I've read most of the fuel pressure posts, but I'm still a little baffled. 1998 Blazer 4.3 4X4. When it is cold, and most of the rest of the time, when the key is turned on, it jumps up to 60 psi as it should. When it is running, it will drop to a steady 50 psi, but under acceleration it will go back up to 60 from the vacuum change. As it warms up to full operating temerature, the fuel pressure remains about 50 psi, but the needle goes wild,rapidly vibrating between 45-55 psi. Eventually, the idle will get worse, and the fuel pressure increasing its wildflutter to 40-60 psi,until it dies. While it is normal operating temp, sometimes upon turning the key on (or back to the off position), it will only be 35 psi, but a few minutes later, right back up to 60 psi. When it only comes up to 35psi,the fuel pumpjust sounds week. I'm wondering if there is anything else that may control the volatge to the fuel pump? Why would it only be 35 psi, but a minute later (on a differnt key cycle) be back up to 60 psi??? The fuel pump is about 1 1/2 years old (but it was an aftermarket replacement). Any ideas???

 

With odd problems like this, I would recommend removing the fuel filter and adapting the threaded connection to the fuel pressure gauge. If you note the same fuel pressure problems, it is a pump problem. If the pressure stays at a steady 60psi or greater, the pump should be fine and you've narrowed the problem down to the fuel pressure regulator.

 

I guess I don't understand what would change, whether testing at the fuel rail or at the fuel filter. Are they not on the same line, just5 feet apart?Since the fuel pressure regulator (FPR) comes after the schrader valve testsite, would it effect the fuel pump pressure? Does the FPR affect the fuel pump pressure, or just the pressure to the injectors? Is it a loop system with some fuel back to the tank, too?

 

The FPR is what controls the amount of pressure in the line. The FPR bleeds off excess pressure, rerouting fuel back to the tank. By testing at the fuel pump, you remove the FPR and the return line from the equation.

 

So testing at the shraeder valve the pump provides pressure to the tester and the regulator....Testing at the fuel filter, the pump only provides pressure to the tester...

 

More precisely, when testing at the fuel pressure test port (shrader valve), the FPR is what is in command of the fuel pressure which should be held at its set point given a specific vacuum pressure. When testing at the fuel filter, the dead-head pressure (the maximum pressure the pump is capable of) of the pump is what you are reading.

Based off of this, fluctuating fuel pressure could be the result of either a FPR malfunction or a fuel pump malfunction. By testing at the fuel filter, you isolate the pump.

 

OK, so after an hour or so in the hardware store trying to configure an adaper for my puel pressure gauge (and some help of JB Weld!), I made one and isolated the fuel pump. It initially read about 64 psi. Knowing it was a heat or time related problem, I turned the key about 200 times to get the fuel pump to run as much as I could in 2 second intervals. After about 200 times and 5 minutes, as well as pre-carpal tunnel symptoms in my wrist, the fuel pump apparently heated up enough to start dropping pressure, down to about 40 psi max. It also got that kind of 'growling' sound as it audibly slowed down. Once I stopped, let it rest for a few minutes, it jumped right back up to 60 psi.

Am I correct that fuel pump runs full time (when the engine is running)? I had replaced the fuel pump 18 months ago. Is that about the life span for an aftermarket discount fuel pump? As soon as I get it running, I'm selling it (already bought an '07 Trailblazer), but I don't want it to be someone elses problem later, either. Any good advice on a fuel pump brand, or anything to stay away from?

 

The fuel pump should never be subjected the the punishment you put it through dead-heading it repeatedly like that. When the truck is running, so is the fuel pump, correct. However, with the FPR, the pump is continuously flowing fuel, just at a constant pressure. The fuel cools the pump. By dead-heading it, you remove the cooling affect because you are no longer allowing fuel to flow through the pump.

I hope you did not hurt the pump if it wasn't the source of the problem...

Did the fuel pressure fluctuation occur more with a low fuel tank level or was it independant of the amount of fuel in the tank?

 

The eventual sound I got by deadheading was exactly the same as the one I would hear after the fuel pressure would drop and the engine would stall with the fuel system in tact. But at that time, it would only come back to 25psi, after a minute up to about 40 psi, and after several minutes, up to 55-60 psi. With it back up to 55-60 psi, it would restart, and after a few minutes of running, would stall, and go through the same psi ratings. Essentially, the fuel pump had the same failure independently as it did when it wasin conjunction with the FPR.

I can't imagine a bad FPR having that broad of range, with the fuel pump sounding like it was struggling, but with such a low psi output. I did relieve the gauge pressure at every cycle, so there was a little fuel passing. I would have been more concerned if the initial pressure was higher. Besides, if the FPR limits the fuel pump to 60 psi, and thats all its putting out, nothing would be getting by the FPR in the first place besides the minimal amout of fuel being burned, right? How else could I have isolated the fuel pump and check it without deadheading it, and still simulate the initial problem I was having???

As far as fuel level, it has gotten progressively worse, even though I currently have 3/4 tank of fuel. It has been undriveable since I towed it home.

My impression is that the fuel pump motor is weak, and gets weaker as it runs and warms up. I'll be replacing it soon, and I'll let you know the results.

 

I replaced the fuel pump and all is well. It maintains pressure all the time. Finally!