84 k30 6.2 fuel system (leak hunt)

[KamperBob]

I'm hoping y'all can help set proper expectations. My 6.2 developed a fuel leak that I'm trying to chase down. From research I've pieced together some information. If I understand correctly, the primary fuel circuit (stock) would be a follows. From dual tanks supply and return lines meet at the selector valve. Then a single pair goes up under the hood. The supply line goes on to the lift pump (mechanical) on the front passenger side of the engine, inside the frame rail. From there fuel flows up to the filter on the firewall. Then over to the IP on top of the engine, under the intake manifold. Presumably entering the IP at the rear. The IP return line (probably also rear) goes back down to the tank selector. Do I have that correct? Or am I off target somewhere? Any visual aids would be a major blessing to lay eyes on everything under a less than ideal work area in eleventy some temps. Thanks a bunch!

 

[Blue Ox]

Pretty much. The return combines all the injector jumpers and the pump (top) return to a crossover tube that bridges across the front of the engine like an upside down V and has a barb near the passenger frame rail near where the supply line for the lift pump goes.

The injectors have a boot on the back return barb and jumper hoses between them that meet the crossover at the front of the engine.

 

[Ricko1966]

Are you leaking fuel or sucking air?

 

[Ricko1966]

Are you leaking fuel or sucking air?

That was a serious question I had a diesel that I couldn't get the IP to prime because it was sucking air instead of fuel. No external leak visible. I just wondered what he is chasing.

 

[KamperBob]

That was a serious question I had a diesel that I couldn't get the IP to prime because it was sucking air instead of fuel. No external leak visible. I just wondered what he is chasing.

Both. I couldn't see the source but it appeared to pee fuel from behind the IP. Which makes me suspect the supply line first.

 

[KamperBob]

Update. Turns out there are multiple issues in play. I guessed wrong that fuel was coming from behind the IP. My bad. This morning before it got too hot I saw more. I was misled in part because my firewall filter has recent history of leaking. So I started there of course. While it was a little wet there could still be an issue there. And a leak along that part of the fuel circuit would explain lost prime and hard starting. But that's minor. I'm focusing on the major fuel leak first.

With the air cleaner out of the way I could see the valley was dry. That pretty much eliminates leaking from the IP connections. Yes? No?

I traced fuel leak indications up the block on the driver side. Seemed to come from cylinder #5 injector area. Cleaned and dried all around with brake cleaner and compressed air. Unplugged fuel solenoid and cranked the engine a few turns. Enough to see fresh wetness on top of that injector. Appeared to come from the jumper hose on top. So I supposed that'd be return fuel from injector #7. The rubber hose itself looked fine.

Headed out shortly to hunt for replacement hose at local auto parts stores. Meanwhile, figured I'd post an update here as the brain trust may have crucial insight. One thing itching my brain is how and why fuel would push out there. Return fuel shouldn't be under much pressure. I mean, if it was, wouldn't those NOT be rubber on barbs without clamps. Yes? No?

Alternatively, suppose that rubber hose connection itself is in fact fine. Then would leakage there suggest an injector problem? The way all the injectors on each bank are daisy chained together for return fuel, I gather those circuits are open flow. At least while an injector is NOT active. Yes? No? I test that for each injector with compressed air. I'll just need a small cone adapter while I'm out.

But. Moreover! The injected fuel pressure is high. Really high. So how/why is return fuel so piddly? That's not intuitive to me. I mean, with high pressure fuel being delivered to an injector by the IP, shouldn't ANY leak also be under high pressure? Hm...

 

[Ricko1966]

From experience I can tell you leaky return lines can cause hard starts and lost prime.

 

[Blue Ox]

It's not much pressure, but it is positive pressure. Remember, the injection pump return is tied into it as well, so if the pump is turning it's pushing fuel into the return.

@Ricko1966 is correct. Leaking returns can cause bleed down.

I replaced all of my injector jumpers with green viton hose from McMaster. The regular hoses get brittle and fail more often.

 

[KamperBob]

Thank you both. Great info! Considering these hoses may be original, it's not surprising that rubber can fail after 42 years.

 

[Blue Ox]

But. Moreover! The injected fuel pressure is high. Really high. So how/why is return fuel so piddly? That's not intuitive to me. I mean, with high pressure fuel being delivered to an injector by the IP, shouldn't ANY leak also be under high pressure? Hm...

The high pressure fuel is being injected into the engine. What returns from the injector is a product of internal leakage, mainly between the needle and nozzle body. Obviously there has to be some clearance there for fuel to lubricate the moving surfaces so fuel returns for lubrication and cooling. However, as the industry used to point out, diesel fuel injection components are the most precisely made production parts in the world. So there's as little clearance there as they can get away with. Hence very little fuel comes out of the return, and certainly not at high pressure.

 

[KamperBob]

The high pressure fuel is being injected into the engine. What returns from the injector is a product of internal leakage, mainly between the needle and nozzle body. Obviously there has to be some clearance there for fuel to lubricate the moving surfaces so fuel returns for lubrication and cooling. However, as the industry used to point out, diesel fuel injection components are the most precisely made production parts in the world. So there's as little clearance there as they can get away with. Hence very little fuel comes out of the return, and certainly not at high pressure.

That nugget was golden. Thanks so much!!