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trouble with a 350 tbi.. again

84CUCV

84CUCV

3/4 ton status
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Jul 1, 2002
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NJ for now
its a 89 suburban. all stock 350. took a bit of cranking to start, but it did start. trying to get it back on the road. put 4 gallons of fresh fuel. would only start with starting fluid. ran to long to be just on starting fluid. then it just dies. not like its losoing fuel. it just shuts off.

any ideas?? thanks
 
any luck? any codes? check fuel presure? If you don't have a fuel pressure gauge and are thinking of replacing parts before diagnoosing I would start with a fuel filter as it seems you were storing it empty and added fuel...

Also when taking a vehicle out of storage, EFI needs good voltage. Is the batterey good?
 
no not yet. truck is about 1 1/2 hours away from me now. hope to get there soon
 
Ignition module? cant even think of what that is. ill check it out
 
84CUCV said:
Ignition module? cant even think of what that is. ill check it out
Click to expand...
It is located on the distributor. It's bolted on the base under the cap and rotor and has a pigtail plugged into it. If you replace it, make sure to follow the instructions in regard to using (or not using) di-electric grease.
 
You may have heard me say this before but have the auto parts store check it three times. If it fails any it's no good. It is the only piece in these EFI systems that is tempermental...
 
Does it run longer than 3 seconds? If you oil pressure switch is bad it, shuts off the fuel pump after 3 seconds. This is designed to protect engine in event of oil pressure loss, but it has a 3 second delay to allow the engine to start since you have low oil pressure when cranking the engine.
 
Big6ft6 said:
Does it run longer than 3 seconds? If you oil pressure switch is bad it, shuts off the fuel pump after 3 seconds. This is designed to protect engine in event of oil pressure loss, but it has a 3 second delay to allow the engine to start since you have low oil pressure when cranking the engine.
Click to expand...

I think the way the oil pressure switch and fuel pump relay work are not well understood.

1. If the oil pressure switch is bad, and the fuel pump relay is good, the engine will start ok, and run as normal.

2. If the the oil pressure switch is good, and fuel pump relay is bad, the engine will be hard to start, but the engine will run normal once the oil pressure switch closes and sends power to the fuel pump.

The oil pressure switch is a back up source of power to the fuel pump, if the fuel pump relay should happen to fail.

dave w
 
dave w said:
I think the way the oil pressure switch and fuel pump relay work are not well understood.

1. If the oil pressure switch is bad, and the fuel pump relay is good, the engine will start ok, and run as normal.

2. If the the oil pressure switch is good, and fuel pump relay is bad, the engine will be hard to start, but the engine will run normal once the oil pressure switch closes and sends power to the fuel pump.

The oil pressure switch is a back up source of power to the fuel pump, if the fuel pump relay should happen to fail.

dave w
Click to expand...

Dave, I think you're right and wrong, I also mispoke earlier. The fuel pump gets its regular power through the oil pressure switch, so it requires oil pressure to run. However, when first cranking the engine there isn't immediately any oil pressure. The purpose of the realy is to provider 2-3 seconds of power to the fuel pump until oil pressure comes up.

So your number 1 above is incorrect, if you have a good fuel pump relay but a bad oil pressure switch, the relay will provide power to the fuel pump long enough to start the engine, but if oil pressure does not come up and trip the oil pressure switch, the fuel pump will lose power when the relay opens after those 2-3 seconds. So in number one above, your truck would start, but would die in a few seconds.

Your number 2 above is correct, with a good oil pressure switch but bad relay, it will be hard to start becuase your fuel pump will not have power until you crank long enough to buid oil pressure and power the pump through the oil pressure circuit.

The purpose of this feature is not to protect the engine from low oil, but to stop the fuel pump in the event of an accident if the engine dies, or you roll over and lose pressure, the fuel pump stops pumping fuel all over.

At least this is my understanding.:rolleyes:
 
I've talk to several different people about how the oil pressure switch and fuel pump relay work. Not everyone agrees with me?:confused:

Here is what I know happens.

Turn the ignition switch to the "ON" position, the computer will energize the fuel pump relay for a few seconds; then de-energize the fuel pump relay if the the computer does not get the reference pulse from the ignition module in the distributor. As long as the computer is getting the reference pulse from the ignition module, the computer will keep energizing the fuel pump relay.

The oil pressure switch is simple, 3 ~ 5 psi of oil pressure and the switch closes and sends power ( redundant) to the fuel pump.

The GM engineers' are concerned about vehicle safety, and designed the fuel pump relay and oil pressure switch system to prevent the fuel pump from running in the event of an accident. If more people looked closely at the fuel pump relay and oil pressure switch as a safety system, then I think they would see why I say what I say about the functional description of the fuel pump system.

