97 blazer starts then stops, new fuel pump, new problem

[funkamongus20]

I just put in a brand new fuel pump (second one in 18 months) and it will start but then stops immediately. anyone? Please?

[old_master]

Is the SES light on? Can you hear the pump run after the engine stalls for a second or two? Have you watched fuel pressure when it stalls to confirm a loss of pressure? It could possibly be that the oil pressure sending unit is not giving the fuel pump current. If there are three wires connected to the OPSU, one of them keeps the fuel pump energized provided there is sufficient oil pressure. OPSU is located at the rear of the engine next to the distributor.

See post #6 in this thread for more details.

[BlazerLT]

ignition switch

Replace it, known issue with the 1997.

[MT-2500]

Originally Posted by old_master

Is the SES light on? Can you hear the pump run after the engine stalls for a second or two? Have you watched fuel pressure when it stalls to confirm a loss of pressure? Could possibly be the oil pressure sending unit is not giving the ECM a signal of sufficient oil pressure. There are three wires to the OPSU, one of which keeps the fuel pump relay energized provided there is sufficient oil pressure. OPSU is located at the rear of the engine next to the distributor.

The oil pressure switch is only on there for or only supplies voltage to/for the fuel pump in case the fuel pump relay fails.
MT

[leadalto]

Originally Posted by BlazerLT

ignition switch

Replace it, known issue with the 1997.

One other thing to try is the fuel pump lead in the harness (different locations on various models). It's usually in the engine compartment and looks like a lead that should go somewhere (like to an accessory that wasn't bought) but doesn't go anywhere.

Mine was in a large harness along the firewall (93 model) but I've read about others that are in a harness along the driver's side.

The fuel pump lead is a direct connection to the fuel pump. The books says to connect it and the + terminal of the battery through a fuse, but that's overkill if you use a light guage jumper which would burn open if you connected through a dead short (in other words, a self clearing fault).

When you connect directly to the fuel pump (this is an independent electrical path to the fuel pump) you can forget about the starter switch or oil pressure switch electrical paths. If the fuel pump doesn't run, in all likelyhood, you didn't make a good connection (which happened to me because of a bad alligator clip) or the wiring to the fuel pump or pump itself is defective.

This is probably the easiest way to troubleshoot the circuit if you can find the place where the fuel pump power lead has been provided as a lead out in the harness.

[old_master]

Originally Posted by MT-2500

The oil pressure switch is only on there for or only supplies voltage to/for the fuel pump in case the fuel pump relay fails.
MT

On S&T series vehicles equipped with a three wire oil pressure sending unit, current is supplied from a 20 amp fuse that is hot at all times, directly to the oil pressure sending unit through an orange wire. The grey wire on the oil pressure sending unit goes to the fuel pump. Once sufficient oil pressure has been achieved, current goes directly to the fuel pump, totally bypassing the fuel pump relay. If this switch sticks in the on position, it will allow the fuel pump to run even after the key is turned off. If this switch does not complete the circuit, the fuel pump will not remain energized when the key is in the "RUN" position and the engine will stall. The purpose of this circuit is to shut down the fuel pump when there is a loss of oil pressure in the engine.

The fuel pump relay is triggered by the ECM through the dark green/white wire at initial key “ON”. If crankshaft sensor output is not present, the circuit is de-energized and the pump shuts off. This typically takes about 2 seconds. In the "START" position the circuit is energized continuously. The only reason this circuit is necessary is to supply current to the pump during cranking until oil pressure is insufficient to satisfy the switch in the oil pressure sending unit.

The fuel pump will continue to run for a few moments after the key is turned off due to residual oil pressure satisfying the oil pressure switch. Once the pressure is relieved, the pump stops. You can verify this by removing the fuel pump relay while the engine is running; the pump will continue to run. On the other side of the coin, you can disconnect the oil pressure sending unit while the engine is running, and the pump will stop immediately.

NOTE: The red "fuel pump prime wire" is connected to the grey fuel pump feed wire inside the fuel pump relay. Applying current to this wire bypasses the oil pressure sending unit and the fuel pump relay. (It applies current directly to the fuel pump.)

[MT-2500]

Originally Posted by old_master

On S&T series vehicles equipped with a three wire oil pressure sending unit, current is supplied from a 20 amp fuse that is hot at all times, directly to the oil pressure sending unit through an orange wire. The grey wire on the oil pressure sending unit goes to the fuel pump. Once sufficient oil pressure has been achieved, current goes directly to the fuel pump, totally bypassing the fuel pump relay. If this switch sticks in the on position, it will allow the fuel pump to run even after the key is turned off. If this switch does not complete the circuit, the fuel pump will not remain energized when the key is in the "RUN" position and the engine will stall. The purpose of this circuit is to shut down the fuel pump when there is a loss of oil pressure in the engine.

