Hi. The check engine light came on yesterday and I had some difficulty starting up and keeping the truck running this afternoon.

1993 F150, 8' bed, dual tanks, 4.9 L, 5 speed manual.

I ran "Engine Off" tests and got the following.

111
171
172
173
171

I couldn't get a good idle for the other tests yet.

The truck would run on the rear tank (barely) and I would slowly bog down on the front tank til it stalled out. I was going to run fuel pressure tests tonight after church. I was thinking the front fuel pump with the check valve thingie in it was the problem.

Any other suggestions?

 

1-1-1 means the KOEO tests passed. The rest of the codes are memory codes.

171 and 172 are consistent with low fuel pressure. The computer is complaining that the system is reading lean (172), and it has maxed out the injector pulse width trying to compensate, and has failed (171).

Code 173 is the opposite of code 172. If the fuel pressure is intermittant, it is possible that the computer runs the injector pulse width up trying to fix the lean mixture, only to have the pressure come back suddenly. The computer has an underdamped feedback loop, cannot react that fast to sudden changes, and will tend to leave the system enrichened long enough to set code 173.

By all means, check your fuel pressures. The (cheap) regulator could be the culprit, instead of the more expensive and painful to change fuel pump.

 

Thanks for the advice fefarms.

I ran the fule pressure checks and for the following:

Engine off:
Front tank 32 psi
Rear tank 20 psi
(should be 50-60 psi)

Engine running:
Front tank 28-30 psi
Rear tank 20-22 psi
(should be 45 - 60 psi)

With the vaccum hose disconnected I dropped 2-5 psi. (It should have went up 5-10 psi)

I think I will try the pressure regulator first and see what happens.

Any comments so far anyone?

 

Fuel delivery is proportional to the square root of fuel pressure. With these pressures, your air/fuel ratio (without feedback compensation) on the front tank should be about 18:1, and on the rear tank about 23:1.

It should run sort-of OK off the front tank, albeit with stumbles, and setting codes. On the rear tank, it would have trouble running at all.

You might try a quick but crude check of your fuel pressure guage by using it to measure the pressure in one of your tires.

Let's hope it is the regulator at fault...

 

Well the regulator replacement got me the same test results as before. I was thinking that since the pressure is low with both tanks, I would try a fuel filter swap as my last easy fix before dropping the tank.

It's been about 20,000 miles since the last swap, so its due now anyways.

The gauge test with the tires read dead on.

Could the something in the dual tanks be causing the problem?

 

Unless you got a load of bad gasoline, the fuel filter should not be clogged at 20000 miles. Doesn't hurt to replace it though, and it is cheap.

A bad check valve in the front tank will cause low fuel pressure when operating from the back tank. But it would also cause fuel transfer from back to front, which you haven't mentioned.

Check the quality of the power and ground to the fuel pumps. It seems somewhat unlikely that both pumps would have gone bad in the same timeframe. Maybe you got a load of gasoline filled with rust or whatever.

 

You know I did have fuel moving from the front to the rear when I ran off the fron tank. It was slow, about 1/4 tank a month or so, but it did happen.

Will changing the front pump also swap out the check valve?

No luck with the fuel filter either. Same results.

BTW - thanks for your assistance with this.

 

Ford bundles the check valve, pump, and shuttle valve into a service part called the "fuel delivery module". All the components are replaced together. I'm not sure what you would get in an aftermarket part.

A leaking check valve will sap the output of the pump. It would be like there was a leak on the pressure line. The fuel transfer rate you describe is not indicative of a severe enough leak to reduce your fuel pressure that much, however. If fuel moves from front to rear, it is the back tank fuel delivery module that has a bad check valve.


A leaking shuttle valve can cause tank-to-tank fuel transfer, but will not impact the available fuel pressure.

From the sounds of it, you may have been running off only the front pump for some time, having given up on the back pump. This makes it more plausible that both pumps have become tired and have low output.