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I've verified all this on my TBI test bench, and that is how I know what is happening. Build your own TBI test bench and you will be able to verify the fuel pump relay and oil pressure system for yourself, and not rely on what others say.:doah:

dave w
 
Alight, I'm tempted to believe Dave's understanding of how this system works.:rolleyes: I don't have a test bench and, as Dave suggests, I am simply relying on what other have said and a little review of wiring diagrams.

Dave do you know why GM added the redundency for energizing the fuel pump? It would seem redundency would not be what you want if this is to shut down the pump in an accident. I guess the "redundency" would work to shut off the pump except in a roll over, where the oil pump was starved, but engine was still running.

So if I understand correctly, the pump always recieving power from both places when running? So if I take off my oil pressure switch and start the truck it will keep running fine?

Thanks for the new knowledge, I've been operating under the old assumption for a while :dunno: But am curious to understand more of what you've learned, I realy like understanding this stuff and you seem to have really dug into it!:bow:
 
Big6ft6,

I might have gone on a bit of a rant about how the fuel pump relay and oil pressure switch work. I've posted a better picture of the Fuel Pump Relay and Oil Pressure Switch schematic I used for my TBI test bench.

Org - Orange wire is battery positive (+) power.
Red - Red wire is Fuel Pump Prime.
Gry - Gray wire is positive (+) power to the fuel pump.
Dk Grn / Wht - Dark Green wire with a White stripe is positive (+) power from the computer to the fuel pump relay.
Blk / Wht - Black wire with a White stripe is chassis ground (-).

dave w

Fuel Pump.JPG
 
Thanks Dave,

All I can tell from the wiring diagram (I'm not an EE :)) is that the relay and pressure switch are wired in parallel to the fuel pump, which I've always understood. What I cannot tell from the wiring diagram is how they operate.

My understanding was that the relay was timed and it opened up shortly after starting the engine. After that, the fuel pump was only energized through the oil pressure switch circuit. So if your oil pressure switch went bad, or you lost oil pressure, your fuel pump would shut off. You'd be able to restart using the relay circuit, but it would die after a few seconds when the relay opened.

What I think I understand from your testing is you've determined that the fuel pump is ALWAYS enegrized through BOTH circuits meaning either the relay or the pressure switch could be removed from the equation and your fuel pump would still run and your truck would drive fine?

In other words If my truck was running and I went out and unplugged my oil pressure switch nothing would happen the truck would keep running and driving fine?

Thanks for your help, this is quite important for me to udnerstand when I'm troubleshooting problems.
 
Big6ft6 said:
Thanks Dave,

All I can tell from the wiring diagram (I'm not an EE :)) is that the relay and pressure switch are wired in parallel to the fuel pump, which I've always understood. What I cannot tell from the wiring diagram is how they operate.

My understanding was that the relay was timed and it opened up shortly after starting the engine. After that, the fuel pump was only energized through the oil pressure switch circuit. So if your oil pressure switch went bad, or you lost oil pressure, your fuel pump would shut off. You'd be able to restart using the relay circuit, but it would die after a few seconds when the relay opened.

What I think I understand from your testing is you've determined that the fuel pump is ALWAYS enegrized through BOTH circuits meaning either the relay or the pressure switch could be removed from the equation and your fuel pump would still run and your truck would drive fine?

In other words If my truck was running and I went out and unplugged my oil pressure switch nothing would happen the truck would keep running and driving fine?

Thanks for your help, this is quite important for me to udnerstand when I'm troubleshooting problems.
Click to expand...

On my TBI test bench, I used an electric drill to spin the distributor. When I turned on my ignition switch, the fuel pump relay would turn on for only a few seconds. Then, with the ignition switch on, I would use my electric drill to spin the distributor which sends a reference pulse (engine is spinning / RPM) to the computer. Once the reference pulse is received by the computer, the fuel pump relay would turn on, and stay on as long as I kept the electric drill spinning the distributor.

So yes, "If my truck was running and I went out and unplugged my oil pressure switch nothing would happen the truck would keep running and driving fine?"

Just remember, some trucks use the oil pressure switch for the oil pressure light / oil pressure gauage also, so unplugging the oil pressure switch could cause the oil pressure light to come on or the oil pressure gauage to show ZERO.

dave w
 
Dave,

Thanks for the explanation, there is definitly a misunderstanding out on "the street" about this. The incorrect info must've started from people hearing the relay timing out when you first turn the ignition on, and assuming that was the extent of the relay's purpose. Thanks to people like you who take the time to look deeper and find out the real answer!

I'm converted! :waytogo:
 
finally got down here. working on it now. fully charged battery. cranks real good now. found a bad fuse and some creative wiring i didnt see before. trying to figure it out. kinda a mess under there.. dont know to much about wiring yet. see how it goes
 
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