Can injector lines be reused once they have been removed?

On a common-rail engine the manufacturer’s answer is usually no: an injector line or other high-pressure pipe undone for an injector or pump job is replaced, not refitted. Delphi and Isuzu say so outright. On older engines with a mechanical injection pump the same cone-and-nut joint is remade as routine, and Bosch’s reference book counts that among its advantages. The engine’s service manual decides first, the state of the cone second.

What do the manufacturers say?

Delphi, which makes common-rail systems, is unambiguous. Whenever work on the system requires a high-pressure pipe to come off, diagnostic work included, the pipe is replaced. Its workshop manuals print the rule in capitals:

“REPLACE HIGH PRESSURE PIPES EACH TIME THEY ARE REMOVED.”

Delphi, Common Rail High Pressure Pipes Replacement

Isuzu agrees. A service bulletin of August 2016 for its 4HK1, 6HK1 and 4JJ1 common-rail engines states: “Fuel injector pipes must not be reused. New pipes must be installed anytime the fuel injector pipes are removed.” Failing to replace them, or to follow the fitting procedure, “may cause severe engine damage”.

Not every instruction is that sweeping. Detroit Diesel’s 2014 bulletin for the DD13, DD15 and DD16 calls the feed lines that supply the rail “one-time use components” that “MUST be replaced any time they are loosened or removed”. The lines from the rail to the injectors are taken off, given a new seal and installed again. The rule belongs to the engine, sometimes to the individual pipe.

Why is the cone treated as single-use?

A high-pressure pipe seals metal to metal. The cone at each end is shaped from the tube itself, and the nut presses it into a seat in the injector, the rail or the pump. Delphi describes the first tightening:

“Whenever one of these pipes is first fitted, the pipe cone deforms to mate precisely to the connector seat when the nut is tightened, creating a tight seal.”

Delphi, same article

So the cone carries the impression of one seat. After removal, Delphi’s wording is careful: “it cannot be guaranteed that this tight seal will be maintained if the same pipe is refitted.”

A second Delphi note adds three reasons. At every refit the cone deforms again, and the steel is by then “work hardened so a tight seal is more difficult to achieve”. Tightening a thread traps “minute particles of swarf” in it; undoing the nut frees them, and the next tightening can carry them into the fuel system.

The bore at the cone can also close up. Delphi shows two pipes in cross-section: clear after a single use, visibly narrower after several.

Why do so many workshops refit pipes without trouble?

Workshops refit injector pipes because the cone-and-nut joint was designed to be undone, and in their experience a sound cone seals again. Bosch’s reference book on diesel injection lists among the joint’s advantages: “Fitting can be disconnected and reconnected numerous times”. That describes the design of the joint; it is not a service instruction for any engine. On older mechanical-pump engines it is also daily practice: lines come off for a pump overhaul and go back on.

The habit carried over to common rail. On owners’ and technicians’ forums one person cites the one-use rule and the next has refitted such pipes for years without a leak. The figure passed around is three or four refits. No manufacturer document we found gives a number, and those who repeat it add the catch themselves: nobody knows how often a given pipe has been off.

Delphi does not say a refitted pipe will leak. It says the seal cannot be guaranteed and gets harder to achieve each time. What changed with common rail is the load: the rail is, in Bosch’s words, “permanently filled with pressurized fuel”, at what Delphi gives as “up to 1800 bar”.

Of the two views, only “replace” is a manufacturer instruction. Refitting is experience, and the workshop that relies on it carries the risk.

What do you check before a pipe goes back on?

Before a used injector pipe is refitted, check its position, cone, bore and fit, in that order. This applies only where the engine’s manual does not call for a new pipe.

1. Mark and cap each pipe as it comes off. It returns to the same cylinder, the same way round: the cone was formed against that seat.
2. Look at the cone under a good light. The seating band should be even all the way round. A step that catches a fingernail (a mechanics’ test, found in no manual), pitting, rust or a score across the band means scrap. Do not dress it with abrasive: that removes metal from the sealing surface.
3. Look into the bore. Compare the opening with a new pipe or the others in the set. Visibly smaller means tightened too often or too hard.
4. Wipe the threads, then offer the pipe up by hand. Both cones should seat and both nuts start by hand, without the pipe being sprung. A pipe that has to be bent to reach is not refitted.
5. Tighten with a torque wrench. Too much and, in Delphi’s words, “the pipe will excessively deform”; too little and “not achieving the correct torque will risk high pressure leaks”. A refitted pipe that weeps at the correct torque is replaced, not given another quarter turn.

When must a pipe never be refitted?

A used high-pressure pipe must not go back on in any of these cases.

• The workshop manual specifies a new pipe. On common rail that is the usual case.
• The injector, pump or rail on the other side of the joint is new: a used cone carries the impression of the old seat. That is our reading of Delphi’s explanation; Delphi wants a new pipe either way.
• The cone or the bore fails the checks above.
• On common rail, the joint leaked before the pipe came off. On older mechanical systems a pipe that leaked from dirt or misalignment can be cleaned and refitted if its cone is unmarked.
• The pipe is bent, kinked or chafed, or nobody knows its history.

What should you do with the pipe in your hand?

Start with the engine’s workshop manual, not with the pipe.

1. Read the removal procedure for that pipe on that engine. If it says new, fit new, and price the pipes into the job beforehand.
2. If the injector, pump or rail is being replaced, order the pipe with it.
3. If the manual allows refitting or is silent, inspect cone, bore and fit as above. Where doubt remains, replace the pipe.

Doubt weighs most where a leak cannot be seen. Where the injector pipes sit under the cylinder head cover (the Isuzu procedure begins by taking it off), a weeping joint is hidden and the fuel ends up in the engine oil.