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Losing FP after cleaning hfcm

14K views 73 replies 10 participants last post by  bismic1 
#1 ·
Tried this on another forum but getting very little feedback....

Pulled the manifold off my HFCM about a week ago and cleaned all that nasty chit out of there. Hasnt been done in the 3 years I've owned the truck, and figured it hadn't been done ever before. Anyways, put it all back together, and anything over 25% pedal, fuel pressure drops from 58-59 down to 50-52. Bury your foot and it dropped down around 40-42. Pump looked original, so figured I'd throw some somewhat cheap insurance at it and put a new pump on. Same scenario afterwards.

Got to thinking...when I cleaned it, I did it on the frame....didn't remove the HFCM from the truck. When I pulled the manifold, the DTRM spring/nipple fell out. Figured maybe I didnt get that back in there right, and it was sending fuel back to the tank instead of back up top. Pulled it completely off the truck today, and made sure I got all that back in there right. Put it back on the truck, went for a ride, and down goes fuel pressure again. Wtf! I'm at a loss here now. I had ZERO issues holding FP before I cleaned that thing, and now it's this nonsense. What am I overlooking? If it were sucking air around the manifold (like maybe the gasket wasn't just right), would it cause this issue? Even though we aren't talking about a very technical piece here, I made sure to take time and clean all surfaces up really well when putting it back together. Just don't know what else would cause my issue....don't see it being a coincidence happening at the same time I cleaned it.
 
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#2 ·
Drawing air through where the filter spins on , yes it would create similar symptoms to what you're describing . That drastic ? Not sure.

Take it off and triple check all the threads are clean and o ring is intact
 
#3 ·
I did that yesterday when I had it apart again. Well, I cleaned the piss out of the threads, and did a quick once over of the o-ring. I might have a spare oring...if I do I'll replace it just for chits and grins.
 
#5 ·
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#7 ·
Me neither. I sort of think it's sucking air somewhere around either the manifold, or the filter cap.

Anyone know if the oil filter and the HFCM fuel filter caps are the same? Yeah look close to the same, but I haven't held them side by side. I have a spare oil filter cap that I'd slap down there and rule out a cracked cap, bad threads, etc...if they are the same anyways.
 
#8 ·
Yes Dave fuel and oil filter caps are the same.

I assume you used a new gasket for the manifold on the HFCM? Try installing a "T" with a vacuum gauge on the suction side of the HFCM. Anything more than 3-5" of vacuum you have a restriction.
 
#9 ·
Yes sir...new gasket.

Not sure I follow you. I assume you mean between tank and pump correct? And if so, how do I go about teeing into that line! You forget, I'm just a dumb ******* from Mississippi!
 
#14 ·
Ok, I started thinking...not sure if that's a good thing or not.... To rule out a restriction between the tank and hfcm..why can't you push a little compressed air back through the lines towards the tank? Or would that even work? Seems like if it were a clogged pick up or something along those lines, a little air would push it back into the tank, and cure my issue. Then if so, I could just drop the tank and clean, because I'd know that's the issue....or vice versa.

Can it be done, or is it safe to do that? Wasn't sure how delicate the screen was, or if there's some type of check/flow valve in the feed line that would keep air from making it back to the tank.
 
#16 ·
Yea, I wasn't planning on emptying a 30gal tank on it. Just figured a couple quick shots of air would rule out any blockages. I mean if that's the case, I have no problem dropping the tank. Just didn't want to go through the trouble if it wasn't necessary you know.
 
#18 ·
10-4. I'm going to put my spare cap on there tomorrow and see if it does anything...doubtful, but I'm running out of ideas. If that doesn't work, I'll try the air. If that doesn't work....hell, I don't know where to go from there. This truck is pissing me off lately. If I didn't have so much invested in the thing in the past year I'd sell it.
 
#20 ·
Yea bud I know what you meant. By spare cap, I meant my spare filter cap....just to rule out it being cracked, damaged o-ring, etc.

You said air to the "return" line....so I can't put air to the feed line from the tank? Seems like that's the only way to clear any possible blockage....not sure what air to the return line would solve....or am I thinking about this wrong?
 
#21 ·
Changed the caps...no change. Grrr.... Wasn't expecting anything grand, but whatever. I guess I can try pushing a little air back to the tank and rule out a restriction. Seems really odd that would be my issue considering this didn't start happening until I cleaned the HFCM. Seems like if this were a problem, I would have had pressure issues before then. Might back the spring off the regulator and pull the return line and push a little fuel through it, just to make sure there isn't any trash in there....again, which I doubt. Probably should bypass my sending unit for the FP gauge just to make sure it isn't at fault here.

Yea, I'm thoroughly frustrated with this, and tired of crawling under the dam truck!
 
#22 ·
Took sending unit off my buddies truck and put on mine, just to rule that out. No change. Backed the adjustment screw out if the regulator so it would do nothing but push fuel through it and out the return. No change. Figures.

Something I noticed...assuming it's done this the whole time. Sitting in the driveway, truck in park. Rev it up over say 2200-2500 rpms, and pressure drops just like when driving. It went from say 58 down to about 52psi, just from revving it in park. I honestly don't get it. Still need to check for a clogged screen in the tank....might get a chance to this weekend. Although I don't see that being the issue here, but I'll try anything at this point.

I'm about ready to find a used HFCM somewhere and slap on it, just to satisfy my curiosity. It's getting old driving by my FP gauge.
 
#23 ·
Well...still pulling what little hair I have out over this. What I've done so far...

New lower cap and o-ring. No change
New o-ring on upper cap... No change
Pushed a little air back through the feed line to the tank....no change
Did the same through the feed line to the upper housing...no change
Slapped on a used HFCM, with my new pump, cap, and used filter (has less than 1000 miles on it)....no change

I'm at a complete loss here. Only thing left I could possibly think could cause an issue is my regulator, which is only like 3 mths old (Aeromotive). Not sure why it would though. Both of my gauges (mechanical on the regulator, and electric in cab...sending unit is tee'd off regulator with mechanical gauge) fluctuate 2-3psi at idle....like the needle constantly is moving. Does the same cruising at speeds at a steady pedal. I don't get it. No reason a stock pump won't hold pressure with a 155/30, especially a new one.
 
#24 ·
I've been fooling with this all day. Swapped on my old cheap FPR, which did the same thing. Rules a bad regulator out. Figured what the hell....lets do a bubble test, just to be safe. Passed that with no bubbles at all. So it's not an injector leaking. With KOEO, as the pump runs it sounds like air bubbles passing through my regulator. (I've sort of thought maybe it's sucking air somewhere). Unhooked the feed line on engine side of HFCM, ran it into a clear jug and cut the key on. Fuel had air in it at first, then clears up until right before the pump shuts off, then gets bubbly again. So, figured I'd rule out anything in tank. Unhooked the feed line on the tank side, ran some hose from the HFCM into a jug of fuel, with the engine side feed still in my other clear jug. Basically bypassing the fuel tank all together. Key on, and it's bubbly through the entire pump on cycle. So, it's sucks air in the HFCM, for some damn reason. Like I said, this one is new to me, I never removed the manifold, so it shouldn't be an issue there. All I can think of is maybe the o-ring on the filter itself is nicked or something. Either that or the one on the pump itself. Gonna put a new filter in there and see what happens. If this whole ordeal is because of a filter o-ring I might lose what's left of my sanity!
 
#26 ·
Assuming you mean hfcm housing? If so, yes, I found one locally. It's on the truck now.
 
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