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Think it's expensive to hire a Specialist?

Our Diagnostic Specialist Ross Kemp (AKA 'Mr Scantec') gives one example of how using Specialist services can be the most cost effective route to successful diagnosis & repair - even when you may initially think it is 'expensive'.

We were recently tasked with checking over a Mercedes C220 Sport for the owner after having had several unsuccessful diagnoses and repairs at other workshops with the same fault continuing to occur.

As with every diagnostic booking, we always ask for as much background information as possible and in this case, the owner kindly supplied copies of all previous invoices that listed basic diagnostic results, recommendations and parts replaced.

The reports/invoices unfortunately (as in a lot of cases) did not list the part numbers relating to the parts replaced, so we had no idea if the replacement parts were genuine approved parts or sub-standard ‘imitation’ parts. However, a quick visual check confirmed that they were in fact genuine Mercedes parts.

The owner’s complaint was that the engine was permanently lacking in power with a permanent engine management warning lamp illuminated. The paperwork supplied reference previous diagnoses/repairs suggested that the only fault being logged (on both previous workshop visits) was a P0299FA Low boost pressure.

Connecting our Mercedes dealer diagnostic tool and carrying out a global quick fault code analysis confirmed that in fact we now had two fault codes present. We could confirm that the P0299FA low boost code was still present (despite previous attempts to rectify this on two occasions), but we also had a permanent fault of P246309 DPF soot content too high.

System knowledge tell us that the DPF soot content fault is merely a subsequential fault caused by the vehicle having been driven with boost issues present. This is caused by the fact that in order to enable a successful DPF regeneration during normal driving operations, an excess of boost over and above normal boost conditions is required to prevent a driver perceived lack of power that would otherwise be present during DPF regeneration.

The above vehicle uses a twin turbo design - a smaller HP (High Pressure) turbo and a larger LP (Low Pressure) turbo. As such, for the engine control unit to achieve correct control of this system the ECU employs a boost/pressure sensor for both LP and HP sides of the system as well as two control solenoids.

Ignoring the DPF at that moment (we already knew that the DPF was not sufficiently blocked enough to cause a lack of boost from the turbos) we set about checking the fault data that was captured when the fault of low boost was last logged in the fault memory - something that is quite commonly not available via after-market diagnostic tools used by most generic mechanical workshops. We could see instantly that at the time of the fault occurrence, the ECU (Engine Control unit) was commanding approx. 2.8 Bar of boost pressure but was in fact only registering or seeing approx. 1.5 Bar and that is why the engine fault lamp was illuminated.

Using critical captured fault data and system knowledge, we could already confirm that the HP turbo was generating boost pressure, as even when the engine faulted we still had approx. 1.5 Bar of boost pressure. It also suggested (although not categorically) that we were unlikely to be looking at an intake or boost leak as the system was holding at least 1.5 Bar.

Therefore at this stage (before having lifted the bonnet or a single hand tool), we were fairly sure that the most appropriate initial testing should be focused on the LP turbo and its control.

Using the Dealer diagnostic tool, we carried out a basic activation of both the LP & HP control rods and could instantly see the HP turbo control rod moving backwards and forwards under our diagnostic control, however the LP control rod did not move at all when commanded.

A further check confirmed that when we requested LP turbo control, in fact no vacuum control was present at the control rod’s vacuum diaphragm. Once we had quickly checked the diaphragm was capable of holding a vacuum together with seeing the control rod move and holding a boost position when we applied our own vacuum to it, we were happy to move on and test the vacuum control solenoid that is responsible for the LP control.

Testing at the control solenoid confirmed that the solenoid had a good vacuum supply and the ECU was correctly commanding the valve to open and close, but the valve had failed internally and subsequently was not controlling the LP turbo.

Once the valve was replaced, system faults deleted and the system retested, we could not only see the LP and HP turbos being controlled correctly, but road testing confirmed no loss of boost and no faults being logged.

Now although the above shows several diagnostic steps - in reality as diagnostic specialists, the above represents a very short period of time and having the correct tooling and training has allowed us to pinpoint the area of the fault before even lifting the customer’s bonnet. This also allowed us to eliminate any unnecessary testing and/or guess work - ultimately saving the owner unnecessary costs.

The total cost for initial diagnostic assessment, diagnostic testing, replacement parts, repair labour, DPF final regeneration and a repair of some (unrelated) damaged wiring (spotted during testing) was less than £400.

A further point as to why using a diagnostic specialist is in most cases a better & more cost effective option, if we look at the previous failed and flawed repair attempts we can see that the owner has paid over £300 to have the vehicle tested and has also paid for the HP turbo pressure sensor and the HP turbo control valve to be replaced - we had established that neither of those parts required testing, let alone replacement – before even lifting the bonnet!

So not only has the unfortunate owner of this vehicle paid out £300 unnecessarily, they also risked causing damage to their highly expensive DPF system by continuing to drive the vehicle - as their workshop (who attempted to diagnose the fault) didn’t advise the customer that continuing to drive the vehicle in this failed state could cause other expensive issues.

Still think it’s expensive to hire a Specialist? Think again.


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