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Diagnosing Air Leaks in the Fuel Line

GM Guy

GM Guy

Manual Trans. 2WD Enthusiast
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Location
NW Kansas and SC Idaho
Hey all,

It appears that I have an air leak in the fuel system somewhere.

I replaced the rubber lines going into and out of the FFM, and put a seal kit in the FFM while it was out when I replaced the IP.

LP is new, AC Delco EP158, new o-rings at the same time.

Is there any common places for air intrusion I can look at?

What is the best way to trace the air leak?

Any input appreciated, thanks!
 
use clear tubing in place of the rubber lines to temporarily let you see bubbles. there's what 4 places with rubber lines?
 
When did this start happening? Was it after you replaced a particular item?

What symptoms do you have: hard starting, won't start, rough idle?...
 
No, it won't let it leak back. Iirc the factory LP has a one way check valve in it to stop drain back. AM General installs external check valves to ensure that never happens, even if LP fails.
 
basically the truck in question, the 93 K2500HD, has had a mild air leak since I got it. I imagine this is what contributed to the IP going bad.

I replaced the IP, and it ran much better, but I still have an air leak, and its getting worse. Now its to the point I gotta crank till I get oil PSI to kick on the LP. I thought it was timing, until it started getting worse.

Is the rubber hose from the tank to the hard line on the frame a common source of air intrusion?

Also, where does a guy find the clear tubing? an online source may be best, as I highly doubt our local guys have it.
 
Is this constant air bubbles from the IP return line or just hard starting? Constant air bubbles are 100% before the lift pump. Hard starting can be anywhere.

Hardware store for the clear lines. Air leaks may not leak fuel.
Any air leak even on the return line can drain the entire system. This is why GM spent the money and submerged the return line in the tank! Leaking or sticking injectors can also cause hard starting and air in the injector lines. Look up hard start video where it hits and then stalls from bad injectors. Note the lift pump can still allow the system to drain out as it will drain from air leak out the return fuel side.The lift pump check valve may also leak some and given enough time give you the hard start air bubble. Usually overnight.

Put a clear line before and after the lift pump.

Off the top of my head air leaks are:
FFM including pinhole in heater.
Fuel filter.
Rubber hoses esp the one from the FFM to IP.
Kinked hoses or restriction boiling diesel aka vapor lock.
Top of in tank pickup assy.
O rings to the hoses on the tank.
Lift pump and connections.
Throttle shaft seals on IP.
Return lines on injectors.
Leaking injectors. (Also they can allow compression in the fuel system during cranking.)

Last: Park nose down and see if the hard start is worse/same/better or you have fuel leaks.

Note: Cranking should run the LP. Check the LP fuse by the power distribution passenger side and fuel pump relay.
 
Injectors were replaced with Bosch German bodies rebuilt by a reputable shop with new genuine Bosch india nozzles. doesnt seem like injectors, as it doesnt even try. new Delco 60Gs at same time as injectors.

Fuel hose from FFM to IP is new, and so is the one from the hard line to the FFM. so basically all I have is the line from the LP to FM at the rubber section between the frame and hard line that goes up the bellhousing, and the one back on the tank.

IIRC the LP coming on while cranking is on 95 only, I had it that 94 and older doesnt come on till oil pressure registers. this one definitely never kicks on till the oil pressure is up. I also have Leroy's relay kit, and the fuse on the OEM harness was clean and good when I checked it when installing the relay kit.

Return lines on the injectors could be a possibility. How heat resistant is the fabric braided style? when I did the injectors, I of course put back the heat shield, but IIRC there was some touching the inside of the shield. Could these have melted and are leaking?

FFM heater is also a possibility. It looked like an original unit when I did the seal kit. I used a luber finer branded OE style metal filter, and it went together well, so I dont think it was the filter. Is the WIF sensor also known to go to crap?
 
I like that. I have to take calls a lot from customers that want to prime up there new fuel systems. I tell them to stuff a rag in the filler neck and blow though the rag.
Did you know the "MAG" wheel valve stem is a bolt on type. I'll probably try that.

ditchpig said:
Had issues with air in system but every line was dry. Modified the fuel cap with a valve stem and put ~8psi on tank. Buy the time 30 seconds I crawled under truck the line between the tank & lift pump was getting wet.



Click to expand...
 
I just found out the fabric covered lines are beyond krap! I pulled one off to put clamps on to stop the damn leaking from them and all of them are dry rotted. They have leaked since day 1 and are getting really bad. I found some real rubber 1/8 lines at the local parts store that has fabric in the rubber not over the hose. These will be going on with clamps and yes, have to pull the damn turbo to do it.

Combo of heat and biodiesel does them in I guess. Not the Bio doesn't bother the old rubber hoses from 1993/1995 I reused sections of to the return rail.
 
I thought about trying to come up with a hard line manifold set up like some other diesels like the Isuzu has. A lot easier said than done. Would be nice to find an injector body from another app that would work but use a 6.5 nozzle.
Sounds like custom machining would be needed.
 
GmGuy, it's been too long since I've looked on a pickup at the rubber lines at the fuel tank (not really I hate changing those) but iirc it is 3/8 line back there- that's why everyone changes the 5/16 line from the FFM to the IP up to 3/8 because that's what it is in the back.

Sae 30r9 iirc is the rating you want.
 
Return lines found in the Dipaco kits get hard and brittle after a year of service. I only use the return line that says Made in Germany on it. No issues with that.
 
WarWagon said:
I just found out the fabric covered lines are beyond krap! I pulled one off to put clamps on to stop the damn leaking from them and all of them are dry rotted. They have leaked since day 1 and are getting really bad. I found some real rubber 1/8 lines at the local parts store that has fabric in the rubber not over the hose. These will be going on with clamps and yes, have to pull the damn turbo to do it.

Combo of heat and biodiesel does them in I guess. Not the Bio doesn't bother the old rubber hoses from 1993/1995 I reused sections of to the return rail.
Click to expand...


I have never had problems with return lines and I have been running 11% since the 90's sometime when it was available for delivery here
 
uniquediesel said:
Return lines found in the Dipaco kits get hard and brittle after a year of service. I only use the return line that says Made in Germany on it. No issues with that.
Click to expand...

These are the exact lines giving me trouble. Looks like they are designed to heat shrink some and then they get loose on the nipple. Noted lots of cracks on the end of the hoses. Maybe clamps would have stopped the leaks on the Germany lines. Regardless new rubber lines that say "Gates" on them installed with clamps and finally no diesel leaks. Overkill? Oh wait this is the 2nd time I had to pull the Fing turbo to do just them. :rage:

I am not the only one to experience fuel leaks on the return lines.
 
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