I think I might have figured out the problem. I've seen where the fuel tank selector valve gets stuck and pumps gas to the other tank as well as other problems.

I'm wondering if the fuel pump was pumping the fuel to the other tank or running out of fuel, would the EEC cut power to the relay?

Has anyone had a faulty tank selector and if so, what happened?

Based on my research so for, a faulty EGR system wouldn't cause the EEC to kill the power to the fuel pump relay.

 

I think you might have figured it out but won’t replace the switch. In that thread or one of your other fuel pump relay threads you said you looked at the switch and it seems fine but you also wrote this:


and


The spring in the ignition switch is what pushes the key back from START to RUN. There should be no reisistance if you turn the key with the switch disconnected from the rod.

No. The computer has no idea that the Dual Function Reservoir is there, what tank is being used or where the unused fuel is going.

On my ‘89 the DFR was stuck on the front tank because the truck sat for almost 10 years. It would not allow fuel to flow from the rear tank.

Correct. They are two completely separate systems.

 

Thanks for the details reply. At this point, after having chased a few rabbits, what seems to be clear is that the EEC is shutting off the fuel pump relay. The thing is that I have no idea why it would do that.

It was suggested that if you have a fuel problem (one of the fuel pumps not getting fuel) that the EEC might shut things down in order to protect the engine.

One solution that looks like a winner, is to jump power straight to the fuel pump relay or straight to the pump (just before the inertia switch). As well as verify that the front tank isn't pumping fuel to the rear tank. It is either doing that, or my fuel gauge just went bad.

So I think the best solution is to open the selector valve and verify it works right (or bypass it) and run a toggle switch to the pump relay itself.

 

I have never heard of the computer shutting off the fuel pump relay to protect the engine. Protect it from what?

How would the older fuel system with a DFR pump fuel to the other tank? That is a common problem with the newer system that uses check valves on each in tank high pressure pump.

 

@13:00 he talks about fuel going from one tank to the other. The valve gets stuck.

I put about 8 gallons in the front tank, then 7 more before a 15 mile drive. After some test idles, it's running out of fuel.

I just ran a toggle switch from the yellow to the brown (fuel pump) and let it run until it stalled. Once it stalled, I switched over the toggle and it was getting lean backfire.

I know that it's starving for fuel, but I'm guessing that the computer shuts down the pump to protect the engine from running too lean. Basically it thinks it's out of gas, but what's happening is that the dual tank system is failing.

If you open the fuel tank selector valve, you can see where it's getting stuck and that causes the pump to move the fuel from one tank to the other.

@16:00 in the video, he shows where the problem is.

One solution is a manual switched brass ball valve. I don't use both tanks, but I do have to pass California smog, so I think I have to have it working, IDK.

At least I know that applying power to the brown wire doesn't fix the problem, it's the tank that is probably empty because the selector valve is defective.

IDK what a DFR pump is, I'm just looking at the videos online.

 

DFR is Dual Function Reservoir, which is the proper name for the "fuel tank selector valve".
The computer doesn't switch the fuel pumps based on lean or rich conditions.
The computer grounds the coil of the fuel pump relay, which powers the fuel pumps, for about two seconds, whenever the key is cycled from the off to the run position.
The computer then sees a signal from the PIP sensor in the top of the distributor, which tells the computer if the engine is turning or not. As long as the engine is turning, the computer keeps the fuel pump relay coil grounded, and the fuel pump circuit receives power.
Do you have a volt meter?
You need to determine if your fuel pump relay is dropping out, at the time the engine stalls.
If the fuel pump relay is dropping out, you need to test the ECM relay, to see if it is losing power to it's coil. That power is coming from the ignition switch.
When the ignition switch powers the ECM relay coil, power flows through the normally open contacts of the ECM relay, to the ECM and to the fuel pump relay coil.
Then the above described process, of the ECM grounding the fuel pump relay coil takes place.
If the Dual Function Reservoir isn't functional, there are tests you can do at it's fuel line connections.

