Just because I've dropped the tank (for cleaning & Hutch mod etc), disconnected fuel lines, drained & cleaned the bowl, added new filter etc. ... the entire fuel delivery route now has quite a bit of air in it.

So I'm curious how all that air is supposed to get purged?

I primed the bowl with fresh fuel prior to installing the filter & cap... but

as I considered all the rest of the air in the lines I realized I didn't know how it's normally handled. Obviously one doesn't want air traveling up to the injectors...so is there some sort of built in purging scheme going on by way of the return line to the tank (via the pressure regulator bypass or something like that)?

Thanks
dave

 

On an unmodified stock fuel system....the Fuel Pressure Regulator is the highest point in the system. The air returns to the tank via that line.

 

OK thanks S&B! -- that's kinda' what I assumed.

So after putting everything back in, I'm also guessing that cycling the pump 4 or 5 times *should be* enough to flush out all the air and then that's all there is to it? (meaning nothing else to worry about)..(?)...

 

I always cycle the key 3 times and let the fuel pump run, most of the air will return back to the tank like Greg said before, if you are near the filter housing when the key is turned on you can hear the air go through the FPR, I cycle it a couple of times and have never had an issue with air on start up after changing the fuel filter.

 

It could be a bit more cackly than usual for typically 50-100 miles on the highway.
You could install a "deadhead" fix... such as the FRx Clay sells at Http://www.Riffraffdiesel.com

Im having an issue wit my fuel gauge sending unit...
Apparently when my gauge reads 1/8th of a tank.. im really out of fuel.

With the FRx installed on Motorbug... it took 2 cycles of the key on fuel pump and she fired right up... ran her up to 3k on the tach... watched the poof of white smoke turn to black and vanish...
All was right in the world.

 

Key on/off will only run the pump for a few seconds. When I do my filter changes, I drain my fuel bowl and replace the element, put it all back together. First key on will place a lot of fuel in the bowl, but to be on the safe side bump the starter. The fuel pump will then run for about 20-30 seconds and purge the air from your system. (I prefer to do this 3 times) If you leave the hood up, you will be able to hear the air escaping from the filter housing and running back to the tank.

Hope that helps ya!

 

Is this considered common knowledge? 'Cause if it ain't, this short thread should be a sticky.

As far as I know (which really isn't very far), there's no mention of doing this in the factory manual. In the past when I've changed the filter in both my '97 and now my '99, I've been very careful to get as much fuel in the filter bowl before I screw the lid on. And I've always had long cranks and poor running until everything works it way out. I almost want to change my filter again just to try it!

 

I very much appreciate those who've weighed in on my original inquiry. It's definitely been very helpful for bettering my understanding (and my overall appreciation) of the 7.3 PSD.

My own recent experience of a no-start condition occurred after driving a short distance (four blocks) and turning the engine off...only to have it not start again barely an hour later (it was running fine just prior to that).

Unfortunately, I don't have AE or any way to scan fault codes yet, so I was left with only reading about problem possibilities on the forum as a guide.

As I began my troubleshooting study here on FTE and after tinkering a bit with it, I concluded that fuel delivery issues were the most likely culprit.

So I changed the fuel filter & cleaned the bowl but that didn't help.

I then proceeded to drop the tank and found clogged strainers. This prompted me to do the hutch mod (with my own stainless-screen adaptation that completely surrounds a self-designed custom pickup foot).

After getting the tank cleaned, re-fitted with the new foot, and then reinstalled... I went through multiple fuel-pump purging cycles (KOEO) before trying once more to get her started again..but once more - no dice.

It was at this point that I became curious as to how air is purged through the fuel system (and thus I posted my question here).

I finally did get her started again (and she runs fine)... but it didn't happen until after I changed the oil & filter (also drained/re-filled HPOP).

But the thing is: I can't be sure that the oil change had much to do with it. The oil level read fine beforehand, and I could see the LPOP was working while cranking. So it's entirely possible (in my mind anyway) that it needed a *multiple* number of times for the fuel pump to be cycled -- and that it was just taking a bit more (much more than I expected) to finally get all the air out of the lines & fuel-bowl following all of the work I did.

Thanks again for everyone's feedback. The explanations from the more knowledgeable guys here has really added to my understanding. Perhaps this will help another freshman like myself if they should have any similar questions as I did.

Cheers ~ dave

 

Getting the Hutch/Harpoon mod is a big improvement for your truck. Got any pics or details on your custom pickup foot? I like seeing homebrew mods!

As for the air intruduced during filter changes, some steps can help. I did fuel filters (pre-pump and bowl) this weekend. Pinch off the hoses with proper tools or gently with visegrips when changing out the pre-pump, and collect the fuel from the bowl and put it back. Cycle the pump once or twice to purge the air. Mine started smooth as silk after one fuel pump cycle. An FRx installed makes it easier of course... if stock I'd cycle the pump 4 or 5 times since it costs nothing and might help.

 

any chance of fuel bleeding down while the truck is parked for a few days putting air back into the system?