Tacoma_IDI
Registered User
I chased around a similar issue on my 1985 6.9 and my 1992 7.2 with the former being the more complex issue. You suspect air intrusion reaching the high-pressure side or the injectors which I think is a good assumption. There are three ways for this to happen that I know of: 1) Fuel leak in the return system (which you've already addressed). 2) Fuel leak in the low pressure side of the system 3) Blockage or lift pump failure in the low pressure side of the system.
A leak or failure in the high-pressure side is possible but unlikely and also the most difficult to troubleshoot so I'd start with what you know. I would NOT recommend fuel system modifications of any type until you're certain you found the cause of the problem. This is a mistake I made with my 6.9. At that point it just makes things more complicated.
Recommendation:
1. Identify fuel leaks of any kind. Check the valley pan very carefully especially under the IP. I use a drip pan under the truck to start with. Suspect areas are: filter gasket, low-pressure o-ring on the IP inlet., steel fuel line "olives" (seals), low pressure lift-pump, tank xfer switch.
2. Does your truck also stumble, stall or is hard to start when already warm? This is often an indication of a leak or blockage on the low pressure side.
3. However, you also seemed to indicate hard starting. In that case I'd check to see if upon initial cranking the lift pump is completely filling the fuel filter. Do this only after eliminating any leaks.
4. Last, consider checking for blockages. If a supply line or return line between the engine and tank is blocked in some way, it can cause the return line to return to the 2nd tank. The fuel tank selection valve only controls the supply lines. The lines inside the tanks are very very close to the tank bottom and anything from tank replacement to a collision can cause the lines to bottom-out on the inside of the tank.
4a. Any easy way to determine if you have a blockage is to temporarily replace the line to the lift pump with a short piece of fuel line from a 5 gal can of diesel. When doing this keep in mind that the 5 gal will disappear super quickly because its also being siphoned into your fuel tank via the return line. (be sure to shut off the truck before it runs dry). If it runs perfectly on the temporary tank, work backwards from your lift pump to find the blockage. Note: A blockage in one tank can effect both tanks because of the return lines.
A leak or failure in the high-pressure side is possible but unlikely and also the most difficult to troubleshoot so I'd start with what you know. I would NOT recommend fuel system modifications of any type until you're certain you found the cause of the problem. This is a mistake I made with my 6.9. At that point it just makes things more complicated.
Recommendation:
1. Identify fuel leaks of any kind. Check the valley pan very carefully especially under the IP. I use a drip pan under the truck to start with. Suspect areas are: filter gasket, low-pressure o-ring on the IP inlet., steel fuel line "olives" (seals), low pressure lift-pump, tank xfer switch.
2. Does your truck also stumble, stall or is hard to start when already warm? This is often an indication of a leak or blockage on the low pressure side.
3. However, you also seemed to indicate hard starting. In that case I'd check to see if upon initial cranking the lift pump is completely filling the fuel filter. Do this only after eliminating any leaks.
4. Last, consider checking for blockages. If a supply line or return line between the engine and tank is blocked in some way, it can cause the return line to return to the 2nd tank. The fuel tank selection valve only controls the supply lines. The lines inside the tanks are very very close to the tank bottom and anything from tank replacement to a collision can cause the lines to bottom-out on the inside of the tank.
4a. Any easy way to determine if you have a blockage is to temporarily replace the line to the lift pump with a short piece of fuel line from a 5 gal can of diesel. When doing this keep in mind that the 5 gal will disappear super quickly because its also being siphoned into your fuel tank via the return line. (be sure to shut off the truck before it runs dry). If it runs perfectly on the temporary tank, work backwards from your lift pump to find the blockage. Note: A blockage in one tank can effect both tanks because of the return lines.
