740 Exhaust Replaced - Now Runs Rough

Hi folks, I had my exhaust replaced today, from the cat back. It looks to be a good job, too. However, since the job was done, the engine seems to run noticably rougher until it has been driven more than 15 - 20 miles. Even then, it still isn't as smooth as it was before the exhaust was replaced. Does anyone have an idea what the problem might be? Thanks, John.

Only time i've known a car to do that is when the exhaust wasn't "sealed" properly... It sounded rougher, until the exhaust heated up (and expanded) which closed the gap... Might want to get the joints checked out... HTH Mick

Thanks for replying Mick, but I don't think a leak is the problem. The exhaust had a hole in it before it was fixed, and it now seems a really thorough job - it doesn't seem to leak anywhere. The exhaust fitter did use a bit of silicone to seal a gasket (I can smell it when it gets warm) but it was all downstream of the cat, so it couldn't have done it or the oxygen sensor any harm... could it?! John.

I'm not 100% certain, but does the 740 have a twin oxygen sensor set up? (i.e. one after, and one before the cat?) If so it wouldn't harm getting the one after the cat checked... (even a visual inspection) The only other problem i've seen anywhere near this is an exhaust on a MPI mini, where the cat was choked... but you haven't had the cat replaced so you would hope that wouldn't be the case..... Hope you get it sorted soon.. Mick

Hi, The 740 and 940 has a single sensor, not dual sensors. The sensor is also located well away from the cat so problems from a cat back exhaust should not affect it.. Des. . . ;)

Maybe it just needs to be 'run- in'? Perhaps I should just wait a few days and hope it goes away? It's not too bad I guess, but the engine did use to be incredibly smooth before and it isn't any more. John.

Was this a 'fix' or a replacement job? If it was a replacement, was it a Volvo supplied system or a 'High Street' system? The non-Volvo systems can give different characteristics (including back pressure) because of the different pattern boxes. I have experienced this some time ago. A little 're-tune' may be needed.

It was a high street jobbie. It had a hole in it before, so maybe now it has too much back pressure? What sort of re-tune would be necessary? Ta, John.

When this happened to me the car went in to the local dealers for a service/tune so I do not really know what was done. This was our 'posh' car several years ago and most of the work on the engine was done in the dealers. I was just being mean and trying to save costs on an exhaust with a 'lifetime guarantee' for a full system. The dealers got the car running well again.