Car revs itself upon ignition and backfires constantly. Had checks at garage but they say the only thing they can think of is it might be a valve under cylinder head, which will cost £300 - £ 400 to do. Had exhaust welded on saturday, rear end and middle. Drove 300 miles okay but problem surfaced on Sunday. What could this be?
If it backfires contsantly then it probably is a a valve or a plug wire going to the wrong cylinder. However we have no info on this car. You might tyr scanning for codes and check fuel pressure though to see if something else is wrong.