No Start -- Fuel Pressure? Oil Pressure?

[mudpie]

I'll start off the same way I started off with other questions I had....

I'm not a Chevy guy. I have some skills, but my family always owned Toyota's, and I'm a Volvo guy. The truck I have now is the first Chevy I've ever owned, so if my questions fall into the category of "Damn, everybody knows that" then I'll apologize in advance.

Also, I did search, but got no results. I assume the search feature is down, since I got no results from a search for "oil pressure". Seems like pretty basic search terms to me.

Truck in question is a '96 C2500 HD 5.7, and it recently developed some issues starting. It cranks and cranks and cranks, and eventually will start. Once it does start, it seems to run fine.

My first thought was maybe a fuel issue. When the truck was cold, I put a fuel gauge on it and cranked it. Cranking pressure is zero, for about 7 or 8 seconds while it cranks, then it jumps to 53 psi and starts. When I shut it off I can hear the fuel pump continue to run for a couple seconds, and the pressure jumps to 60 psi. Then the pump shuts off, pressure immediately drops to 30, and then slowly drops to zero.

Then I warm it up, and nothing changes. Same cranking and cranking, and eventually starting.

I noticed that the oil pressure gauge on the dash was reading zero for the 7 or 8 seconds that the truck was cranking. Eventually the oil pressure would start to rise, and when it got to an indicated 45 psi the truck would start. So, now I'm thinking maybe the fuel pump doesn't turn on until there is oil pressure, and I'm thinking this is an oil pressure problem, not a fuel problem.

The last couple days the truck has gotten to the point where it will only start when cold. Same cranking, same fuel and oil pressure issues. I'll start it, drive it a couple miles to warm it up, and when I shut it off and try to re-start it the oil pressure only goes up to an indicated 35 psi, and it won't start no matter how long I crank it. When it's running, oil pressure is a pretty consistent 45 psi.

My thought is the oil pump. However, I've seen multiple threads on various forums where people talk about oil pressure readings in the 20's on vehicles with a 5.7 Vortec, and they run fine.

I don't want to just throw parts at the truck and hope it solves the problem. Any suggestions on what I should be looking at? Or, could it be as simple as just an oil pump?

Any input appreciated.

 

[skylark]

Your testing indicates a weak fuel pump. It should be 55-60 almost instantly when you turn the key. It should also hold pressure when you shut the engine off.

 

[mudpie]

Thanks for the input. I appreciate it. Any advice on whether dropping the tank or lifting the bed would be easier?

 

[mudpie]

Now that I think about this a little, I'm curious about the how and why part. If my fuel pressure is the same when the truck is cold or fully warmed, why would it start cold but not warm?

 

[mudpie]

New fuel pump, and now it won't start at all

 

[Black_cirrus]

Check your Fuel Pump relay, Chevy runs the relay, then as a back up the oil pressure switch will pass current to the fuel pump once the engine has oil pressure. You'd think they would shut it down with no pressure but believe me seen it many times. Sounds like I'd check the relay, and if you've been driving it like that for a bit replace the Oil Pressure sensor because they don't handle the current for long. And I hope you don't have my last personal truck fuel pump issue, someone poked the wire to the fuel pump over 5 years ago and corrosion killed the wire. Poke the wire seal it up!

 

[CAPTNDAVIE]

I wish you luck. I have a similar problem with a Vortec 7400.

 

[tunedbytuna]

Have you checked the injectors? Or the fuel pressure regulator? If it's bleeding down I would question if you have a leaky fuel injector or a bad pressure regulator.


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[mudpie]

Finally got a break in the weather....

Fuel pump works. I can hear it. Put a jumper wire across the relay terminals just to verify. Truck still won't start.

Verified spark with my timing light. Beyond that, I'm not sure what to check. I'm just not familiar enough with GM.

Any suggestions appreciated

 

[steampunk1375]

Fuel pressure regulator I bet. Its bleeding off line pressure too fast after shutdown. Also try cycling the key four or five times just to let the fuel pump run. So turn the key on until you hear the fuel pump turn off and do it again like 5 times then try and start it.