* Selector Valve Failure: The most probable cause is a malfunctioning fuel tank selector valve. This valve controls which tank the fuel pump draws from. If it's stuck or not switching completely, it might be forcing fuel into the pressure line of the *incorrect* tank. The pressure builds because there's no path for the fuel to return. This can also lead to the fuel pump running constantly (as it tries to keep up) and ultimately damage the pump.
* Clogged or Restricted Return Line: Even if the selector valve is working, a blocked or severely restricted fuel return line in the rear tank can prevent fuel from returning to the tank after the pressure builds. Debris, corrosion, or a kinked line could be to blame.
* Faulty Fuel Pump (less likely): While less likely to be the sole cause *if the problem only occurs after switching tanks*, a malfunctioning fuel pump could be contributing if it's not able to regulate pressure correctly or its internal check valve is failing.
* Wiring Issue (less likely): A short or miswiring in the system could improperly energize the fuel pump, causing it to pump fuel into the wrong line. This is unlikely to be the sole cause given your description but possible in conjunction with a valve issue.
Why no MEC (Mechanical Emergency Check Valve)?
Your 1993 F-150 likely *doesn't* have a dedicated MEC valve as part of its design in the same way some newer systems do. The safety function might be incorporated into the fuel pump design, and/or rely heavily on the proper function of the selector valve and the return line. The absence of a standalone MEC makes the correct operation of the selector valve and the return line even more critical.
Troubleshooting Steps:
1. Check the Selector Valve: This is the priority. Try switching the tank selector back and forth several times. Listen for any unusual sounds. If possible, visually inspect the valve (may require removing some components) to see if it's moving freely. Often, these valves can be replaced relatively inexpensively.
2. Inspect Fuel Lines: Carefully examine the fuel lines, both supply and return lines of the rear tank, for any kinks, blockages, or corrosion. Pay special attention to the return line.
3. Test the Fuel Pressure: Use a fuel pressure gauge to measure the pressure in the system. If the pressure is excessively high even when the engine is off, this points strongly towards the selector valve or a return line problem.
4. Listen Carefully: When switching tanks, listen closely for any unusual gurgling or whooshing noises from the fuel tanks. This might help pinpoint the source of the problem.
5. Professional Help: If you're not comfortable working with fuel systems (it is highly flammable!), it's best to take your truck to a qualified mechanic. They have the tools and expertise to diagnose and fix the issue safely.
Remember, working on fuel systems requires caution due to the fire hazard. Always disconnect the battery's negative terminal before starting any work.
