Injector leak-off line: what it does, why it leaks, and when the injector is to blame

On a common-rail engine the injector leak-off line returns the fuel that has passed through each injector to lubricate and cool it. When the line leaks or a connector keeps coming off, look at the joint first: clip, O-ring, plastic connector. Wet fuel alone does not condemn an injector. That takes a leak-off test, which compares how much fuel each injector sends back.

What does the leak-off line do on a common-rail engine?

The leak-off line collects the fuel that leaves each injector and feeds it into the low-pressure return system. Pico Technology, which makes diagnostic oscilloscopes, explains why:

“Common rail diesel systems use the fuel flow through an injector as a means to lubricate and cool its internal components.”

Pico Technology, guided test: CRD injector back-leakage

That flow is by design. The fault is a change in the quantity.

An article in Professional Motor Mechanic, describing a Laser Tools test kit, calls excessive fuel returning to the tank a common reason for pressure loss, because the pump can no longer build enough pressure. The result is “poor or no engine starting, rough running and poor acceleration”, most obvious at start-up and at idle.

Tractors with mechanical injectors have a different line and a different test, covered in our article on tractor leak-off hose leaks.

Why does the line leak, and why does a connector come off?

The first place to look on a leak-off line is the joint on top of the injector. The same article notes that the return connectors are “secured with wire or metal clips”. Highway and Heavy Parts (HHP), a truck parts supplier, warns that “these plastic fittings can be brittle” and has their O-rings inspected after a test. For diesel on or near an injector Delphi gives two possibilities: a damaged injector, or “the seal may have worn through”.

Not every line is a push-fit. On Isuzu’s 4HK1 and 6HK1 truck engines the service bulletin shows a leak-off pipe held by banjo bolts, refitted “with new gaskets”.

For a connector that keeps coming off, our one field account is a single vehicle on the Defender2.net forum. The owner of a 2013 2.2-litre diesel writes that the connector on injector number 4 “is popping off the injector body”, with a diesel smell in the cabin and fuel on the ground. Replies say the connector may be worn and that most are held by “plastic clips which break easily”. The thread ends with no cause found.

Nor is the line unpressurised on every system. According to Pico, some systems, piezo-injector ones for example, “malfunction without a minimum positive pressure in the injector fuel return system”. No source links that to the clips; our own inference is that on such a system a missing clip or a damaged O-ring shows up sooner. We could not open a manufacturer document saying that excessive back leakage pushes a connector off.

How is a leak-off test done?

In a leak-off test a clear chamber is connected to the return port of each injector, the engine idles, and the fuel collected in the chambers is compared. The article’s sequence:

1. Warm the engine up. A cold start can give inaccurate results.
2. Look first. Remove the engine cover and check for leaking pipes, loose hoses and chafed wiring.
3. Remove the clips and keep them safe.
4. Close off the disconnected return line. On some engines it shares a connection with the fuel pump, and fuel can be forced back out when the engine starts.
5. Fit the tubes in injector order. Keep them away from pulleys and belts.
6. Idle for a few minutes. Stop when the chambers are 50 to 75% full and compare the levels.
7. Refit and check. Refit the connectors with their clips, run the engine for a minute or so and check for leaks.

How do you read the result?

A leak-off test compares the injectors with each other, and the one that stands apart is the suspect. The sources do not agree on how big a difference is a fault.

None of these is an engine maker’s figure. Pico measures pressure pulses, not volume, and notes that the return also depends on the pump, the pressure control valves and rail pressure.

Is the test done the same way on every engine?

A leak-off test is not done the same way on every engine. The first limit is return pressure.

The second limit: the return is not measured injector by injector on every engine. In a Cummins bulletin on fault code 4713 for the ISB6.7 and B6.7 (CM2350), what is measured is the combined drain flow, and unless the injectors are isolated, “collective drain measurements will not be able to identify the source of the malfunction”.

So the injectors are capped off one at a time and the combined flow is measured in millilitres over 30 seconds, three times, and averaged. The bulletin’s example gives 145 ml with no injector blocked, 120 ml with number 2 blocked and 125 ml with number 6. The two injectors that reduce the flow most are replaced. The figures are an example; the limits are in that engine’s service manual.

Line or injector: what order should you work in?

To tell a leaking leak-off line from a faulty injector, look at the outside first, then at the return quantity.

1. Fuel at a return joint. Check the clip, the O-ring and the plastic connector, or the gaskets on a banjo joint.
2. Dry outside, but hard starting or a rough idle. Read the fault memory first, as Delphi’s diagnosis does, then measure the return by the manufacturer’s method.
3. One injector stands apart. Delphi says the measurement “indicates which injector may be faulty and in need of replacement”, so one is replaced instead of the whole set.
4. A connector that looks sound and still comes off. The forum’s suggestion, not a manufacturer instruction, is to compare the return from all four injectors.
5. Harder to start the longer it stands. The return line may be letting air in, the subject of our article on air in the diesel fuel line.