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Just out of curiosity, how much fuel is in your tank? Has the pick-up foot ever been changed/ updated? These are prone to breaking apart and pieces getting wedged into the pickup tube. The foot when broken can make the pump starve for fuel around 1/4 tank.
Just out of curiosity, how much fuel is in your tank? Has the pick-up foot ever been changed/ updated? These are prone to breaking apart and pieces getting wedged into the pickup tube. The foot when broken can make the pump starve for fuel around 1/4 tank.
I would have to look, but probably between 1/4 and 1/2 tank.
Just out of curiosity, how much fuel is in your tank? Has the pick-up foot ever been changed/ updated? These are prone to breaking apart and pieces getting wedged into the pickup tube. The foot when broken can make the pump starve for fuel around 1/4 tank.
Great post. I would still be concerned over the P0231 though:
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The 6.0L has a feedback circuit on the fuel pump supply power. This is simply a circuit spliced into the voltage feed to the fuel pump. The splice is usually located close to the PCM. The PCM monitors this feedback circuit to ensure that there is proper voltage being sent to the fuel pump. As it activates the fuel pump relay, it expects to see battery voltage on the feedback circuit. If it sees voltage that is too low or no voltage at all, it may set P0231.
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I would still like to see a picture because I still am not confident we are "on the same page", especially when there is only one fuel pump, it is part of the HFCM, and it is the one I linked to. Maybe it is simply that he has been buying a new complete HFCM, and didn't know that the pump can be bought separately.... I dunno.
I would also like to know if his pump has voltage without the key in the "on" position. The posts weren't clear on that.
Battery voltage, actually. The best way to do this is to disconnect the lead for the alternator and glow plug power at the passenger battery positive terminal so the glow plugs are not pulling voltage down. Key-on, check the voltage at the battery, then check the voltage at the inertia switch, WITH THE HFCM PUMP PLUGGED IN. You are looking for a relatively close voltage between the battery and the inertia switch. You can have bad contacts at the fuel pump relay that will allow a good voltage reading when the HFCM pump is disconnected, but under load with the pump connected, will drop very low.
Some aftermarket harnesses from China are wired incorrectly when purchased from Amazon or eBay.
If you want to test whether the HFCM pump is working, jump the connectors directly to the battery and ground to see if it runs. I've done bare wires, but you can also use a connector from the older harness, even if you cut it out of the harness and later splice it back in.
P0231 is certainly suspicious for a fuel pump relay issue. Jumping 12v directly from the battery to the pump or inertia switch is a pure test of whether the relay is the problem.
Last edited by TooManyToys.; Oct 30, 2025 at 06:51 AM.
Battery voltage, actually. The best way to do this is to disconnect the lead for the alternator and glow plug power at the passenger battery positive terminal so the glow plugs are not pulling voltage down. Key-on, check the voltage at the battery, then check the voltage at the inertia switch, WITH THE HFCM PUMP PLUGGED IN. You are looking for a relatively close voltage between the battery and the inertia switch. You can have bad contacts at the fuel pump relay that will allow a good voltage reading when the HFCM pump is disconnected, but under load with the pump connected, will drop very low.
Some aftermarket harnesses from China are wired incorrectly when purchased from Amazon or eBay.
If you want to test whether the HFCM pump is working, jump the connectors directly to the battery and ground to see if it runs. I've done bare wires, but you can also use a connector from the older harness, even if you cut it out of the harness and later splice it back in.
P0231 is certainly suspicious for a fuel pump relay issue. Jumping 12v directly from the battery to the pump or inertia switch is a pure test of whether the relay is the problem.
good morning amd thank you for the advice. I bought 3 new for oem relays, but I'm unclear which relay is actually for the hfcm. I saw one thread showing the relay on the fuse block under the hood, but then I have read several other stating the relay is non serviceable on the fuse panel under the dash. Can you provide destil on that please? I will test the voltages as you described.
Great post. I would still be concerned over the P0231 though:
.
The 6.0L has a feedback circuit on the fuel pump supply power. This is simply a circuit spliced into the voltage feed to the fuel pump. The splice is usually located close to the PCM. The PCM monitors this feedback circuit to ensure that there is proper voltage being sent to the fuel pump. As it activates the fuel pump relay, it expects to see battery voltage on the feedback circuit. If it sees voltage that is too low or no voltage at all, it may set P0231.
.
I would still like to see a picture because I still am not confident we are "on the same page", especially when there is only one fuel pump, it is part of the HFCM, and it is the one I linked to. Maybe it is simply that he has been buying a new complete HFCM, and didn't know that the pump can be bough separately.... I dunno.
I would also like to know if his pump has voltage without the key in the "on" position. The posts weren't clear on that.
i apologize for not being clear. I did buy a complete unit. It was actually on ebay not amazon.
The fuel pump relay is is permanently mounted in the CJB or fuse box. The only way to fix it is to replace the entire fuse box, use circuit board, medics to repair the fuse, or if you’re good with wiring, splice in your own relay to bypass the relay in the fuse box.
Mark, have you done a write up with pics and parts of the external fuel pump relay? It is something I want to do before the wife and I head to Alaska in the not to distant future. Thanks
Thank you all for the input. I will be running 12v straight to the pump tomorrow and follow up with results. Anyone know where to get a good fuse panel if it does require replacement?