I encourage you to measure the voltage as far back as possible along the wiring going to the two pumps. High resistance connections, poor grounds, a bad tank switch, or a bad fuel pump relay could result in a low effective voltage at the fuel pump motor, which would result in low fuel pressure output. This would be a common mode failure affecting both pumps.

 

Well I got about 12.6 volts for less than a second at the pumps when the system was priming.

The thing that confuses me is that the pumps are at 6.9 volts after that initial system prime. SHouldn't the voltage frop to zero until the trucks starts running?

All grounds are good on the connections. They zeroed on on the resisgance checks.

Testing the tank switch seems easy enough. How do I go about testing the relay?

 

I also jumpered the fuel tank pumps off the battery. I still get the same pressure readings. I am thinking now it is the check valve.

 

The 6.9 volts could be a back-feed from the FPM line going to the computer. (The computer is connected to the fuel pump power line, so that it can sense the voltage being provided as a diagnostic aid.)

You can force the fuel pump to run by jumpering the FP line in the diagnostic connector to ground (or to SIGRTN). This would let you test the voltage more easily.
Measure the voltage at the tank with the pump continuously running. If the voltage is OK, the fuel pump relay is OK.

 

Well, I got the bed off the truck and can get to both fuel pumps now. I jumpered the pumps and still got the low pressure readings. I think I am done for today. The bed was alot of work, but both tanks were full. I think tommorrow my next steps will be to run both pumps, one at a time and look for overflow in the other tank. If I am reading you right the tank with the bad pump will accept overflow from the other.

Tank voltages were ok when they were just tested.

 

Perhaps you should check for debris in the fuel line in front as well as behind the fuel filter. My 92 F150 decided to act up in a drive across Iowa and the service station found that the fuel line connection had deteriorated and found chucks of rubber and silicon the previous owner used to seal the connection blocking the fuel flow. It is now acting up again (will idle but not run under load) so now I will look at the fuel line connection on the engine side of the filter. Good luck...I am interested in how you solve your problem..

 

Fefarms what's that fuel test, where you disconnect the supply fuel line and put it in a full gas can? Then recheck the fuel pressure, pressure will read ok if the the intank is bad. Ford said thats how they double check my front fuel pump to see if it was in fact the pump that was bad on my 1987 F150. This test may not work on your 1993 truck, fuel system is set up different then mine.

 

Grounding the FP wire engages the FP relay, forcing the fuel pump to run. If you look at the picture of the self test connector on this page:
http://www.troublecodes.net/Ford/

the FP wire is pin 6 of the connector, which is in the bottom row of 4, all the way to the right.

In Mr. Plapczynski's case, he has low fuel pressure even with NO volume demand. This suggests that the problem does not lie with fuel line blockage. A partially blocked fuel pressure might show OK pressure KOEO, but have no pressure under load when fuel has to actually flow through the lines. With the pressure below 30 PSI, I should think the regulator would completely close the fuel return. I'm not certain, however. He could try pinching off the fuel return line to see if the pressure comes up at all.

The reading of 6.9 volts has been bothering me. The test for voltage at the pump needs to be done with the pump connected and running. The idea is to have current flowing through the wires so that we can check for high-resistance connections. It is not sufficient to just pull the connector off and probe the voltage with the fuel pump not in the circuit, even though that is more convenient. We need a voltage test back at the fuel pump with the load in the circuit.

For Mr. Plapczynski's case, we could hope that all of the problems lie with the back pump. It has low output (20 PSI), and also a bad check valve (bleeding off pressure from the front pump and allowing fuel transfer front to back). But the rate of fuel transfer is low enough that I would not give high odds on this curing the front pump as well.

Finally, the post 1989 trucks have a high pressure pump inside each tank. It is not the same arrangement as the 1987 trucks, which had a lift pump in each tank, a transfer valve, and a single high-pressure pump on the frame rail.

A fuel flow volume test is a good diagnostic aid, regardless of the pump arrangement.