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REMANUFACTURED INJECTORDiesel Fuel Injectors
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Diesel Injector Leak-Off Test

Injector Compatibility Checker

Quick Answer

A leak-off test measures how much fuel each injector returns through the spill line. Connect clear hoses from each injector return to separate containers, idle the engine for 30–60 seconds, and compare volumes. The injector returning the most fuel is likely worn or damaged. This is the single most effective DIY diesel injector test.

Symptoms Indicating Injector Failure

Fuel in the return line is excessive

A worn injector nozzle allows high-pressure fuel to bypass the needle and exit via the return line.

Hard starting after sitting overnight

A back-leaking injector drains the fuel rail pressure, requiring extended cranking to rebuild pressure.

Rough idle that smooths out at higher RPM

At idle, the lower injection pressure makes a leaking injector more noticeable than under load.

Diesel smell around the engine

Excess fuel in the return circuit or external leaks from worn injector seals can produce a noticeable fuel smell.

Step-by-Step Testing Methods

Standard Leak-Off Test Procedure

Easy
  1. 1Locate the injector return rail or individual return pipes (usually on top of the cylinder head).
  2. 2Disconnect the return lines — have rags ready for small fuel spills.
  3. 3Attach a length of clear 6mm hose to each injector return nipple.
  4. 4Place the free end of each hose into a separate, identical container (e.g. small measuring cups).
  5. 5Start the engine and let it idle for 30–60 seconds.
  6. 6Switch off and immediately compare the fuel volumes in each container.
  7. 7A good injector returns very little fuel. A bad injector returns significantly more.
  8. 8If one container has 2–3 times more fuel than the others, that injector is suspect.

Tools needed: Clear 6mm fuel hose, 4 identical containers, spanners, rags

Cranking-Only Leak-Off Test (Engine Won't Start)

Easy
  1. 1If the engine won't start, perform the leak-off test during cranking instead of idle.
  2. 2Connect the hoses and containers as above.
  3. 3Crank the engine for 10–15 seconds (do not exceed 30 seconds to protect the starter).
  4. 4Compare the fuel volumes.
  5. 5This method is useful when the engine is too rough to idle or refuses to start.

Tools needed: Clear hose, containers, spanners

Tools Required

Clear fuel-rated hose, 6mm internal diameter, 1m per injector
4 identical graduated containers or measuring cups
Spanner set (10mm–17mm)
Clean rags or absorbent pads
Hose clamps or cable ties (to secure hoses)optional

When Replacement Is Necessary

Replace an injector that returns more than double the fuel of the best-performing cylinder. If the engine won't start and all injectors show heavy back-leak, the full set likely needs replacing. On high-mileage engines (150,000+ miles), consider replacing all injectors as a set once one has failed.

Compatible Injector Parts

0987654567
0987654567Bosch
£99.883450
3450Bosch
£99.903415
3415Bosch
£99.903407
3407Bosch
£99.903533
3533Bosch
£99.903467
3467Bosch
£99.903392
3392Bosch
£99.903390
3390Bosch
£99.903413
3413Bosch
£99.9045584736378
45584736378Bosch
£99.904343454321
4343454321Bosch
£99.903538
3538Bosch
£99.90

Affected Engines

OM651 N47D20 CAYC DW10C CRBC

Vehicle & Engine Combinations

AUDI A6CAYC
AUDI A6CRBC
BMW 1 SERIESN47D20

Related Fault Codes

P0087P0201P0202P0263P0300

Related Injector Problem Guides

Hard StartRough IdleWhite Smoke

Frequently Asked Questions

How long should I run the engine during a leak-off test?
Idle the engine for 30–60 seconds. This is long enough to produce a measurable difference between injectors without overfilling the containers.
What if all injectors show heavy leak-off?
If all four injectors return a large amount of fuel, the entire set is likely worn and should be replaced. This is common on engines with 200,000+ miles.
Can a leak-off test damage the injectors?
No. The test simply captures fuel that is already returning through the spill line. It is completely non-invasive and safe.
Do I need to remove the injectors for a leak-off test?
No. The leak-off test is performed with the injectors installed in the engine. You only need to disconnect the return lines.

Other Testing Guides

How to Test Diesel Injectors – Complete GuideHow to Test a Diesel Injector – Step-by-Step MethodsCommon Rail Injector Testing – Diagnosis & Test Procedures

Need a Replacement Injector?

All our remanufactured injectors are fully tested on a dedicated bench, calibrated to OE specifications, and supplied with a 12-month warranty.

View All InjectorsReplacement Cost Guide Call 020 3355 6581

Explore Diesel Injector Guides

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More common questions from UK installers

Specific testing, packaging and report questions our customers ask before posting injectors to us.

What does an injector test report actually show?

Every Remanufactured Injector test report shows four numbers per injector: idle delivery (mg/stroke), full-load delivery (mg/stroke), back-leakage (ml/min) and a spray-pattern grade. Anything outside OEM tolerance is flagged red. The report is a single A4 PDF emailed within 48 hours of receiving your injectors — no phone-call interpretation needed.

Do I need to send all four injectors for testing?

We recommend sending all injectors from the affected engine even if you only suspect one. Back-leakage is a relative measurement — a single injector tested in isolation cannot be compared against the others on the same engine. Testing the full set costs £80 and rules out borderline units that would otherwise come back as a workshop return weeks later.

How should I package injectors for testing?

Wrap each injector individually in bubble wrap, keep the cylinder number marked on the side (e.g. 'Cyl 3'), and place upright in a small cardboard box with packing material. Royal Mail Special Delivery or DPD Tracked is sufficient. Our address is on the confirmation email — please do not include fuel; pump the injector dry first or bag it separately.

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