2000 Gmc Sierra Fuel Pump Problems

[wargle]

I have a 2000 GMC Z71 with 114K miles. I have been having fuel guage problems. When I fill up, the guage goes to empty then suddenly back to full. This happens until the fuel level drops to about 3/4 tank, then it seems to read normally. Also, I have been having a "surge" when under acceleration. I checked fuel pressure with KOEO and it read 52#. with engine running it reads 48#. With engine running and unplugging vacuum to FPR, the pressure reads 58#. The haynes manual says the spec pressure should be 55-62#. I believe the fuel level sending unit in the tank is bad. Do I have a weak fuel pump also? I have read that there could be problems with the connector at the fuel tank. If I drop the tank and the connedctor checks out, should I replace the fuel pump assembly? Any other advice would be greatly appreciated.

[abracadabra]

I have a 2000 GMC Z71 with 114K miles. I have been having fuel guage problems. When I fill up, the guage goes to empty then suddenly back to full. This happens until the fuel level drops to about 3/4 tank, then it seems to read normally. Also, I have been having a "surge" when under acceleration. I checked fuel pressure with KOEO and it read 52#. with engine running it reads 48#. With engine running and unplugging vacuum to FPR, the pressure reads 58#. The haynes manual says the spec pressure should be 55-62#. I believe the fuel level sending unit in the tank is bad. Do I have a weak fuel pump also? I have read that there could be problems with the connector at the fuel tank. If I drop the tank and the connedctor checks out, should I replace the fuel pump assembly? Any other advice would be greatly appreciated.

 

not sure about the haynes spec.  48 is definitely low

 

This is something i've seen posted before for testing fuel pressure:

 

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 75-85 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 and fuel pressure testing guide line.

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 64-65 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 75-85 or more.

And if it has a fast leak down after shut off you have a leak in system.

Post back fuel pressure readings.

[wargle]

I have a 2000 GMC Z71 with 114K miles. I have been having fuel guage problems. When I fill up, the guage goes to empty then suddenly back to full. This happens until the fuel level drops to about 3/4 tank, then it seems to read normally. Also, I have been having a "surge" when under acceleration. I checked fuel pressure with KOEO and it read 52#. with engine running it reads 48#. With engine running and unplugging vacuum to FPR, the pressure reads 58#. The haynes manual says the spec pressure should be 55-62#. I believe the fuel level sending unit in the tank is bad. Do I have a weak fuel pump also? I have read that there could be problems with the connector at the fuel tank. If I drop the tank and the connedctor checks out, should I replace the fuel pump assembly? Any other advice would be greatly appreciated.

 

not sure about the haynes spec. 48 is definitely low

 

This is something i've seen posted before for testing fuel pressure:

 

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 75-85 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 and fuel pressure testing guide line.

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 64-65 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 75-85 or more.

And if it has a fast leak down after shut off you have a leak in system.

Post back fuel pressure readings.

 

 

 

 

 

[C6Racer]

Your pressure reading at idle should be at about 60 psi. My fuel gauge had the same problem as yours, but I dealt with it as that had no effect on how my truck ran. When the original pump finally gave up the ghost at around 130k miles I removed it and saw that the sliding contacts for the fuel level sender had burned halfway off! Sure enough, since I put the new pump in that problem hasn't resurfaced. Not to mention my pressure went back up to normal. If you regularly replace your fuel filter(every 30k-40k miles) then the low fuel pressure is a sign that your pump may be on its way out. Any kinks in the fuel lines are HIGHLY unlikely if all you do is change your oil. If you've done major work, like removing the transmission or fuel tank then kinked lines may be a problem.

[mooreapi]

Wargle,

The gauge behavior is as abracadabra described, the contacts of the level sensor wear out. I had the same problem with several of my fuel pumps (I have replaced it 5 times in my truck). The only way to fix it is to replaced the sender, which is not available separately, so therefore you have to replace the whole pump/sender assembly. The reason it occurs only when above 3/4 tank is that you must only run your truck partway down before refilling. This is why mine wore out, I filled my truck up every day that I worked (5-6 days per week) because I had to make sure I had a full tank at the beginning of each day (ask me how embarrassing it is to report to a client that I lost a subject because I ran out of gas ) This wears out a section of the sensor, then when you get down to the less worn area, it works fine.

 

While the fuel gauge behavior is annoying, there is no other effect. I would definitely check the connectors, because that could also be causing the gauge behavior, and likely also the surging.

You might also want to replace the fuel pressure regulator, which is also prone to failure.

 

Dropping the tank is a PITA. I swore if I have to do it again, I'm cutting a hole in the bed above the tank!

If this is the original fuel pump, you might consider just replacing it now, they are highly prone to failure. Remember to replace the fuel filter if you do change the pump.

 

Good Luck,

Matt