[92 2.3l] Fuel Pump Voltage Issues - ECM Voltage Drop?

[Nwasyont]

So I've been having acceleration issues with my ranger. I took it to a shop, and they said 'we'll replace the fuel pump.' I said 'I'll do it myself.'

Basically, I've narrowed it down to the green wires after the fuel pump relay.
I've tested the relay, and also got a newer one to test the circuit, and both work properly.

With the ECM connected, the voltage on the circuit drops to ~6.5v after the fuel pump relay. I bypassed the fuel pump relay with a wire and read the circuit between the inertia switch and ECM, and before the fuel pump relay. It's reading 12v without the ECM connected. Whereas with the ECM and no bypass wire, it was reading ~6.5v.

I popped open the ECM and there is no apparent damage.
This one has me stumped.

Could the ECM be drawing 6volts away from the fuel pump circuit?

I've also taken the bed off, and did a voltage test on the fuel pump connector: It reads around 6.5volts, I've read it's supposed to be .5 under battery voltage. So it is low, and explains the 172 lean code I've been getting, as well as the bogged performance.
Battery voltage is around 12.5. So it's not the battery.

Any leads on what this could be? A faulty ECM? Maybe a faulty ground? Everything else seems to be working pretty well in the truck, except for this.

 

[adsm08]

Is this engine running, or engine off.

The pump only gets full power for a few seconds KOEO, then the PCM shuts is down.

It also has the ability to duty cycle the pump relay, which will look to your meter like lowered voltage.

Check your voltages with the engine running if you haven't. This will be the most accurate test.

 

[Nwasyont]

No, this is was with key on, engine off. I'll definitely try it with the engine on tomorrow. Funny that I didn't think to do that.

 

[adsm08]

No, this is was with key on, engine off. I'll definitely try it with the engine on tomorrow. Funny that I didn't think to do that.

Yeah, I think your 6.5V KOEO is normal after the priming cycle.

I would probe only at the relay or the inertia switch to see voltage. I wouldn't pierce the wire under the bed. It seems you have already load-tested the wire's full length, so you can be confident that the reading you get at those points is what is making it to the pump.

 

[RonD]

The ECM Monitors fuel pump voltage, that monitor circuit will show 5 to 8volts when key is on(ECM on), it has no amps just volts.

So what you see is normal, 12volts just after key is turned on, for about 2 seconds, the 5-8volts after that if key is still on.

When ECM Grounds fuel pump relay(turns it on) it expects to see 12volts on that Monitor wire(5-8v wire), if it doesn't then it will set a code

1992 is an OBD1 system so there will be a ViP/OBD connector in the engine bay
There is an image of it here: http://www.therangerstation.com/tech_library/OBD_I.shtml

On that one drawing the FUEL PUMP slot is IDed
That slot is connected to the ECMs Ground wire for the fuel pump relay
Thats how the ECM turns FP Relay on, it grounds its coil.
If you put a jumper wire in that slot and Ground the jumper, then when you turn on the key the fuel pump relay will close and stay closed as long as the key is on
So you should see 12v at inertia switch full time(key on), doesn't hurt anything, its what ECM does, grounds fuel pump relay, but ECM doesn't ground it full time until engine RPMs are above 400

It is a good way to test ECM, and fuel pump relay
Also fuel pump wiring since you have 12volts full time without engine running


The 2 second only 12volt power is a safety feature, if a fuel line should break you don't want gasoline being pumped out until battery dies, lol.
If fuel line breaks engine would stall, under 400RPM, so ECM shuts off power
Most accidents would trigger inertia switch, but what if it didn't.................electric fuel pumps are great, but...........

In the older carb days electric fuel pumps were activated by oil pressure switch, so no oil pressure no 12v to pump.
Carbs had enough fuel in float bowl for start up, then there was oil pressure and electric fuel pump started up

 

[Nwasyont]

Thanks for the replies.

Alright, just got back in from testing it with engine on.

When I turn the key on the multimeter jumps up to around 12v for a few seconds, then back to 0v.

Edit: Just tested what RonD mentioned and got 11.5v at the fuel wire on the inertia switch.
With the engine on, and ECM connected, it runs at 14.8v, is that too high? I thought it was supposed be around 12v. I switched out both relays to make sure, and both ones read 14.8v.

