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Low Power Mercedes-Benz by Mark Lemay

Autosphere » Mechanical » Low Power Mercedes-Benz by Mark Lemay
Mercedes-Benz ML. Photo: Mercedes-Benz

A twin-turbo ML400 required diagnosis to restore its full performance potential.

This vehicle was brought to us from another shop with a customer complaint of low power and the Check Engine Light on. A quick scan showed codes P008A (Low pressure/fuel pressure too low), P0243 (Electrical fault in low pressure turbo output) and P0229 (turbocharger 1 boost pressure too low). A low fuel pressure problem and a low boost problem are indicators here.

This vehicle features a twin turbocharged V6 engine with Direct Injection. The initial question we had was, is the low fuel pressure causing the low boost or is the low boost somehow causing the low fuel pressure concern or were we dealing with two separate concerns?

Hard to tell

We started out by road testing the ML after we cleared the codes and during a roughly 20-minute test drive the vehicle performed normally, maybe slightly down on power but it was hard to tell as we had nothing to directly compare it to at this moment in time. However during the road test the low boost code set again. Scan data shows the boost pressure under WOT (fig 1). Obviously, the turbochargers are not producing and boost. We checked our information system (Alldata and Direct Hit). They had nothing other than maintenance information on this vehicle. No wiring diagrams, no system operation, no diagnostic troubles codes, nothing. We decided to pay for a short-term subscription to Mercedes-Benz— this process was difficult, but we were able to access the M-B aftermarket site after 10 days of back and forth with the OEM. Once we learned how to navigate the site we found that M-B does not provide trouble code charts or definitions. They do however provide system information and wiring diagrams. Not perfect, but better than nothing.

Scan data showed low turbo boost. Photo: Mark Lemay
Fairly simple

Turbo control on this vehicle is fairly simple. The two wastegates are spring loaded to the low boost position. An electric vacuum pump provides the vacuum required to close the wastegates and vacuum to the wastegates is controlled by a pulse width modulated solenoid. When the engine is started the PCM applies vacuum to the wastegates and that allows vacuum to be bled off to control boost. We noted on a test drive that the vacuum to the wastegates was not strong and was shut off after a few second when we went to Wide Open Throttle (WOT). This is the PCM’s response to the low boost condition. When the PCM cannot get the desired boost it shuts the turbos down by removing vacuum from the wastegates. We brought the vehicle back to the shop and started to check the vacuum system for the turbochargers. We found that the driver’s side turbo wastegate had a leaking vacuum diagram. A new wastegate was installed and the vehicle was road tested to verify the repair. Now the vehicle produced good boost and had a lot more power.

After replacing a waste gate boost was restored… Photo: Mark Lemay
Stalled again

About 10 minutes into the road test and having observed that both turbos were functioning properly, (Fig 2) the vehicle stalled and the Check Engine light came back on. A code scan revealed that the turbo boost code was gone but the low fuel pressure code was back. We were able to observe low side fuel pressure with the scan tool and the data showed low fuel primary fuel pressure. (Fig 3). At this point it looked like we had two separate issues going on.

…yet another issue proved to be low fuel pressure. Photo: Mark Lemay

It turns out that this particular ML has a three-phase brushless fuel pump similar in operation to the Motor Generator operation in a Hybrid Electric vehicle. The fuel pump is controlled by a fuel pump control module mounted in the spare tire well in the back of the vehicle. We checked powers and grounds and the three phase circuits of the pump using a scope and low amp probe and determined that the fuel pump was faulty. We also noticed during our testing that the fuel pump module was really hot to the touch. The pump was drawing very high amperage and rather than take a chance on the module, we replaced the module as well.

Once everything was replaced, the car was road tested to make sure there were no additional problems. It is fairly unusual to have two separate problems causing a low power concern but that proved to be the case on this ML 400. Sometimes, you never know until you begin the actual diagnosis.

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