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Discussion Starter · #1 ·
We have just acquired a 36000 km 456 that has been sitting for some time and it exhibits the fault of going onto 6 sometimes. All it requires so far is to switch off and then restart whereby it goes back onto 12. We contacted the previous owner who replaced both pumps with new Bosch units. At this stage we have not removed and checked the pumps or the tank as I understand the new fuel can dissolve, or at least break down the rubber sleeve that suspends the pumps within their brackets. From the exhaust colour it seems it is consistently the left bank that has the fuel issue. Has anybody encountered this and dealt with it? I have replaced the relays too with correctly rated units as I am aware these really are a service item which should be replaced every 10 years. My money is still on the partially blocked, by rubber pieces, left hand fuel pump but my experience makes me doubt that as it is always able to perform properly once restarted for sometime. I also if it were rubber pieces blocking the pump for both pumps to be affected as they are side by side basically. Being an old bugger with an innate suspicion of the reliability of anything electronic I am wondering whether it may be electronic, such as a diode, or a dry solder joint within the ECU which is overheating. The relays certainly seemed to make it idle better but fear it was pure hope and imagination. Owing to other commitments (13 other classics and grand children) I haven`t given it a road test yet.
 

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I purchased a 1999 550 three years ago. The previous owner had driven 300 miles in 6 years, so our cars were in a similar state when we purchased them. There are several items I resolved that may be applicable to your 456:
1. Fuel system - when the fuel filters were changed, black rubber debris was discovered. The fuel pumps were removed, the fuel tank drained and lines were flushed (as well as the injector spray bars) and the culprit rubber fuel pump vibration isolators replaced. After 20 years they just disintegrated. Be careful with the fuel line attachments to the pumps -- they are expensive.
2. The ECUs that manage the catalytic converter temperatures have a tendency to crack and allow water to enter and corrode the inner electronics. These are rectangular "boxes" that are mounted on the inner finders near the rear of the engine bay on both sides (one for each bank). They start as a 5-sided plastic box. The electronics are inserted and a resin top coating applied. It's this top coating that cracks. They are approximately $550 per ECU. The catalytic converter sensors also failed giving false readings to the ECUs.
Hope this helps --- There have been times I wished I had the back seats in your 2+2.
 

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Discussion Starter · #3 ·
Thanks Reese, these are all things we are going to go over. We managed to track down the person who imported the car into New Zealand all those years ago and gained useful information. The car was bought not going and the NZ owner replaced both fuel pumps (Bosch units) which got it going but exhibited the symptoms discussed - go onto 6 cylinders, switch off, then on again and back onto 12. It may run on 12 for a couple of days or allow 50 or so kms before going back onto 6. It can drop to 6 then go back onto 12 again when cruising. At first I thought it could be a diode in the ECU overheating, or a faulty relay but am inclined to feel it is still a fuel feed problem caused by dissolved chunks of rubber.
We`ll keep you posted.
 
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