Air in fuel lines

[Mad Maxine]

So I replaced the sending unit in my rear tank last week, and ever since then I'm getting rough idle on start-up. It only runs rough for a few seconds, and then it smooths out and runs fine until the next start-up. I've opened the schrader valve on the fuel filter a couple of times to try to get rid of the air bubble(s), but it keeps coming back. I guess it's not all that big a deal, but would like to know how to get rid of that last bit of air in the line....or maybe I'm sucking air in somewhere...

 

[Wvdirtroad]

I've had luck with air so far by just opening 1-2 injectors a few turns. As for air intrusion, I would double check the lines from your FSV to your tank(s). Assuming you still have the viton/plastic lines, they are pretty stiff and can split/ break fairly easily in my experience. I know on my year truck the FSV is tucked behind the front tank and hard to get to.

 

[franklin2]

If you are driving it down the road, the system will purge itself, no need to crack the injectors.

Usually if you have a air problem on the suction side of the system(fuel switching valve, sending unit, etc.) the pump will suck some air while driving and you wil get a slight to moderate surge to the engine.

If you have a air problem on the return side of the system(fuel filter and lines, injector return lines, return line on injection pump) when you first start the engine from a cold start, it will fire up and then quit. After some cranking it will re-fire and then be ok.

 

[Mad Maxine]

That's what I thought. But I've driven it 60 mi. or so since the sending unit install, and it still acts a little horsey on start-up. Doesn't die, just surges and runs rough for a few seconds, and that's only after it's been sitting for a bit -- I'd say 10 minutes or more. If I run in real quick somewhere--like to the Stop-N-Rob, and crank her back up, runs fine. Seems kinda like an air bubble(s) that rise up...(that's just what I imagine in my noggin ).

I did notice, and I posted this in another thread, that the Goldenrod fuel filter I installed has an air gap at the top, visible thru the plastic bowl. The fuel level is way down on the lower 1/3 of the filter. However, it seems from what I've read that this is normal for that type of filter. I dunno.

 

[gandalf]

That's what I thought. But I've driven it 60 mi. or so since the sending unit install, and it still acts a little horsey on start-up. Doesn't die, just surges and runs rough for a few seconds, and that's only after it's been sitting for a bit -- I'd say 10 minutes or more. If I run in real quick somewhere--like to the Stop-N-Rob, and crank her back up, runs fine. Seems kinda like an air bubble(s) that rise up...(that's just what I imagine in my noggin ).

I did notice, and I posted this in another thread, that the Goldenrod fuel filter I installed has an air gap at the top, visible thru the plastic bowl. The fuel level is way down on the lower 1/3 of the filter. However, it seems from what I've read that this is normal for that type of filter. I dunno.

Goldenrod filter: mine does the same. The bowl runs partially full whenever the engine is sucking fuel. That doesn't present a problem for me. I think that's okay.


Runs a bit rough on startup: From all you've done and all you describe, that sounds like a drainback. It sounds like the fuel is draining back toward the tanks when the fuel pump is not pushing it. I'd start looking at the IP end of the system for just a very slight air intrusion. In fact, I'd start at the individual injectors. It doesn't need to leak fuel in order to allow air in.

Remind us again what you have for a lift pump.

 

[Mad Maxine]

I'm running the stock mechanical lift pump. Looks to be (fairly?) new. Don't know if it's a FoMoCo or Brand-X.

 

[Wvdirtroad]

Did you put a new pickup strainer on inside the tank, or just a piece of hose? Possible the line is sucking to the side of the tank if you did use line/ didn't cut a v relief. I had that issue the first time around working on the tanks.

 

[Mad Maxine]

Did you put a new pickup strainer on inside the tank, or just a piece of hose? Possible the line is sucking to the side of the tank if you did use line/ didn't cut a v relief. I had that issue the first time around working on the tanks.

I did. The sending unit I bought from LMC came with a new strainer.

On another note -- but the same issue -- I notice that when the truck sits for longer periods of time (days) she won't crank over easily. It takes several tries to get her to fire up, and then, of course, she runs rough for a bit and then smooths out. So, I guess the lines aren't holding pressure (maybe) and fuel is bleeding back??

 

[IDIoit]

have you deleted the WFS?
i believe yours has a canister inbetween the fender and the power brake booster.
if you do, delete the stock one and get an aftermarket water fuel separator.

 

[Mad Maxine]

have you deleted the WFS?
i believe yours has a canister inbetween the fender and the power brake booster.
if you do, delete the stock one and get an aftermarket water fuel separator.

Yes. I got a Goldenrod filter. Of which, I have had some concern because there is an air gap at the top (bowl is not all the way full), but I am told by other Goldenrod owners that that is normal.

 

[OLDBULL8]

Check the Olive seal in the hard line (one 3/8" seal at the top) from the lift pump to filter and the other two seals (two 5/16") from filter to IP. When you installed a new lift pump you disturbed that line, those seals get hard and brittle and leak air in.

Put a clear line from #1 injector to filter with a loop in it, that will show from which way the air is coming in.

 

[Mad Maxine]

Check the Olive seal in the hard line (one 3/8" seal at the top) from the lift pump to filter and the other two seals (two 5/16") from filter to IP. When you installed a new lift pump you disturbed that line, those seals get hard and brittle and leak air in.

Put a clear line from #1 injector to filter with a loop in it, that will show from which way the air is coming in.

I didn't mess with the lift pump, just the tank/sending unit. I mentioned the lift pump earlier in the thread just to say that I'm still running a mechanical lift pump and that it appears to be relatively new (just got the truck a couple of months ago).

The things I have done so far in the fuel supply chain:
--Replace the rear tank sending unit
--Remove the fuel selector valve and splice in the rear tank directly
--Install a Goldenrod water separator/filter
--Replace the fuel filter

I'll do the clear line thing, tho.

 

[OLDBULL8]

If those seals have never been changed, they are damn near 30 yrs old, they get hard, don't leak fuel but let air in if someome has just bumped them, no big deal to change them, but it will eliminate a possible cause. You can cut a 1/4" long piece from the proper size hose and use that for a seal or a couple of O'rings stacked.

 

[IH444t]

I have the same problem with my truck, I'm subscribing to this thread

 

[Brad S.]

Put a clear fuel line from the fuel pump to the inlet on the filter head...
Just to monitor air maybe coming from the tanks...???