Big Bertha Needs More Surgery

Big Bertha Needs More Surgery

We left Oaxaca and headed towards Tehuacán. All started out well, but Big Bertha steadily lost power and progressively exhausted more and more thick black smoke. By the time we reached Tehuacán we were having trouble exceeding 40kph and climbing hills. All was definitely not well.

We found a wild camp and the next day Dave started diagnosis with the help of his network of MAN experts. Big Bertha has covered more than 320,000km now and is not ‘as new’. Black smoke is symptomatic of a rich fuel combustion and so diagnosis focussed on what might be restricting air flow: air filter, air paths, turbos, turbo wastegate, EGR, exhaust brake and exhaust.

After a day’s work and testing Dave found that the catalytic converter inside the exhaust silencer was completely blocked. The problem had got out of hand because as the converter started to block, the mixture got richer, leading to more soot, accelerating the blocking process. Soot ‘runaway’ you could say! Dave disconnected the exhaust box and we were able to drive to the very nice campsite in the Cactus Botanical garden, just outside Tehuacán (but why was the catalytic converter blocking in the first place…?).

The first step was to find a way to clean the catalytic converter. After some WhatsApping (thank goodness for the translate function) we found an exhaust work shop (Mofles Ruiz), owned by Javier, that was willing to help us. Javier removed the exhaust box and sent it off to Mexico city for professional cleaning. In the meantime, we parked up in the Walmart carpark and Dave removed the exhaust downpipe and took it to Javier’s workshop to fabricate a new exhaust gas temperature sensor mounting (it was breaking off).

Having received the silencer back from Mexico City, Javier reinstalled it and we bid our farewells.

However, Big Bertha was still discharging smoke (white/grey) and on our journey up over the Cortés Pass (3,600m) towards Mexico City, it got progressively worse. At the same time, Big Bertha’s oil level was rising. Enough was enough; if we carried on , the catalytic converter would block again and we would be back where we started.

This obviously being a fuelling or turbo issue, we contacted Bosch Service in Mexico City and made an appointment for diagnosis. Luckily, José, the boss, speaks fluent English and Dave was able to communicate effectively with him. His initial diagnosis was that BB was had two problems: contamination of the engine oil with diesel fuel (hence the rising oil level) and a rich combustion due to over fuelling. The likely culprits being the high pressure fuel pump and/or injectors. The root cause being bad fuel.

The pump was removed, disassembled, inspected, rebuilt with new parts and tested on the Bosch diagnostic machine. On inspection, there was evidence of ‘fuel wash’ on its drive gear – meaning fuel was seeping from the pump, through its drift shaft seals to the drive gear and from there into the sump. And the electromechanical pressure regulator was also damaged (and replaced).

The six injectors were removed, disassembled, inspected, rebuilt with new parts and tested on the Bosch diagnostic machine. During one test, using high pressure to ensure the injectors were not leaking or ‘dripping’, one injector failed. We initially thought that it was a rebuild fault, but after disassembly and a second rebuild, the test failed a second time. Further inspection of the injector nozzle under a microscope revealed a crack. The nozzle was replaced and the injector then passed testing. Could this be the root cause of our fuelling issues that we have experienced over the past year or so? We surely hope so!

Our test drive was positive – no smoke. And we will monitor the oil level going forward – we’ll let you know. Thank you Bosch Service Maremesa!