Air Bubbles Coming from Schrader Valve What could it be?

[nicksorenson]

I just replaced all of the fuel return lines (injector bypass lines) with clear fuel line. Looks good so far. Here's what I noticed when I finished the job:
Parked the truck up a steep slope to purge the air and lots of air in the return line to the Fuel Filter. Once it ran about 1-2 minutes the air completely cleared out.

Then....

I waited 5 minutes and started it again and it still runs rough at start up, but... there is no air in the fuel filter return and injector bypass lines.

So...

It's got to be somewhere else I'm getting air. I pulled the valve core out of the schrader on the fuel filter housing and hooked up a clear line. LOTS of bubbles came out of that line. At first the truck was sputtering like crazy and NO fuel was coming out at all (Only Air). Then it got bubbly for a minute and then only fuel but it took about a minute or so. But I'm sure this is why it starts rough at first. Where do I look?

 

[Agnem]

The classic problem is 1/4 fuel level in the tank, and a perforated pick up tube. Do your test from each tank to see if it is just one tank or the other. The next place to look if from both tanks, is the fuel selector switch (temporarily bypass it to rule it out), then suspect a cracked or otherwise leaky fuel line.

 

[nicksorenson]

It seems to be a problem in both tanks.... And it’s only upon Startup after 5 minutes or more of sitting with the engine off that it’s missing when I start it. If I let it sit any period of time, it looses prime and air is in the fuel filter. All lines on the injector caps and fuel return look fine with no air. The schrader valve will put out mostly air for a minute or two when I start up.



If the IP was leaking somehow could that put air in the filter while sitting? How likely is that? My guess is maybe the fuel tank selector valve, or lift pump has a leak somewhere. But would I see leaking diesel around it if that's the case?

 

[nicksorenson]

Ok... I just did one more diagnostic. I put clear line from the fuel supply line to the lift pump inlet and the fuel looks like Carbonated water going into the lift pump. LOTS of air in that line.

So here's what I'm going to look for... is there anything I could be missing?

-Fuel Supply Switching valve???
-Fuel Pickup in Tank???

Anything else that could be at fault? I've read there is a fuel water separator on this Fuel system but I don't see one. From what I see, it's part of the fuel filter itself. Am I correct? It's the 94 IDI turbo in my signature if that helps.

 

[nicksorenson]

That last post was while I was parked on a steep hill. I drove it back on flat ground and the bubbles from the fuel supply to the lift pump seemed to stop. Now they are very small and only ocassional. But...

to update...

The truck still runs rough if I let it sit for 20 minutes. It's loosing prime. I'd think if the glow plugs were bad it'd run rough. But since I had the engine up to temp, letting it sit 20 minutes I'd think the engine should still be warm enough to not need GPs. Also, the schrader coughing up all that air is an indication that the IP is getting mostly air upon initial start up. Again the fuel injector overflow return lines all have no major air bubbles. Where else can I look?

Can I put clear line on where the metal lines from the Lift Pump to the Fuel filter and from the Filter head to the IP are??? How???

Could my IP be leaking air? I don't see any major fuel leakage below the IP.

 

[icanfixall]

The lift pump is sucking in air. It may be the suction side fitting or the lift pump itself. Has your motor oil gained without you adding any lately.

 

[nicksorenson]

Unless it's just a super small amount it hasn't gained any oil level.

The suction side of the lift pump has clear line and I don't see significant amount of air coming in there now. It's tiny bubbles ocassionally but nothing steady or big. But somehow air is getting in that fuel filter when it sits. I looked also at the connectors on the fuel filter housing (where the sensors are plumbed) and they look/feel dry with no sign of diesel leakage.

I would like to see what's coming out of the lift pump. Is there a way to put clear tubing there? The only other thing I can think of is the line going to the IP. There are only so many places air can be getting into the filter housing.

As far as lines going to the Filter I can see the following:

Filter Supply (from Lift pump)
Fuel Return
IP Feed line going out

It's got to be one of those lines supplying the air. I can see the Fuel return line looks pretty clear. I do notice that what bubbles are in there are at times going one way and at other throttle speeds going the other direction. Not sure if this is normal.

The lift pump is sucking in air. It may be the suction side fitting or the lift pump itself. Has your motor oil gained without you adding any lately.

 

[nicksorenson]

Anybody know where all this air could be getting into my fuel filter?

 

[Agnem]

You could put a clear line before the filter. I'd sooner try another fuel pump though first. Your problems definitely sound suction side, and the pump is the last stop. Now would be a good time to go electric....

 

[swampdigger]

On my truck, I have similar problems. It's the fuel supply line coming off the tank selector valve. If my truck is idling, and I start shaking the line, it'll start running rough.

I'm considering installing the in-tank pumps and selector valve off a '91 F-150 I have for parts. Never have air leaks again, only fuel leaks.

 

[nicksorenson]

I'm going to put a Napa 3.8-5.7psi fuel pump in tomorrow hopefully and see what happens. Yesterday I put clear tubing from the fuel line from the tank to the lift pump input and didn't see any major air there after it purged itself. I shut down and still no major air bubbles (all very small and very few). But it's still starting on air (rough for about 30 seconds now to 1 minute) It's getting better though. I changed all the fuel injector return o-rings and replaced those lines with clear 1/4" tubing.