The fuel pump relay is triggered by the ECM through the dark green/white wire at initial key “ON”. If crankshaft sensor output is not present, the circuit is de-energized and the pump shuts off. This typically takes about 2 seconds. In the "START" position the circuit is energized continuously. The only reason this circuit is necessary is to supply current to the pump during cranking until oil pressure is insufficient to satisfy the switch in the oil pressure sending unit.

The fuel pump will continue to run for a few moments after the key is turned off due to residual oil pressure satisfying the oil pressure switch. Once the pressure is relieved, the pump stops. You can verify this by removing the fuel pump relay while the engine is running; the pump will continue to run. On the other side of the coin, you can disconnect the oil pressure sending unit while the engine is running, and the pump will stop immediately.

NOTE: The red "fuel pump prime wire" is connected to the grey fuel pump feed wire inside the fuel pump relay. Applying current to this wire bypasses the oil pressure sending unit and the fuel pump relay. (It applies current directly to the fuel pump.)

True the oil pressure switch and the fuel pump relay power feed is hot all time feed by the orange wire EMC fuse no 9.
But the oil pressure switch is just along for the ride if the fuel pump relay is working right. And is only on there to supply power to the fuel pump if the relay quits.
But it does feed power to the gray fuel pump wire when the oil pressure is up.
True the oil pressure switch or the fuel pump relay either one can power the fuel pump.
True the relay primes the fuel pump with key on for 30 seconds prime. And also on engine cranking.
But after engine start up the computer turns on and keeps the fuel pump relay on and sends power to the fuel pump with the engine running.
And also the oil pressure switch feeds power to the fuel pump after oil pressure comes up. But it is only feeding power to the fuel pump that already has power if the relay is working.
Or in other words they work side by side to power the fuel pump.
And either one can keep power to the fuel pump if the other one quits.
False you can unplug the oil pressure switch and the fuel pump will quit.
Why because the relay supplies power to the fuel pump at all times while the engine running.
You can unplug the oil pressure switch and the fuel pump will keep running off of the relay. You can unplug the relay and the oil pressure switch will keep the fuel pump running.

Way back in the 60’s on big trucks when electric fuel pumps first came out they used the oil pressure switch to power the fuel pump with a primmer button to prime up the fuel pressure.

In the early 80’s when GM first came out with the computers and fuel injection a few cars also used the true oil pressure switch as the only power to the fuel pump.

But on later gm cars and trucks the fuel pump relay is the main power for the fuel pump.
And the Oil pressure switch is only on there as a backup power in case the relay quits.
Also the oil pressure switch is not used on cars and the newer C and K and S series trucks are doing away with it.

MT

[old_master]

Why would General Motors design a redundant circuit for the fuel pump which would totally bypass the oil pressure sending unit’s ability to shut down the fuel pump in the case of oil pressure loss?

If the fuel pump relay is removed while cranking, the fuel pump will not operate unless the engine can build enough oil pressure to satisfy the oil pressure sending unit switch. If the oil pressure sending unit is unplugged while cranking, the engine will stall after the key is turned back to the "RUN" position and fuel pressure is depleted.

[MT-2500]

Originally Posted by old_master

Why would General Motors design a redundant circuit for the fuel pump which would totally bypass the oil pressure sending unit’s ability to shut down the fuel pump in the case of oil pressure loss?

If the fuel pump relay is removed while cranking, the fuel pump will not operate unless the engine can build enough oil pressure to satisfy the oil pressure sending unit switch. If the oil pressure sending unit is unplugged while cranking, the engine will stall after the key is turned back to the "RUN" position and fuel pressure is depleted.

Old Master it is hard to explain with the key board.
Why GM designed it it that way I do not know. The only thing I found someplace in the repair manuals is they states the Oil pressure switch is not designed as a kill or saftey switch to shut off the fuel pump.
I will look around and see if I can find it and post it when I can.

But first on the oil pressure sending unit it is not designed or put on there to shut down the fuel pump in case of oil pressure lose.
It is put on there to keep the fuel pump running in case the fuel relay quits.
When oil pressure drops it will shut off it's end of power to fuel pump.
But as long as the engine is running the fuel pump relay supplies power to the fuel pump from the relay.
Think of them as a 3 way electrical switch powering a bulb.
The oil pressure switch and the fuel pump relay run side by side.
Either switch can turn on the bulb and/power the fuel pump.
But neither on can kill power to the fuel pump as long as the other one is working or supplying power to it..
But in the case of oil pressure switch and fuel pump relay both are feeding power to the fuel pump with engine running.
So neither one can shut off the fuel pump by thereself.

