Funky fuel pressure

[K85 Octane]

I have a FAST system with a Walbro intake pump, with factory pickup. Running PSI is normally 47-50psi.

Damn thing is having a hard time running and actually died on me a couple times in a McDonalds drive through.

Pressure with key only gets up to ~25psi and drops off much faster than normal
Pressure while idling runs up and down from 0-20psi
The fuel pump makes a funny hiss sound every 5 seconds
Any bump of the accelerator makes it stumble bad, sometimes it clears out and will rev nicely, but struggles once it's back idling.

Not sure what's up. I don't have any fuel leaks anywhere. I also have a full gas tank. This just started today and there really isn't anything that's been done lately to cause it.

Whatcha think?

 

[4X4HIGH]

Drop the tank and check the hose between the pump and sender, i'll bet it is split open like many have done.

 

[sweetk30]

yep i have seen that before x2

 

[eagle mark]

Yeah what they said... also is there still a vent and return? Loosen cap and see if it works.

The hiss every 5 seconds sounds like it's trying and then sucks air.

Could also be a filter? If you have an external pump you need a pre filter and high pressure filter after pump. High pressure filters for EFI have a much finer micron rating.

 

[K85 Octane]

I removed the cap when it started happening, was hoping it was something simple like a blocked vent line to the carbon canister. Guess not. Yes, the system has a return line, but that's after the regulator, so the pressure should still be good, or so I would think.

The Walbro pump is placed into the stock in-tank pickup. I have a reusable screen type fuel filter. Like this:

Guess I'll be pulling the sending unit out. Glad I cut a hole in the floor a while back otherwise I'd be dropping a completely full tank

 

[eagle mark]

So the Walbro pump is in tank? Has the filter on bottom?

I really doubt that filter you have is rated at micron filtering needed for EFI injectors? Ones like that I've seen let enough crap past to fill a float bowl of carb with crap.

Still not your issue...

Return line should be after or at regulater. Of course a faulty regulater could be an issue?Could adjust or pinch return line and pressure should raise real quick. If not your back to pump.

HAve you checked voltage at pump? Grounds?

 

[K85 Octane]

I really doubt that filter you have is rated at micron filtering needed for EFI injectors? Ones like that I've seen let enough crap past to fill a float bowl of carb with crap.

regardless if this is my issue, now I'm concerned

I read it filters down to 40 microns. There are some on Summit that filter down to 120 microns. SS mesh vs. paper filters



should I really change it or am I ok?

 

[eagle mark]

If the old memroy still works 40 micron before pump which is like a clear filter used on carbs and 10 micron after pump...

 

[K85 Octane]

well, the intake pickup has the sock on it

the concerned filter is between the TB and pump

the smaller number the better? Seems almost the other way around looking at some of the prices. Ok

 

[eagle mark]

I'm old, I may be wrong...

Nope I was right...

10 Micron filters are recommended for use as a post filter in EFI systems

40 Micron filters are recommended for use as a post filter in carbureted systems

100 Micron filters are recommended for use as a pre filter in carbureted and EFI systems

 

[K85 Octane]

Fudge!

Before I buy another one, I found the filter that came with the FAST kit. The only reason I'm not running it.....was I didn't realize it came with one So I bought a Summit Racing filter. I think that one goes down to 40 microns. Everything on Summit only goes down to 30. Not sure what microns level filter was supplied with the kit, I'm sure it's a good one. I'll try running that. I'll also try squeezing the return line and see what happens. Still haven't done anything about it, found other stuff to take care of.



thanks

 

[K85 Octane]

Cause I hate reading something and not getting the answer. This needs an update.

This problem fixed itself some how. So I left it alone. Few months later, it was back. I had already replaced the external filter with the supplied FAST filter.

Checked voltage. Pre/post relay 12.6ish
Checked vacuum to regulator, good hose though stiff, needs replacing
Squeezed return line, no change in pressure
Removed pump, put in pot with gas, thumb over outlet, lots of pressure
Removed lines from all connections. Blew compressed air through engine side of hoses. Emptied into container at rear. Blew out fuel rails too.

I noticed when I did the return line, it felt clogged. It sprayed back at me, hiccuped and then blew through without a problem. I found nothing but gas in the container I put everything back together (now 1am) and started it up. Ran perfect Took it for a test drive and performed flawlessly.

So, I don't know if it was the return line. Nothing was in the container, so that confuses and worries me. It's got a decent amount of road time plus last weekends offroad trip. Runs great. I should also mention the fuel lines are as far as they could be from the exhaust. Runs down the backbone and along passenger side. Exhaust is completely on the driver side.


So I'll leave like I came in

 

[eagle mark]

No you actually found the problem! Your return line is kinked or collapsed!

If you pinch return line now pressure should rise. Has too! If not run a return line to a gas can and see how much is flowing, check pressure, pinch and see no flow and check pressure.

 

[K85 Octane]

I'll do that. The return line isn't too hard to get to. I'll double check my lines too.

 

[72blazerguy]

Pressure with key only gets up to ~25psi and drops off much faster than normal
Pressure while idling runs up and down from 0-20psi

Whatcha think?

I just installed one of these systems. You should be getting good pressure while at idle and at key on. Mine was running 90+ psi with a restricted return line.

I'm thinking you have a supply problem and not a return problem (or maybe a regulator problem). The system is supposed to run around 43 psi and that Walbro pump they send definitely puts out some gas. If your return line is restricted, pressure should go way up. FWIW

 

[eagle mark]

He's right, I think that is where we started. I just read the OP last post and he did give clues to a return line issue.

But clearly not enough PSI on feed side!

 

[K85 Octane]

Well, I blew out the regulator and every other fuel system related piece. I'm just gunna run it till something happens again. Not sure what is going on but it feels strong atm. I'll be sure to update this down the road with any hiccups.

 

[K85 Octane]

This is still causing me grief.
How can I test the regulator? It is adjustable with a bolt and lock nut on top of the unit.

I'm also looking for a factory vortec fuel pump to replace the walbro unit. I won't get my 50 psi but it will do better than 20.

I'm just so confused about what it could be. Starts up some mornings with 50 psi, I'll get to work pop the hood and find it at 20. Pinch the return line and nothing changes. I haven't yet checked voltage at the pump, only at the relay under the hood. There are also times where it will start up at 40psi, then drop lower. I can definitely feel the difference off idle when the pressure is low, sometimes stumbling when it's down around 10 and fluttering.

 

[K85 Octane]

Ok, not sure which wire does what yet. Does anyone know?

I measured voltage directly at the sender with engine running. At time of test I had 25 psi.
Grey wire had 13.6
Purple wire had 3.5
Both tests used the black ground wire going off the sender, stabbed into for ground.

I went ahead and bought an AC Delco pump from a TPI car, 50-60 psi. Working on it tonight, o god I hope it fixes the problem.

 

[K85 Octane]

So far, so good. Steady 50psi but I won't get too excited as this thing fooled me once already. I will say though, the Walbro pump is both heavier and WAY efing louder than the Delco unit. The pressure gauge has also never been more steady.

Fingers crossed