Truck Repair: Starting problems with a 90 Chevy 2500, oil pressure switch, fuel pump relay

Question
Thanks for the advice. I have two bits to add. I tried the Carb fluid approach again to no avail. Would the bad relay cause the engine to not turn over at all? Secondly, where  is this fuel pump Relay and the Crank Sensor located? Being a novice, are they accessible for me to pull out an replace without the help of a mechanic?

Thanks,
Greg
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Followup To
Question -
I recently had problems starting my 90' Chevy Cheyenne 2500. It has the 5.0L V8 and 90k on it. I thought at first it was the fuel pump and after banging on the gas tank to get the car started (a trick I read about), I took the truck to have a new fuel pump installed. The shop that did the work replaced the Fuel pump but couldn't get the engine to start. They claim its something in the electrical system, but after 8 weeks of having the vehicle, couldn't diagnose what! Eventually I had enough with them and picked the truck up. The shop had managed to start it with the help of some carb cleaner (the truck is EFI I thought?) and when I got it home, parked it and tried to restart it, again it would not. It just cranks and cranks.. Any suggestions?
Answer -
Hello Greg,
Yes, it is EFI.
If it started with carb cleaner, and then continued to run, I would suspect that the fuel pump is running once it gets power.
I also suspect the fuel pump relay may not be working, and it is running the fuel pump on the backup system, which is an oil pressure switch.
The relay itself could be the problem, or the crank sensor, which tells the ecm the engine is rotating, and it is ok to run the fuel pump.
The oil pressure switch sends battery power to the fuel pump whenever oil pressure is above 4 PSI. I feel the carb cleaner, which is acting like starting fluid, is running the engine long enough to build oil pressure, and then the oil pressure switch takes over.

Van

Answer
Hi Greg,
First lets talk about "Turn Over".
When you turn the key to start, if the starter engages, and the crankshaft is rotating, that is what is commonly referred to as turning over.
The engine can turn over and over as long as the battery will stand it.
It is also sometimes referred to as "cranking over", but cranking is also sometimes used to mean starting, such as the engine cranked.

So much for definitions...

The fuel pump relay is probably mounted on the firewall, under the hood, in front of the passengers area.
There may be a long plastic cover over it, and several other things. It should be marked on the cover, although maybe hard to see.

The relay is very easy to replace. The crank sensor is harder to get to, but not too hard to replace. Have a parts house show you one, and then look for it on your engine.

Now...have you checked to see if there is spark at the plug wires?

Have you checked the fuses, like especially the one marked Inj.

And under that cover under the hood, is a fuse holder for the fuel pump fuse. Make sure it is ok. You can jump power to that fuse, and you should be able to hear the pump in the tank running.

There is one problem that the mechanics might not have ever run across. The tank unit, that the fuel pump is mounted on, has a plastic section. The connector for the power is on the outside of that plastic part, and the power is carried through that plastic part by rivets, that connect to the wires inside the tank that run to the pump.
I have seen those rivets get loose, and not make good contact, thus the pump won't run.

So you need to listen and see if the pump is actually running.  It should run for two seconds when the key is turned to run, and then it will stop, unless the engine is "cranking", or running, at which time it will continue to run, until there is no longer oil pressure, and the engine is not "turning".
Turn the key off for a couple seconds, and then back to run, and the pump should run for another two seconds...etc.

If you jump power to that fuse under the hood, and have the pump running, try to start it then. it is ok to let the pump run...it runs all the time when the engine is running....ALL the time.

Another possible problem is the ignition switch...it might not be sending power to the pump all the time. It might send it in run, but not in start.

Van