The fuel pump relay is first turned on by the key on 30 second prime.
Then when engine is cranking the fuel pump relay is turned back on while cranking.
Then when ignition system kicks in or engine running the PCM/VCM computer comands the fuel pump relay on and keeps it on as long as there is ignition or injector pulse while the engine is running.

That is why the oil pressure switch can not kill the engine or fuel pump.
The fuel pump is being supplied voltage by the fuel pump relay as long as the engine is running.

My wiring diagram poster machine is broke right now or I would post you a wiring diagram and some place in the repair manuals it states the info about the oil pressure switch is only on there to keep the fuel pump running in case the fuel pump relay quits.

Here is a good test.
If you are working on your blazer or another one just unplug the oil pressure switch and hook a test light to the fuel pump prime wire and you will see it will start and run normal without it.

MT

[blazee]

Originally Posted by MT-2500

Old Master it is hard to explain with the key board.
Why GM designed it it that way I do not know. The only thing I found someplace in the repair manuals is they states the Oil pressure switch is not designed as a kill or saftey switch to shut off the fuel pump.
I will look around and see if I can find it and post it when I can.

But first on the oil pressure sending unit it is not designed or put on there to shut down the fuel pump in case of oil pressure lose.
It is put on there to keep the fuel pump running in case the fuel relay quits.
When oil pressure drops it will shut off it's end of power to fuel pump.
But as long as the engine is running the fuel pump relay supplies power to the fuel pump from the relay.
Think of them as a 3 way electrical switch powering a bulb.
The oil pressure switch and the fuel pump relay run side by side.
Either switch can turn on the bulb and/power the fuel pump.
But neither on can kill power to the fuel pump as long as the other one is working or supplying power to it..
But in the case of oil pressure switch and fuel pump relay both are feeding power to the fuel pump with engine running.
So neither one can shut off the fuel pump by thereself.

The fuel pump relay is first turned on by the key on 30 second prime.
Then when engine is cranking the fuel pump relay is turned back on while cranking.
Then when ignition system kicks in or engine running the PCM/VCM computer comands the fuel pump relay on and keeps it on as long as there is ignition or injector pulse while the engine is running.

That is why the oil pressure switch can not kill the engine or fuel pump.
The fuel pump is being supplied voltage by the fuel pump relay as long as the engine is running.

My wiring diagram poster machine is broke right now or I would post you a wiring diagram and some place in the repair manuals it states the info about the oil pressure switch is only on there to keep the fuel pump running in case the fuel pump relay quits.

Here is a good test.
If you are working on your blazer or another one just unplug the oil pressure switch and hook a test light to the fuel pump prime wire and you will see it will start and run normal without it.

MT

I've read the same thing, although I must admit, I haven't actually tried running with the oil pressure switch disconnected to see if it's true. I remember reading it, but I don't remember where. It also stated that the oil pressure switch closes at about 4psi, well below the required amount to safely run the engine. The engine only needs 6 psi at idle, but should have at least 20 to drive safely.

[MT-2500]

Originally Posted by blazee

I've read the same thing, although I must admit, I haven't actually tried running with the oil pressure switch disconnected to see if it's true. I remember reading it, but I don't remember where. It also stated that the oil pressure switch closes at about 4psi, well below the required amount to safely run the engine. The engine only needs 6 psi at idle, but should have at least 20 to drive safely.

10-4 on that Blazee
I will scrach around and see if I can find it again.
And if you have any more information on the fuel pump relay and oil pressure circuits and functions post them for us.
It is hard for me to explain it with just the old keyboard.
Thanks
MT

[blazee]

Well, for shits n giggles, I decided to go out and test it. Instead of fumbling with the oil switch harness in the dark, I planned on testing the relay wiring to see if the coil was energized while the engine was running. As most of you know... if the coil is energized then that means the contacts are closed and its suppling power to the pump. I started the blazer and pulled the relay to do the test, and the engine died. Going by the schematics, the only possible explaination for this happening is that the oil pressure switch is redundant to the relay, and my oil pressure switch or circuit is bad.

[old_master]

Thank you for your efforts! I stand corrected on the redundant circuitry. Seems like a rather foolish move on GM's part to one: program the ECM to override the OPSU and two: leave the wiring and switch in place raising the cost of the vehicle. Oh well, I suppose they've done other foolish things too.
Thanks, OM

[Brian R.]

Originally Posted by blazee

Going by the schematics, the only possible explaination for this happening is that the oil pressure switch is redundant to the relay, and my oil pressure switch or circuit is bad.

Is it possible, pulling the fp relay power caused the ECM to shut down the engine?

[blazee]

Originally Posted by Brian R.

Is it possible, pulling the fp relay power caused the ECM to shut down the engine?

I don't think so. It would have to be something that's internal to the ECM and not shown on the schematics. It would also have to be something that changed at some point because I know some guys have pulled theirs and the engine didn't die.