Mystery Intermittent Fuel Leak

[RogerB]

I have had this fuel leak off and on for months, and I cannot determine where it is originating. It is on my 72 FI 3.0 CSL. It appears to be coming from around the area of the rear fuel injectors and is leaking down the driver's side of the block. I don't drive the car much, so I have been starting it periodically to see if the leak is still there. I thought it had stopped until I started it yesterday. The side of the block was wet with fuel, and there was an approximately 6-inch diameter puddle on the floor underneath the car. I switched off the engine and let it set until this afternoon. I started it and everything was dry again.

This has been going on for quite sometime and had not leaked during test starts for the past few months, but it did leak again yesterday as I said above. I cannot determine where it is coming from or when it will or will not leak! Any thoughts on how to correct this?

 

[Sven]

Could be from some old hoses. I would take off the intake log and 'C' runners, then remove the wires from the fuel pump and connect 12 v battery leads to the pump terminal. Run the pump only and look for the leaks. You are going to have the pull the intake to replace any bad hoses anyhow. If you find only one bad one, I would still replace them all.

If in doubt replace all the rubber fuel lines in the engine bay (including the short ones from the injector to the rail).

 

[nashvillecat]

Some thoughts in addition to Sven's good advice.

If your fuel injectors are original and have the original hoses that feed each injector from the fuel rail - they are all likely due for replacement. Over time even the best rubber components can become brittle and subject to leakage. Of course, if you injectors have newer hoses, your leak could be something as simple as a loose hose clamp (see arrow). Another possibility, although unlikely, could be an injector that is leaking. If possible, I would locate the source of the leak before removing the manifold because some of the hose is replaceable by removing one or two manifold runners.

Bear in mind that once the fuel pump is energized, the system is under significant pressure. This pressure can quickly transform a tiny leak into a biblical flood posing a substantial safety risk.

 

[RogerB]

Everything was replaced by the PO on this engine several years ago to the best of my knowledge. The PO was very particular about the restoration of this car. The fact that it may go months with no leaks and then for no apparent reason it starts leaking and then just as suddenly it stops leaking is what baffles me!

 

[rsporsche]

Roger,

have you given any thought that ethanol could have eaten up some of the small rubber? i only ask that because i have had that issue with some of my stihl yard equipment - in fact i replaced some fuel lines today on my string trimmer. stihl warned that using fuel with ethanol would cause the original fuel lines or rubber parts to break down from the inside.

just wondering ...

 

[Stevehose]

Given the gravity of a fuel leak I would cluster bomb this problem by replacing all rubber hoses under the hood and check all clamps and fittings.

 

[RogerB]

Roger,

have you given any thought that ethanol could have eaten up some of the small rubber? i only ask that because i have had that issue with some of my stihl yard equipment - in fact i replaced some fuel lines today on my string trimmer. stihl warned that using fuel with ethanol would cause the original fuel lines or rubber parts to break down from the inside.

just wondering ...

Good thought Scott! I've encountered some of the same problems with the evils of ethanol in some of my small engines despite using additives to prevent this. However, the on again off again leaking is still baffling. They are either compromised or they are not?!?! They are under high pressure anytime the engine is running, so they would theoretically leak any time it is running and not just intermittently??!!?!?

 

[RogerB]

Started it again today and no leak! WTF

 

[jefftepper]

One thought that comes to mind is that the rubber hoses expand slightly when warm/hot effectively swelling the leak shut. When cold, they shrink slightly allowing the leak to show itself. Pay attention to the ambient temperatures when you are testing to see if there is a pattern. If your goal is to locate the leak vs just replace everything rubber in the fuel delivery system, start with tightening all rubber fuel hose clamps under the hood and see what you've got. If one of the hose clamps has lightly sliced a hose allowing for a slight leak this might reveal where the leak is located.

 

[bert35csi]

Everything was replaced by the PO on this engine several years ago to the best of my knowledge. The PO was very particular about the restoration of this car. The fact that it may go months with no leaks and then for no apparent reason it starts leaking and then just as suddenly it stops leaking is what baffles me!

Were hose clamps used to attach the short rubber hoses to the base of the injectors? If this is the case, then fuel will most likely leak at the bottom of the hose(s). For whatever reasons, the injectors' barbed nozzles will inevitably leak when hose clamps are used to connect hose to injector. Clamps should be only used for the connections between the injectors and the round nipples of fuel rail.

Believe the earliest Bosch injectors had crimped fittings for the hose/injector connections. This was later changed to the press fit brass "bowls" fittings (as shown above). This is the only surefire hose attachment method to prevent fuel leaks.