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This issue has been the cause of much controversy over the years. Please note that we as a club can only recommend a course of action. It is up to you, the individual if you would like to take the advice or ignore it.
The E51 has what is known as a Quad-Cat system. This means that there are 4 catalytic converters in total. Two for Bank 1 of exhaust manifold of the V6 engine, and then another two for Bank 2.
The issue from this system is that over time the first catalytic converters from each bank can deteriorate over time and can break down. The debris from the broken ceramic material then created a blocked in the secondary catalytic converters further down the exhaust. Once a blockage has occurred, pressure builds up, exhausts gasses and debris can back flow into the engine and create terminal damage to the engine.
Some owners have luckily had warning signs which allowed them to react and fix the issue before the engine died. These symptoms can range from rattling sounds in the exhaust area (under the cockpit), drop in revs, low power etc.
Other owners have not been so lucky, and the blockage has happened quickly and destroyed the engine before they had time to stop. In this instance, the only options are engine rebuild/replacement or sell the car to a breaker/scrap yard.
The below diagram shows the flow of exhaust gas. From the engines exhaust manifolds, into the primary cats, through the secondary cats and into the centre box.
The best solution to this problem is to remove ALL FOUR catalytic converters and replace them with TWO high flow sports cats. Now this can be a very expensive procedure but gives 100% safety from this issue.
Other options are available, but must be done at the owners risk.
You can remove the primary cats completely and replace with a straight pipe, leaving on the rear cats in place. Alternatively, they can be cut open and have the internals removed, welded back up and replaced on the vehicle. This will, as above, give 100% protection from this issue. Due to the location of the primary cats, this is a very labour intensive operation and will also be costly, although cheaper than the previous option.
The final option is to remove the secondary cats and replace the centre box of the exhaust. Removing the secondary cats allows for debris from the potentially failing primary cats to “pass through”. The same theory applies to the centre box. The standard centre box in the E51 is another “debris trap” and must be replaced with a “straight through” type box.
Before any of the above options are performed, you MUST inspect the cats to see if they have already started to break down or not. If there is already signs of damage or degradation, you need to inspect the secondary cats and centre box for blockages.If any debris is found and blockage looks to have happened, even if it's minor, you need to be aware that there may be SOME damage already done, although it may not cause terminal failure of the engine.
The standard of unleaded fuel in the UK is changing in 2020. E10 (10% ethanol) unleaded is being introduced as the standard unleaded fuel to help combat emissions.
The problem is that ethanol is corrosive to certain types of rubber and can, over time, erode fuel lines and seals in the fuel system.
Although the vehicle will drive perfectly fine if using E10, over time there MAY be degradation which can lead to substantial failures of the fuelling system, be it the pipework, the fuel pump, or anything in between.
Premium fuel in the UK (97+RON) is remaining at E5 (5% Ethanol).
If you are concerned about the damage that E10 fuel may do to your car in the long term, you must switch to using premium unleaded if you do not already use it.