Yet another air in fuel thread!!!!

[sailormike]

Hey everyone
New guy here but not new to diesels. trying to get a 86 f350 going and its giving me fits. did the electric fuel pump, all new fuel line from tank to mech pump, new mech pump, new quick fitting on the supply side of tank, capped off fuel return line at filter, replaced all o rings on return caps. only thing I haven't done is replace the sending unit itself. still dies after about two three mile on the road, when it does the filter is full of air .But it will run all day in the yard
HELP please!!
BTW its a dump bed and not the original motor or tank

Thanks for any help

 

[laserjock]

Is the tank venting properly?

 

[sailormike]

Good question, I thought about that. since its not OEM and the cap is supposed to be a venting cap, that's something I have not checked. Thanks for the response. ill double check that tomorrow
Other thoughts??

 

[sailormike]

one thing I didn't mention is, I pulled the sending unit out and the check valve crumbled like a 2 week old cookie. I did not replace it because all the new fuel line/e pump I put in is all downstream of the tank to the mech pump. gravity should eliminate the need for the check valve . also the oem water sep is abandoned.

 

[79jasper]

Why are you running the electric pump with the mechanical pump?

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[sailormike]

I wasn't to start with, but I was still getting air and figured it cant hurt, eliminates that much more possibility that the mech pump might be pulling air from somewhere. also it seemed to be the popular way from other threads ive read here. I wired it to the ign so that it runs when the key is in the on or run position. so now the fuel supply is pressurized instead of suction starting at about 2-3 feet from the tank

 

[sailormike]

The only thing I can think of left to do is replace the sending unit, but I just cant see where there would be any air coming from it

 

[gandalf]

You're running both the original lift pump AND an electric? That's not a real good idea. Either one, working properly, will be perfectly adequate. If you have an electric feeding into the stock lift pump you're putting your engine at risk. The stock lift pump has a rubber gasket separating the fuel flow from the crankcase. IF that gasket ruptures, and you're pressurizing the fuel through that stock pump, you will be feeding fuel into the crankcase. That's not a good thing. Check the oil level in the crankcase, see whether it is too high.

Also, you might want to put a clear return line on the engine, from the IP and from the fuel filter. That will help you narrow down the air infiltration possibilities.

You said that the water separator has been taken out of the system. How was that done? Simple bypass around it?

 

[79jasper]

^ what he said.
You definitely didn't read that here.
I would try clear line before the filter also.

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[sailormike]

See your point about running both pumps at once. ill put a toggle switch on it and only use it for bleeding, the mech pump is brand new. the separator is bypassed by PO, so now its just a line from the tank straight to the mech pump with the elec pump inline about 2-3 feet downstream from the tank. the fuel line is 3/8 marine (heavy wall) and runs inside the frame straight to pump, since the tank is mounted higher than a OE tank it will siphon all the way.
On the return line I capped off the return at the filter and at the last injector cap, so now it just goes straight to the tank via the tee at the back of engine, so air in the return shouldn't make any difference. I work on marine diesel and there were a few mfg's over the years that tried the same setup util they realized it was just a bad design (almost impossible to bleed well) of course they don't use the setup with the plastic caps and oring
Going to try some clear line from tank to lift pump today and see what that reveals.

Has anyone had problems with the quick connect fitting on the sending unit??
I put a new (aftermarket) fitting on it yesterday, but I'm starting to think I'm better off just clamping the hose to the metal tube going inside tank??

 

[79jasper]

Yeah, I had a hell of a time with the quick disconnects. Lol
No problem with clamping hose straight to it.

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[OLDBULL8]

The return line off the filter that you eliminated should be on there. It's purpose is to bleed off any small amount of air that is left in the very top of the filter , the fitting has a very small orifice in it. At idle, with the return blocked off, it won't make a difference, but at just about any speed, your using more fuel that can mix with any air left in the filter. You should really put the filter assembly (shower head) pickup in the tank as it sets only a 1/4" off the tank bottom. The duck bill on the return line in the tank helps to keep the return fuel from aerating.

 

[icanfixall]

After reading all this best suggestion I can offer is the issue with the tank suctions. If they are broken off you can't draw the last 1/4 tank of fuel because the now suction level is at the 1/4 tank level. Sad horrible engineering issue but easily repairable too. I drove this way for years. finally dropped rear tank and added 3 inches of fuel rated fuel hose. Front tank you just remove the sender and add same length of hose.
Now something was mentioned about the red duct bill suction rubber had issues.. That is ONLY for the fuel RETURN line to the tanks. Has nothing to do with suction. The new factory suction head shown in Bill's post has the correct upper suction valve. that is there in case you need fuel after some water has froze up the suction head. I never felt it was useful because how can that much water enter the tanks and nor ruin everything fuel related.

 

[sailormike]

ICANFIXALL: I did just that, added a piece of hose to 1/2" from the bottom with a 45 cut on the bottom and filled the tank to be sure. No difference
OLD bull, I think we are on the same page, have to b there is nothing left to replace!. After two days I finally found a sending unit with the "shower head in your pic 60 bucks on ebay. I will definetly try re attaching the hose you suggested because the symptoms are exactly as you described. it will run all day in the driveway until you get out on the road.
so today I discovered that the the e pump the PO put on was only rated for gas and had 5/16 fittings instead of 3/8, so I replaced it. Also ran clear hose from the tank to e pump and all the way to the lift pump, no air at all, so all is good right? NOT. after putting the right hose back on I went for a ride, went about 5 miles and checked the filter and it had a little air, blead it out and only made it about 2 more mile and the filter was slap dry!!! Only reason I'm goin to order the whole thing is the fuel level sender is shot too. will let you know if reattaching the return line does the trick tomorrow

Thanks everyone, I'm really not a newbee with diesels, only with these engines. boat diesels come much more naturally to me. not to mention this truck has been a dump bed work truck its whole life and with a junkyard motor that has been molested over the years

 

[sailormike]

One more thing OLDBULL, and others. If I reattach the return line back to the filter ( as it was) with the small orifice, what part of the system causes that part of the return to push or pull that residual air out of the filter thru such a small hole? when I checked it at the gas station and found a little of bit of air it was a very short time before the whole system was drained