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Small exhaust leaks suddenly a huge issue?


elbekko
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Hi guys,

Weird things going on, as always.

Coming home from Seven Sisters with the loaded trailer, the engine started running worse and worse. Getting off the channel train, we hooked up the OBD reader, and saw Bank 1's fuel trims maxed out (long-term +25%, short-term +25%). Stopped to unplug the lambda sensor, and then at least it was only the long-term trims affecting us.

Over the past few weeks I've been trying to make sense of things. I drove for a while with the sensor unplugged and the trims reset, and that seemed fine (and I really, really didn't feel like finding out wtf had broken this time :glare:). Plugged the sensor back in, and immediately the same problem. And seemingly worse with every restart, so I think the Motronic ECU adds the STFT to the LTFT on every shutdown or something, even though OBD says 25% is max, I'm pretty sure it's going well past that.
At startup the lambda would be switching until a few minutes into the drive, then it started trailing off towards 0v, and staying there. Bank 2 working normally.

So on Friday decided to have a proper look at it. Started by tracing wiring and grounds, all good. Replaced the sensor, same thing. Ok, there must be a massive exhaust leak upstream then. Found one of the manifold gaskets with a small leak, replaced both with a slathering of exhaust paste, looks good. Drive home with the OBD tool attached, bank 1 is switching fine...

And now bank 2 does nothing. Same symptoms. Lambda is switching fine on startup, then trails off towards 0v when under load..

Very strange that it switched banks, but ok, the same problem must be on the other bank as well then? Weird, but I had a fix for bank 1, so do the same. All gaskets look fine, somehow wiggle out the manifold without taking off anything but spark plug leads, and find a few tiny cracks in the manifold. Weld those up as best I can (one ran between the tubes...), refit everything with exhaust paste and new gaskets.

Switching fine on idle, start driving home, once on the motorway... trailing back off to 0v. And switching between 0v and 0.120v. Get off the motorway at home, back to switching normally.

So, err... what? How on earth could what now can only at best be a pinhole leak in the manifold where I couldn't reach with the welder, cause that big an issue with the lambda?

All I can think of is there must be a blockage downstream in the exhaust. I don't think it's a plugged cat, as the problem switched banks. I went around with a small hammer while she was on the lift yesterday and couldn't hear anything rattling. And could an exhaust blockage that isn't bad enough to feel power loss really lead to a lambda sensor reading lean?

All help appreciated, I'm about ready for a can of petrol and some matches.

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