 

Any time the battery is connected, you should have 12 volts at the purple dots, when tested to battery negative.
When the key is on, 12 volts goes to the red dot and powers the ECM relay coil.
Powering the ECM relay coil, moves the normally open contacts inside the relay and closes them, sending power to the green dots.
The blue dot is the fuel pump relay ground. When the ECM grounds that wire, the fuel pump relay coil is energized and power flows to the yellow dot.

Did you install a toggle switch between the yellow and brown wires at the fuel pump relay?
The yellow is coming from the ignition switch mounted on your steering column.

You can test the wiring at the relays with wiring piercing probes or a straight pin or you can back probe the terminals of the relay sockets.

 

They have called the crossover valve at least 4 different things over the years
Here is a 89 PCED calling it the Mechanical Selector Valve / Reservoir
The Dual function ones also had a fuel filter inside them that was only used on ones like yours
Early broncos and Rangers had the dual function one with the fuel filter inside
You will hear me call it a crossover valve most of the time

There is a bunch of diagnosis concerning these crossover valves in the PCED

 

After he messed with the valve in the reservoir it started sending fuel to the wrong tank. It wasn’t doing that before or after he played around with it again. It must have been stuck in between which he later corrected. Are you having that problem? You haven’t said that before. Is some of the 15 gallons in the front and now showing in the rear tank?

I posted the screenshot from your other thread where you bypassed using the key and manually moved the ignition switch from START to RUN and the truck ran fine for 35 minutes one time and 25 minutes another time until you shut it off. IMO, if the problem was the reservoir it would not stay running. You also stated that it was difficult to move the switch and that there seemed to be an “extra” spot between RUN and OFF where the truck won’t run but has power. Where did you tell us the test results from the ignition switch tests from yet another of your threads other than “it looks good”

 

That's the best test information I've ever seen for the valve.
Anyway, if he put a toggle switch between the yellow and brown, I think more electrical testing is needed, to determine if the fuel pumps are being properly powered.

 

Do you have a volt meter?

Yes

>You need to determine if your fuel pump relay is dropping out, at the time the engine stalls.

I did. The power to the brown wire at the fuel pump relay loses it's power. It (the brown wire) has power when it's running, and that power is gone once it stalls.

>If the fuel pump relay is dropping out, you need to test the ECM relay, to see if it is losing power to it's coil. That power is coming from the ignition switch.

The ECM relay is the one right next to the fuel pump relay, AKA ECC power relay? It's the one with the brown wire and fuze link to the red wire?

I'm glad you pointed out the spinning engine part, that sensor going bad could be an issue.

 

Yes, the EEC relay is the one with the yellow wire (purple dot), being fed from the brown fusible link.

Jumper "A" to "E" at your diagnostic plug, located in the same general area as the EEC and fuel pump relay.

Then start the engine. This jumper provides the ground for the fuel pump relay coil, and effectively tests the functionality of the EEC and fuel pump relays.

If the engine stalls, as previously, check the white and light blue wire (red dot), at the EEC relay. Power should be constant there, as long as the key is in the run position.
Then, if power is there, move to the red wires ( green dots), and check for power.
If the red wires have power, check the yellow wire (purple dot), at the fuel pump relay.

If the engine doesn't stall, with "A" and "E" jumpered, you may have an issue with your PIP sensor or computer or faulty, corroded grounds.
If the engine stalls, with "A" and "E" jumpered, and you have voltage at the white with light blue, red, and yellow wires, as tested above, you probably have a fuel pressure issue.
Possible causes could be bad fuel pumps, bad wiring to the pumps or a bad fuel tank selector valve (DFR).

 

Quick update. The rear tank has no pump and hasn't been used in about 20 years. It's full of gas and the front tank seems to be empty, the fuel gauge doesn't work, but it has a hallow sound to it.

I guess I have to drain the tank to remove the selector valve, so I'm going to pump out the gas from the rear tank.

 

No. That is not needed to replace the DFR.

 

Quick update #2
Inside the reservoir of the selector valve was 1 green O ring sitting at the bottom and one black O ring partially off.

The rods that move the plate move pretty well, but I'm not sure how far they are supposed to move, I'll have to look that up.

So for some reason, the O rings inside the reservoir blew out. The plunger for the black O ring snapped back in once I pulled it out.

I wonder if I need this selector to pass California smog check, anyone know for sure?