Can't figure out - alternator or fuel pump or something else?

[phantom564]

A few days ago my charging system warning light lit up on the dashboard and the in-dash Voltmeter was reading below 12V. Took my 1996 K1500 (5.7L 350) to a parts store and had the alternator tested because the battery is only 11 months old. Guy at the parts store load tested the battery terminals and told me to replace the alternator. I replaced the alternator and the truck fired right up. I took a 300 or so mile trip and yesterday the charging system light came back on and the truck just shut down. Tried to restart it but it just cranked and wouldn't turn over. Tried jumping the battery and nothing happened.

When the tow truck crew got there, one guy crawled underneath my truck with a hammer and started beating the daylights out of my gas tank. When I tried to start the truck, it fired up just fine. He told me the fuel pump was bad and needed replaced.

Any thoughts? Was replacing the alternator unneccesary and should I replace the fuel pump now, too?

[wafrederick]

To confirm the fuel pump is bad, check the fuel pressure first with a fuel pressure gauge and it should be 55-60 psi.That is the specs for fuel pressure.The new alternator might be defective also.

[gmtech79]

Yea sounds like a fuel pump. Are you sure the engine hadn't already died before you noticed the light because it will come on when the engine is not running but the key is on. Check pressure and volume.

[MT-2500]

Originally Posted by phantom564

A few days ago my charging system warning light lit up on the dashboard and the in-dash Voltmeter was reading below 12V. Took my 1996 K1500 (5.7L 350) to a parts store and had the alternator tested because the battery is only 11 months old. Guy at the parts store load tested the battery terminals and told me to replace the alternator. I replaced the alternator and the truck fired right up. I took a 300 or so mile trip and yesterday the charging system light came back on and the truck just shut down. Tried to restart it but it just cranked and wouldn't turn over. Tried jumping the battery and nothing happened.

When the tow truck crew got there, one guy crawled underneath my truck with a hammer and started beating the daylights out of my gas tank. When I tried to start the truck, it fired up just fine. He told me the fuel pump was bad and needed replaced.

Any thoughts? Was replacing the alternator unneccesary and should I replace the fuel pump now, too?

Proper testing is the answer.
If your volts dropped under 12 volts and alternator light was on.
Then you had a charging system problem.
It sounds like they tested it and found the problem on that end.
Now if the truck cranks and does not start. You have another problem.
Proper testing again is in order.
When it no start check for spark and fuel.
If spark and no fuel check for 12 volt power and ground to fuel pump.
If voltage and ground and fuel pump does not run then you have a fuel pump problem.
If fuel pump runs check fuel pressure.

Hard to start cold.
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.
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.
Post back fuel pressure readings.
MT

[phantom564]

Thanks for the help...haven't been able to find a fuel pressure gauge to borrow yet...might have to buy one and when I do, I'll post the readings.

[phantom564]

Well I think the problem was solved for me...sort of.

The truck decided to die on me...the fuel pump wouldn't turn on...so I had it towed home and am currently in the process of replacing the pump, sending assembly, and filter. I'm also gonna replace the lines since they are pretty bad, and paint the fuel tank to keep rust out.

Thanks for the help!