Perfect Timing

Using fault codes as vital clues leads Andy Crook to the root cause

Published:  19 November, 2014

A customer presented us with a VW T4 van demonstrating a lack of power and difficulty starting hot or cold - in fact, it wouldn't start at all once it was turned off. A visual check revealed little evidence, due to the number of covers and shields in place. A quick code read was taken and the following diagnostic trouble code (DTC) was logged in the ECU memory - P1248 Start of injection timing regulation: Control deviation.

Codes are often helpful, in fact, they are often very accurate. This often results in technicians taking the code at face value and ordering whatever component is being flagged by the DTC. In this situation, it would be easy to simply blame the timing solenoid, as this is the component that controls the injection advance. However, you need to make sure you are getting to the real problem and not just treating a symptom. You should be thinking:

Then an appropriate test routine can be devised and logical tests carried out to confirm or deny the presence of faults.

Delve deeper

So, let's take a look at each of these questions...

Does the code match the symptoms?

What else could cause this code to be logged by the ECU?

What sensor(s) are responsible for monitoring this code?

What actuator is responsible for this output?

What mechanical functions are influenced, or influence, the operation of this sub system?

What tests should be carried out to determine the root cause and in what order? Well, in my experience, whatever you choose to test last is bound to be the cause, so I tend to perform the tests in the order of accessibility. The first on the list is questioning the customer i.e. 'Have you run out of fuel recently?' In this case, with the symptoms being present at the time of testing it is unlikely to be the cause of the DTC but it is worth looking through the service history and checking if the symptoms coincide with any previous work.

The timing solenoid itself is easily accessible and is fed with a live and a switched ground, it is best checked with an oscilloscope when the engine is running. This tested OK, as did the crankshaft sensor and needle lift sensor.

The root cause

The cam timing and pump belts were next on the checklist and the pump belt was found to be saturated in oil. This had resulted in stripped teeth. After removing the pump belt it seemed logical to send the pump away to be checked and calibrated.

With the oil leak and two separate water leaks repaired with new seals, hoses and clips, the belt was replaced. The timing was checked using the VCDS (VAG-COM Diagnostic System) timing checker and then adjusted. You can see the injection timing on the vertical axis versus fuel temperature on the horizontal axis in Figure 2.

The Van was taken on a test drive and checked for correct performance, DTC's and leaks. The root cause was the oil leak, which resulted in the damaged drive belt. I have checked the DTC list and I can't find a single code that replaces the technician's ability to analyse, test and diagnose based on the evidence gathered. The DTC is just one piece of evidence, it should not be the only piece you rely on if you want to find the root cause.

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