Return line question

Hello,

Still trying to find the source of air intrusion. O-rings etc all replaced. New return lines..... When I leave the truck for more than a few hours there is air trapped up in the air filter - when I bleed it only air comes out for a while.

My question: The steel return line connection which is circled in the image (sorry I don't know the name) - this is very wobbly and loose on my truck. Should this be nice and snug? Wondering if this is a potential source of air getting in.

Apologies for the bad quality of the diagram - it's the only one I can find online.

Since I'm writing, the symptoms are: Starts fine but then dies after a few seconds. Then have to crank it for ages to get it to fire up, when it idles rough for a short time then smooths out. And if I bleed the schrader valve on the fuel filter at this point then air comes out for a while.

Thanks as always.

Others may know better; but, air intrusion in return lines should not affect the running of the engine.

Return lines are fuel on it's way back to the tank and can be nearly all air; they should have nothing to do with the engine nor the fuel filter or water separator.

The Draw line is the only one you need to worry about air intrusion.

But, like I said, others may know something I don't.

Thanks, yes, I see your point. I'm wondering, though, if air can get in at this apparently loose spot when the engine is off then maybe it is bubbling up to the highest point, which is likely the air filter? The truck runs fine when it's going, but is hard to start and each time it's hard to start I find air in that filter if I bleed it.....

I've checked all the usual suspects for intrusion. Tried parking it nose higher and tail higher and no difference. Looks like a common problem but I sure would like it solved.

Should this connection be loose or a good tight fit?

And if it's not this connection, where else should I look for air coming in, other than fuel return lines between the injectors, o-rings etc. The lines between the lift pump and injector pump all seem dry.

Thanks again.

Most redo the fitting, I've seen where a rusted fuel sending unit has a pin hole leak on top of it to cause air intrusion

Rupert8 said:

Thanks, yes, I see your point. I'm wondering, though, if air can get in at this apparently loose spot when the engine is off then maybe it is bubbling up to the highest point, which is likely the air filter? The truck runs fine when it's going, but is hard to start and each time it's hard to start I find air in that filter if I bleed it.....

I've checked all the usual suspects for intrusion. Tried parking it nose higher and tail higher and no difference. Looks like a common problem but I sure would like it solved.

Should this connection be loose or a good tight fit?

And if it's not this connection, where else should I look for air coming in, other than fuel return lines between the injectors, o-rings etc. The lines between the lift pump and injector pump all seem dry.

Thanks again.

Click to expand...

What you are describing is commonly called fuel drain back or fuel bleed back

Did you replace the hiding return hose on the very back of the engine?

The view on an 85 6.9 van chassis (hose removed but red line represents it)

That fitting is different depending on the year of your truck. If it has a steel hard line leading from the IP return to that fitting, you will have a rubber “olive” seal that is supposed to seal the connection. There’s a good chance that olive seal is cracked and old (mine was hard as a rock and I had to chip it out of the fitting).

I forgot to add that there are other olive seals in the fuel system, so you might as well replace them all. Russ (Russrepair.com) includes them in his return line kit, but I can’t remember if he sells them separately as well. I bought viton ones when I did it instead of standard rubber. Look up Parker vibra-lok 60VLV-5 (5/16” size) and 60VLV-6 (3/8” size). I can’t remember how many of each size I needed, but I think I bought 3-4 of each size and had extra left over.

McMaster Carr -- ferrules/olives ---->



Also -- ClassyDieselDesign return kit gets rid of that return line coming off the filter head -- deletes it altogether and plugs off that spring thingee valve on the filter head also.

Rupert8 said:

Should this connection be loose or a good tight fit?

Click to expand...

Since nobody directly answered this, yes. It should be tight assuming that you have a nut (that covers an olive) there. I've seen them get loose on their mounting point, but that won't affect the fuel system.

more importantly than anything else thats been asked of you.
does your 86 still have the water/fuel seperator?
this is #1 source of air intrusion on all factory trucks pre 1988

IDIoit said:

more importantly than anything else thats been asked of you.
does your 86 still have the water/fuel seperator?
this is #1 source of air intrusion on all factory trucks pre 1988

Click to expand...

100% I just removed mine today completely. I also removed my fuel selector valve as it was junk and just taking free rides around town. I'm going to install a 40 gallon tank in the rear with sump when I get past this motor swap.

Also @Rupert8 delete the fuel return to filter head. If return lines aren't perfect it can put air into the filter head before it goes into the IP.

Cubey said:

Did you replace the hiding return hose on the very back of the engine?

The view on an 85 6.9 van chassis (hose removed but red line represents it)

You must be registered for see images attach

Click to expand...

No - I didn't, thanks. Totally missed that. It will be checked out this weekend, or before if possible.

ROCK HARVEY said:

That fitting is different depending on the year of your truck. If it has a steel hard line leading from the IP return to that fitting, you will have a rubber “olive” seal that is supposed to seal the connection. There’s a good chance that olive seal is cracked and old (mine was hard as a rock and I had to chip it out of the fitting).

Click to expand...

Great info! I'll check this too. I need to research what an olive seal is but I don't doubt that this forum has that somewhere!

IDIoit said:

more importantly than anything else thats been asked of you.
does your 86 still have the water/fuel seperator?
this is #1 source of air intrusion on all factory trucks pre 1988

Click to expand...

No! It has an after-market one. Thanks. I've heard these are often problematic.

IDIBRONCO said:

Since nobody directly answered this, yes. It should be tight assuming that you have a nut (that covers an olive) there. I've seen them get loose on their mounting point, but that won't affect the fuel system.

Click to expand...

THANK YOU! Mine is pretty loose. Yes, it has a nut. I've tried tightening it but am wary of doing damage. I'll look into replacing it. If I understand correctly, the nut being loose will potentially affect the fuel system. That fuel filter is getting full of air pretty quickly. I'm having to remember to bleed it once a day before switching off the engine.... Not ideal.


I REALLY appreciate all these responses. It's hard to know where to start sometimes.

A couple questions:

I know it's a band-aid but if I can't find the leak, would installing an electric fuel pump at least bypass the problem - or would I still end up with the fuel filter half full of air?

If I suspect the fuel tanks selection valve is letting in air, is there a way of bypassing it temporarily?

Again, thanks to all. What an incredible source for information and help.

WrenchWhore said:

100% I just removed mine today completely. I also removed my fuel selector valve as it was junk and just taking free rides around town. I'm going to install a 40 gallon tank in the rear with sump when I get past this motor swap.

Also @Rupert8 delete the fuel return to filter head. If return lines aren't perfect it can put air into the filter head before it goes into the IP.

Click to expand...

Thanks. Regarding the return line to the fuel filter, just cap that at each end? Any chance of air then making its way through the injection pump? That wouldn't be good. Or building pressure elsewhere?

You seem unsure about olives. I've found that pictures often explain things best. Below is a picture of an old olive, on a hard line, and a new olive beside it, waiting to be installed.