Fuel pump dying or larger problem?

[goser]

'01 Blazer: since I bought it a few months ago the sending unit has only worked intermittently and it's been hard to start. Once it's going it runs fine. At one point the fuel pump was whining so loudly for a few days that I could hear it over the engine at the front of the truck. Fuel pressure gauge showed low 50's psi on key on, then immediately fell back to zero. It seems pretty obvious that the pump is inches from death, but the instant leakdown got me wondering if there could be a leak somewhere or a bad pressure regulator as well. I don't see or smell any gas under the truck. Before I drop the money on a delco pump, is there a way to determine if the problem is solely in the pump? I remember something about testing the pressure while blocking off the return line, but I'm not sure which of the other two lines (or both?) are returns. I'm also not sure how to block them off without damaging them. Thanks for any advice you can offer.

[MT-2500]

Originally Posted by goser

'01 Blazer: since I bought it a few months ago the sending unit has only worked intermittently and it's been hard to start. Once it's going it runs fine. At one point the fuel pump was whining so loudly for a few days that I could hear it over the engine at the front of the truck. Fuel pressure gauge showed low 50's psi on key on, then immediately fell back to zero. It seems pretty obvious that the pump is inches from death, but the instant leakdown got me wondering if there could be a leak somewhere or a bad pressure regulator as well. I don't see or smell any gas under the truck. Before I drop the money on a delco pump, is there a way to determine if the problem is solely in the pump? I remember something about testing the pressure while blocking off the return line, but I'm not sure which of the other two lines (or both?) are returns. I'm also not sure how to block them off without damaging them. Thanks for any advice you can offer.

Have you checked your fuel filter?
Just key on is not a very good fuel pump test.
Two fuel lines go to engine injectors fuel suply.
The smaller one is return.
You can plug or pinch off return at rubber part.
Tool places make a fuel line pincher.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 90-100 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 65-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 90-100 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

[goser]

Thanks for the reply MT. I changed the fuel filter today... hopefully I haven't disturbed the delicate balance of gunk that's been keeping it going this long

After priming it again a few times the key-on engine-off reading was 54psi. Running at idle was 49psi. Unfortunately the hose on my pressure gauge isn't long enough to get it out from under the hood so I don't have a running reading. The leakdown isn't as instantaneous as it was before; now it leaks down to 20psi in 10 seconds and 5psi in 1 minute.

I still can't find a place to pinch off the return line... the lines are steel up until I lose sight of them down the firewall and by the time I pick them up again at the back of the transmission there are three lines. I think I could make an adaptor to hook the pressure gauge right to the front of the fuel filter; would that give me a good idea of the direct pump pressure?

[MT-2500]

Originally Posted by goser

Thanks for the reply MT. I changed the fuel filter today... hopefully I haven't disturbed the delicate balance of gunk that's been keeping it going this long

After priming it again a few times the key-on engine-off reading was 54psi. Running at idle was 49psi. Unfortunately the hose on my pressure gauge isn't long enough to get it out from under the hood so I don't have a running reading. The leakdown isn't as instantaneous as it was before; now it leaks down to 20psi in 10 seconds and 5psi in 1 minute.

I still can't find a place to pinch off the return line... the lines are steel up until I lose sight of them down the firewall and by the time I pick them up again at the back of the transmission there are three lines. I think I could make an adaptor to hook the pressure gauge right to the front of the fuel filter; would that give me a good idea of the direct pump pressure?

Yes hooking direct to fuel filter in line will tell you if pump is weak or good.
Just do not let it run at full pressure over a few seconds.
You should see 95-105 direct pressure.
Your pressure checking shows about 10 or more lbs low.
You need full 64-65 lbs for a good cold start and 60/66 lbs engine runnning.

If line pressure is low at scrader valve.

Two things to check.
Block off return or hook direct to fuel filter in line and see what pressure comes up to.
If not over 90-95 -105 your pump is weak.
But if it come up 75-100 with return blocked the pressure rgulator is bad or bad leak at injectors or housing/lines.

[goser]

Thanks again for the prompt reply. I removed the fuel line to the engine and hooked the pressure gauge right to the end of the fuel filter and got the same results as at the schrader valve. If I understand correctly this confirms that the low pressure and leakdown problems are within the pump itself. Thanks!

[MT-2500]

Originally Posted by goser

Thanks again for the prompt reply. I removed the fuel line to the engine and hooked the pressure gauge right to the end of the fuel filter and got the same results as at the schrader valve. If I understand correctly this confirms that the low pressure and leakdown problems are within the pump itself. Thanks!

Yes Time for a New OEM type Delphi fuel pump.

If thet try to sell you a aftermarket or Airtex pump just pitch it back over the counter at them and find someplace that has Delph pumps.