Help! fuel return lines

[adyszel]

1985 6.9 F-250 C-6
I recently redid the head gaskets in my engine...
I'm getting major air pressurizing in my return fuel line near the fuel filter. Makes for a hard start if I don't start it on the first try. If I do start it on the first try, it runs with too much blue-white smoke leading me to believe its got air coming through the lines. When I turn it off after running it for a minute or two I can hear a hiss when I take off the return line from the filter elbow-- mostly air, very little diesel. Can a leak at a injector cap or return line really cause this much air intrusion? (I replaced 2/3 of the injector o-rings that seemed like they would not reseal-- i know that was poor judgment. I also have new return hoses) My neighbor has a slow leak from an injector cap but he can still start his truck reasonably well. Is there some other place that could cause such an obvious air leak? I replaced the sleeves and o-ring in the fuel filter to injection pump metal line connection because that seemed to be eaten up pretty bad. I had heard theres a check valve on the injection pump return... can this leak badly enough to cause major air leaks?

 

[CDX825]

If you have a constant air coming from the bleeder valve then my guess would be its in your supply line. First two things that come to mind are the water separator and fuel pickup.

 

[hesutton]

Plus 1 on what Justin said. Your fuel pickups may be broken off in the tanks (very common) and you are sucking air/fuel into the lines even though the gauge reads 1/4 tank or so. Also, the stock water separator on the '83-'86 IDI's are notorious for air leaks as well

Heath

 

[adyszel]

Clarification

When i say constant air i mean out of the return line that comes from the fuel filter outlet elbow (i think this design changed in later models) I believe my elbow doesn't have a check valve but I'm not sure. So I believe its letting in a lot of that air. I had fuel coming out of the schrader, didn't check the pressure but I may do that next.

 

[CDX825]

So you have the line pulled off when its running? I'm kinda confused what you mean. I know what elbow your talking about just not how your checking it.

 

[adyszel]

Justin - basically I ripped the return line off the elbow after trying to start it. I heard discharge of air. Thinking about it, it could be coming from either direction -- the fuel filter or the return line bc i don't think it has a check valve.

After I started it started seemed fuel starved, I put about 3 gals of my off-road diesel (shh) in it. So i think that would rule out the pick up issue, right? The gauge said the fuel was a little low but those POS gauges change with the weather (literally). Also, my f/w separator is aftermarket. (Fleetgaurd i believe.) Its got a pump on it that could leak but i torqued it so its very tight and have pledged not to try to use the pump again. It also has a filter that I don't know when it was replaced. Also I have not replaced the main fuel filter. I will probably replace these but I don't think that was the problem. Also, I had the truck running well for a few days when it was warm but the starts were hard. I keep thinking its the return line since those are problematic, but I guess I should start at square 1 and check supply PSI.

 

[CDX825]

Ok well we can rule out the water separator.

How much fuel do you have in the tank? Seems you need at least 1/3 tank to get above the break off point.

If I understand right the only time the return line leaks cause problem is when the engine is not running an there is no longer fuel pressure to keep the air out. Now the supply from the pump back is under a vacuum so air will get pulled in while the engine is running.

When mine was acting odd I hooked a hose from a tire pump up to my bleeder valve while the engine was running. I had constant air from it. Thats how i figured it was on the suction side of the lift pump.

A leak on the return side will cause hard starting though as it will let the fuel drain back when the engine is not running.

 

[adyszel]

Great info. I really wasn't sure about the return line effects. Tomorrow I'm going to check the fuel pressure I think I have a tester that hooks into the shrader valve.

 

[adyszel]

Ok tonight I went out and messed around with the truck. The schrader valve is constantly blowing air when open it. If I pump my manual pump on the aftermarket water separator and push open the schrader valve i can get diesel to come out after a few pumps. If I turn the engine over a few times I slowly lose that prime and air returns. My diagnosis is a fuel lift pump.... what do you think? I mean if the manual pump gets fuel there and theres no huge leak is there any other possible explanation? I also put another three gallons in today so the gauge is reading near full, so hopefully that eliminates the possibility of a broken pick up.

 

[CDX825]

Its certainly possible.

 

[punkmechanic]

could be a lift pump, but if air is coming from the motor side not the fuel side then you have a bad injector. I had no idea that it could happen till we had one at work. the fastest way to find it is to put balloons (I know, just bear with me) over your injectors where the feed lines screw on. make sure they fit tight (we used water balloons). now crank the motor 10 or so times and see if any of the balloons are aired up. they shouldnt be. if they are the injector is allowing combustion pressure into your fuel system.

essentially that cylinder is shoving combustion pressure in and fuel back to the tanks.

sounds crazy but I have seen it take a truck headed for scrap and when we gave it back the guy was flat floored.

punk

 

[adyszel]

thats a very interesting thought. With how fast the line is pressurizing I'm half tempted to try that. The only argument I have against that is that I've actually had the engine running and its a consistent lack of fuel throughout all cylinders, ie if one injector was that f'ed up I'd expect a dead cylinder.

 

[punkmechanic]

unless the one cylinder was allowing enough into the fuel system to fill the ip with gas and not fuel. then your truck shuts down.

to check the lift pump, pull the line that goes from the filter housing to the Ip. use some clear tube and put one end under the level of fuel in a bucket or tub. crank the motor and see if the fuel level rises (it should) and also look for air in the line (it should not).

punk

 

[typ4]

1 injector will screw up the system enough to have your symptoms. I like the balloon idea ,never heard that one.

 

[adyszel]

ill prob try testing the fuel pump first tomorrow via punk's method. That should be easy than testing each injector.