A bewitching tale

Frank Massey explains how a fixed problem isn't always the solution

Published:  06 June, 2016

A couple of interesting workshop repairs have taken my fancy in the recent months, the first involves a BMW 530 common rail diesel.

The vehicle was booked in with a typical problem, a blocked DPF; however with every credit to the owner a genuine BMW replacement had recently been fitted. This brings me to a statement I have made many times and is critical to all DPF related failures. Simply removing the soot or replacing the DPF is not the solution.<

Soot storage is a symptom of incomplete combustion and or failure of the passive regeneration process. Passive regeneration is conducted at temperatures ranging from 350ºC to 500ºC The process also requires an active DPF loading maintaining adequate temperatures whereby the soot is converted into CO2.<

Not simple

This process cannot be maintained without a number of system critical functions, the correct operating environment and a suitable minimum quantity of quality fuel. DPF regeneration is suspended with low fuel levels. When passive regeneration fails, storage of soot is unavoidable.<

The problem with storage is compounded when the previous requirements have not been reached. So what’s the problem with continuous active regeneration? Thermal regeneration demands much higher temperatures 500ºC to 700ºC are common. This not only reduces fuel consumption but produces ash as a by-product. The effective life of a DPF is limited by the very environment in which it subjected to; this is obvious when replacement or cleaned DPFs, fail in very short drive cycles.<

The DPF can easily be damaged or suffer performance reduction with inappropriate cleaning and aggressive regeneration methods. With a nominal expected life of 80,000 miles, consideration should be given not just to its ability in soot removal but also reducing CO hydrocarbons and NOx.<

So, what went wrong with our house guest? The DTC indicated an EGR control defect. The system on our vehicle is both simple and normally robust. A vacuum controlled valve with PCM command through a solenoid. The first test focused on the suction pump, a good pump should provide – 1 bar or 30 inHg.<

Ours reached -25 inHg not a bad result given the vehicle age. The next test was to check the suction at the EGR diaphragm; however this requires a little simple maths. The suction value is a percentage of the pump divided by the actuator duty. So our 25inHg divided by 70% under test does not equate to 5 inHg, so where has it gone?<

Contamination

The sensible action is to remove the manifold to access the actuator and control hoses. These were good, so next a methodical point to point suction test. The suction through the actuator confirmed a restriction, it’s a typical fault, in fact a couple of days later in Ireland the very same occurred on a Volvo s40! On with the new valve, the differential across the valve was better but still low at the EGR. Further tests at the suction manifold were interesting, four outlets, the one not used had excellent suction available, the other three had both a reduction and delayed response. A quick ream with a 3.5mm drill restored full pump suction. So the problem was contamination, later confirmed by the owner who some time earlier replaced the vacuum hoses due to ageing. While the manifold was off I used the Pico to check duty and current through the actuator just in case.<

Next if there is soot in the DPF then there is soot in the EGR. Once removed, the condition and wear demanded its replacement. With no other faults, we can now concentrate on the DPF soot content.<

At 650mb on load with a genuine BMW DPF we stand a chance of recovery. In the DDE module under ECU function there is an option for it to request DPF regeneration. It demands an engine at normal operating temperature, a minimum of 20-litres fuel and 30 minutes of open road driving. That’s 30 min out, 30 min back plus the time reaching our local motorway, the M65 takes you through the witch country.<

Flying high

I actually conducted two regen cycles whilst riding my broomstick, and took the opportunity to monitor both exit temperatures and pressure. Temperature on light throttle was 280ºC, a little low! With a pressure of 40mb, at high load temps raised to 380ºC with 195mb, all very good news.

Just to finish, and reflecting on a recent 5th Gear review on fuel quality, low grade fuel retards the combustion process resulting in an increase of unburned fuel together with all of the nasties that come with it.

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