The truck runs, I gave it gas at 2000rpm for 2 minutes and the voltage stayed at 14.8v. When I let off the gas, the engine dies. This is usual for how the truck performs, I have to 'baby' the throttle to make sure she doesn't die, and often times have to press pretty far down on the gas pedal to keep it from dying. Other times it will stall when stopped in traffic and I have to, again, keep giving it gas to prevent stalling. More so in cold weather.

I've already switched out the fuel filter, and cleaned the MAF, IAC, and EGR. Vacuum holds on the EGR. Fuel pressure regulator is dry and no fuel. New spark plugs, and plug wires. Air filter is clean. I have yet to check for vacuum leaks, and want to make a smoke machine to do so.

The tech at the shop was saying he was getting intermittent fuel pressure. So, is it the fuel pump then? I have one ready to pop in, just wanted to be thorough before installing it.

Oh, and I don't seem to see a 'socket' or location for the 02 sensors. It should be at the end of the exhaust manifold, near the catalytic converter, correct? If these are missing would that cause issues?
Edit: the muffler is pretty bad and needs to be replaced, giant hole in it. Any issues from that, apart from sounding terrible?

 

[adsm08]

14.8V is high, but not catastrophically so. Most vehicles will run between 13.5 and 14.5 with the engine at idle.

A missing O2 sensor can certainly cause your issue. I've had a very similar problem when the O2 ground on my B2 wasn't making good contact to the cylinder head.

For a 2.9 the O2 sensor should be just before the cat, where the two sides of the Y-pipe come together.


When you say "fuel control relay" do you mean the fuel pump realy, or the fuel pressure regulator? The pump relay is an electrical component that is nowhere near any fuel, so it shouldn't have gas on it. The pressure regulator is a mechanical component with a vacuum diaphragm and a spring controlling an orifice that changes the fuel pressure in the rail, and it isn't actually a relay of any sort.

 

[RonD]

Battery voltage is 12.3-12.8volts
Battery is ONLY used to start the engine

After engine starts vehicle voltage should(needs to) stay between 13.5v to 14.9volts
Alternator is now powering ALL electrics in the vehicle, and also recharging and keeping battery charged for the next time vehicle needs to be started.
So no, 14v-14.9volts is not too high for fuel pump, it is normal just after starting the engine when battery is drained.
After battery is recharged, 5 minutes or so, alternator's internal voltage regulator should drop system voltage down to under 14volts, 13.5v-13.8v is normal
Over 14volts for long periods will "cook" a 12v battery, cause shorter life


Remove IAC Valve and plug in its 2 wires
Turn key on
IAC Valve should open all the way for starting
Unplug its wires, it should close all the way
Plug wires back in and it should open all the way again
Repeat as you see fit to make sure IAC Valve is working

IAC Valve is how computer keeps engine running when you stop, Idle Air Control Valve
IAC gets 12volts with key on, computer Grounds its other wire for start up, so valve opens all the way.
After startup computer will Pulse the Ground wire, which in essence lowers its voltage so valve closes a bit to set idle RPMs

So you should see Battery Voltage at IAC Valve key on, full 12v and full ground

 

[Nwasyont]

Update: So I found the o2 sensor, it was smaller than I thought and higher up on the exhaust manifold. Looks okay.

I tried out the IAC test. The IAC valve does move when I turn the key to crank the engine. Had a buddy turn the key and it moves. How much movement is good? It did move a few centimeters.
With key on, and no cranking, it didn't move, is that bad?

To make sure IAC motor is working I did apply 12v to it and grounded, and the valve moves.

I tested the connector, the positive side is getting ~11.5volts. Not sure how to test the negative side, or even if it's necessary.

-

yeah, I meant fuel pressure regulator, just edited the post.

I'll look around for an o2 sensor. The engine was rebuilt, and was running fine up until recently. I already checked along the exhaust from the manifold to the muffler, and didn't seem to notice any o2 sensors, but i'll check again.

I'll try out the IAC test, also.

 

[Nwasyont]

Installed a new fuel pump, and the car started up immediately. Hasn't done that in awhile. I pulled an IAC from the yard, and switched it out, pretty much no difference in performance. I ordered a new one, just in case.

Now, the next major project is switching out the old rusted out muffler and pipe.

Any info on a leaky power steering pump? Seems like it's just a loose clamp on the tube.

Interesting note: I was running the engine without butterfly clips on the old fuel pump. With the new pump, I started the engine, and fuel spurted out. Surprised me. A nice illustration of what a dying pump and new pump can show. (I did install butterfly clips onto the fuel lines for the new one)