough fuel for the engine to fire. On my way home from work I stopped at a Kragen to pick up a fuel filter and while I was there I asked if they had a loaner fuel pressure gage. The clerk got to asking why I wanted one so I explained the problems I was having and he agreed that it sounded a bit like a fuel filter problem he wasn't 100% certain but we agreed that even if I ended up replacing the pump it would be a good idea to replace the filter. They didn't have a fuel pressure gage to loan so just I brought the filter home and got it swapped out this afternoon.

After replacing the filter the truck started right up, I let it run at an idle for a few minutes and turned it off then back on with no problems, then revved the engine to about 3K RPM for a few minutes then off and back on with no problems. Each time I turned the key off I listened to hear the fuel pump running and it did consistantly. For kicks I pressed the schrader valve with the engine stopped and sprayed fuel 18 inches to the firewall.

Feeling like a Master mechanic I gave myself a pat on the back and after a shower headed out in the now trouble free blazer to a scout meeting 3 miles away. After the meeting I went out to start the truck and drive home and the engine turned and turned but never fired... the fuel pump doesn't make noise after the key is turned off and there's only a drizzle at the schrader valve. So the blazer is sitting in a church parking lot until tomorrow when I'll have my wife drop me off and try to start it again, so I can drive it home, or worse, have it towed and try to figure out what's really wrong. Do fuel pumps have a curfew? Does my blazer know that I'm not supposed to be driving around past 8:00PM?

While I was at Kragen I asked for pricing on the fuel pump and the guy mentioned that for a lot less hassle and about half the money I could get a Holley inline fuel pump and leave the original pump in the tank. The inline pump put out 48 psi if I recall correctly and the guy said it would work just fine I'd just have to run wiring to the new pump and cut the fuel line somewhere to install it. I have a full tank of gas right now and I'm not really supposed to be working on my car in the apartment's parking lot under our leas

 

Stick with the pump in the tank. 48psi is not enough pressure for your fuel system. Minimum operating pressure is 50psi, the norm being 55-60psi.

When you say 'key is off', does this mean that it is in the off position? If so, the fuel pump should NOT run with the key in this position. The fuel pump should only run with the key in the 'ON' or 'Start' positions. If your pump isn't turning on, then you either have a bad pump or a bad electrical connection somewhere in the system. My vote would be for the beginnings of a failing pump. Typically fuel pumps just quit working, but it could be something to do with heat buildup and once it cools off, it'll work... I don't know if that's what's happening, but sounds atleast plausible...

 

When the key is turned from the run position to the off position the pump will run for a couple of seconds to pressurize the line for the next start, at least as I understand it. It's stopped doing that consistantly now. Aside from the fuel pump relay is there a switch somewhere that could have bad contacts? Anything else I could test before dropping the tank?

 

Nope... Not from my understanding... The pump shouldn't need to run to have fuel pressure for the next start simply because the fuel system still should be pressurized already. If your system leaks down, it'll be pressurized in the first few seconds of turning the key to the 'on' position, but you may have to wait for it to pressurize (ie. not just try to start it right away). A leak free fuel system should have anywhere from 50-55psi of pressure in it at any point in time after being shut down completely. But gummed up injectors or leaking o-rings in the injection spider may cause the pressure to drop, sometimes to nothing at all. It'll take a bit of time for the pump to repressurize the system at the next startup, but you have to allow the pump to do so.

When the key is off, everything should be off aside from RAP (retained accessory power) items (radio & windows until a door is opened or 5 minute elapses)... Keeping the fuel pump on even for a few seconds after the key is off could be a safety hazard...

Oh and I just read a TSB that stated that the proper fuel pressure for the 4.3L with the key on & engine off should be 60-66psi when tested.

 

So what is the humming toward the rear of the vehicle that I usually hear after turning the key off, I always thought it was the fuel pump and since it's silenced lately I assumed that it meant that the fuel pump has gone bad?

I was searching old posts and came up with something about the ignition switch and a security feature where the fuel pump won't start under certain circumstances. Can anybody elaborate on that one? Could I have a faulty ignition switch?

 

I just got back from a mechanic's shop, where I had asked him about the possessed fuel gauge that Blazers (including mine) suffer from. He knew immediately what I was asking about, and showed me the fuel pump/sender he pulled from a blazer only yesterday (I should have asked him if I could have it). Besides a very badly designed sending unit, the power wires to the pump had melted the insulation, and were intermittently shorting out, killing the pump. What generates heat like that, enough to melt insulation? Bad grounding, or cracks in the insulation allowing arcing. Somewhat scary, for something saturated in an explosive. He also said the same thing I read here in this forum; there is no permanent cure, get used to counting your miles between fill-ups. Now with that said, my advice is this; of all the components on my blazer, I would trust the fuel pump the least. You may get lucky and have it be the fuel-cut-off switch, but I doubt it.