Need a "how to" lesson

[Frank V]

Dealing with the P0251 issue. Before bowing before the IP and FSD gods and abusiing my credit card I'd like to eliminate for sure the air in the pump issue but can't seem to find a 'how to' in confirming if I have air bubbles in my IP or how do I purge them out of there if I do.
I started with the classic engine hic-up or outright stoping. Did all the electrical clean up, checked lift pump, changed filter (boy it was nasty looking). I will run fine for a few days then the SES light up and I get the 0251. The next start up the SES light is out. Ideling at a stop light and suddenly it starts running real rough and then smooths itself out. Do I plop down $360 bucks for SSdiesles FSD cooling kit or is it the pump? What to do? Anyone have a tutorial on how to check and/or remove bubbles.
This is its second pump with less than 30K miles on it.

[DmaxMaverick]

If you want to "see" your bubbles, install a section of clear tubing at the IP return line, in a loop. To stop the bubbles, fix the leak. Either way, the pump or PMD is not a suspect. The leak will be between the IP and fuel tank. Usually the fuel filter assy (damaged assy, or poorly installed filter). If the engine starts, and runs, there's nothing to do about the air, other than fix the leak. Bleeding the system only aids in starting, and the system will bleed the remaining air as it runs. If you have an intermittent problem that shows up while the engine is running, look for a large leak, between the lift pump and fuel tank, or a failed/failing lift pump.

[Frank V]

Thanks for the fast reply. Am I wrong to assume that if there is an air leak, I would also have a fuel leak somewhere? If not an air leak, am I looking at at replacing the FSD module or the whole IP?

[DmaxMaverick]

An air leak will not necessarily have a corresponding fuel leak. Much depends on where the leak is, and the size. A leak between the fuel tank and lift pump will rarely show an external fuel leak. A leak inside the tank (in the pick-up tube) won't show at all without specific testing. A leak between the lift pump and IP (including the filter assy) will generally, at least, seep fuel, but not always. It doesn't take much air to foul the system. It will take a significant amount of air to cause an outright stall, once the engine has been running for a while. This will generally indicate a fuel supply stoppage, or an electrical/electronic failure. With a healthy IP, the engine will run, albeit poorly, with a failed/unpowered lift pump.

PMD and/or IP?? Hard to say. Eliminate the air possibility, then run through the diagnostic steps for pump testing. However, if all the externals and electricals to the IP are healthy, DTC's will usually indicate an internal problem with the IP.