Starting problem

[texas tough]

When the ignition switch is turned to the ON position, the fuel pump relay activates the electric fuel pump for 1.5-2.0 seconds to prime the injector. If the ECM/PCM does not receive reference pulses from the distributor after this time, the ECM/PCM signals the relay to turn the fuel pump off. The relay will once again activate the fuel pump when the ECM/PCM receives distributor reference pulses.

The oil pressure sender is the backup for the fuel pump relay. The sender has 2 circuits, 1 for the instrument cluster light or gauge, the other to activate the fuel pump if the relay fails. If the fuse relay has failed, the sender activates the fuel pump when oil pressure reaches 4 psi. Thus a failed fuel pump relay would cause a longer crank, especially in cold weather. If the fuel pump fails, a no start condition exists.

 

[delta_p]

So the truck would run with a good oil switch and failed relay down to 4 PSI oil pressure? Bad on GM

 

[Schurkey]

So the truck would run with a good oil switch and failed relay down to 4 PSI oil pressure? Bad on GM

Please explain how this is "bad".

In aerospace, it's called "Redundancy", when critical systems are provided by multiple delivery paths. WITHOUT the oil pressure switch, when the relay fails, the truck dies instantly--or never starts.

With the oil pressure switch, when the relay fails, you're given notice that something is wrong (extended cranking time) but the truck starts and runs as good as ever. All you have to do is notice that cranking time is longer than normal, and find out why.

I still have issues with the way it's explained by Texas Tough; but until I can find a wiring diagram, it's at least close enough if not entirely correct.

 

[delta_p]

It's correct, practically verbatim as in a service manual.

I mean it's good the redundant backup to the relay. I just always thought it closed contacts at a higher pressure like around normal idle oil with some margin alllowed below. 4 psi is what I thought a oil dummy light coming on was.
But in terms of redundancy, I thought it was an oh **** backup. Like barreling along at 75 and relay fails and suddenly driver were to lose brake boost, power steering, whatever. Maybe that's not a valid failure. But yeah, experiencing long cranking times to diagnose is a good thing too.

I have a spare switch/sender I'd like to checkout on the bench for the 4 psi for the hell of it.

 

[dave s]

I chased my tail for years with what I thought was the similar issues on my 98 5.7. Turned out to not be the fuel pump, FPR, Spider, etc., but the ignition wire in the steering column. The part that plugs into where the key goes fails. It used to crank and crank until the battery would die. I'm not saying that's your problem, but just a different avenue.
Best of luck!

Thanks for the info!

 

[dave s]

Yes it is.

GM spec for fuel pressure is 9 - 13PSI as mentioned previously but these trucks work better if it's 11 - 13. Low fuel pressure will cause hard starting. As @Schurkey said you should see pressure for 2 seconds when you turn the key on. That will show that your pump relay is working.

Does this mean just turning the key to on? Or while cranking as well? I'll aim my phone at it and check just as soon as I get it back together.

 

[Schurkey]

From the moment the ignition switch goes to "On", you should have a ~2-second pulse of fuel pressure.

Does not matter if the key immediately goes to "Crank" within that two-second prime pulse.

After you've had the 2-second pulse, you'd need to turn the key off and wait some short period of time before the computer 'resets" and provides another pulse when the key is turned. You can't just turn the key on and off and on and off and get 2-second pulses.

Some vehicles--7.4L, maybe high altitude--can have a "Hot Fuel" module that gives a longer priming pulse. Maybe seven or ten seconds. I've never worked with them.

 

[texas tough]

if

So the truck would run with a good oil switch and failed relay down to 4 PSI oil pressure? Bad on GM

it icranks over at 4 psi, I think foot to the carpet it should at least triple ?? ha

 

[texas tough]

So the truck would run with a good oil switch and failed relay down to 4 PSI oil pressure? Bad on GM

I think the reasoning is, if your cranking, and the ecm sees that there is at least some fuel pressure during cranking, it will let the motor start .\
Vehicle manufacturers NEVER say its to avoid a safety concern (that would invite liability if a failure occured}... GM says the secondary path thru the oil switch is to prevent the motor from locking up.

 

[evilunclegrimace]

I think the reasoning is, if your cranking, and the ecm sees that there is at least some fuel pressure during cranking, it will let the motor start .\
Vehicle manufacturers NEVER say its to avoid a safety concern (that would invite liability if a failure occured}... GM says the secondary path thru the oil switch is to prevent the motor from locking up.

The ECM does not see fuel pressure. I think what you meant to say is if the OP switch closes when the system builds enough pressure the truck will start.