Starts well every time - lumpy idle til warm - then smoothes out but will regularly misfire every 5 seconds or thereabouts. Just had head rebuilt. Any thoughts would be welcomed. Its my sons car so will get more precise details. Thanks all!
if head has been off, would suggest that the timing be checked.
Does it do it at idle only, and is it a hiccup or a full-blown misfire?
I only ask because I spent a couple of weeks once trying to diagnose a hiccup at idle which later turned out to be normal behaviour as the fuel mix is adjusted by the ECU.
Lumpy when cold may well be timing as Pete suggests, which could make the warm hiccup more of a cough.
I've been told that at cruising speed say 50 in top gear it hesitates every 5 seconds or so??? Also one of the plugs seems a little blacker than it should... not sure if it is oily tho. Piston rings? But then why only a hesitation every 5 seconds?
I had this problem when a wire from my AFR gauge was unplugged meaning the lambda wasnt functioning properly
Just had this checked out and it was the oxygen sensor shorting out! Dont know where that is but the short was sorted and its all good now! Advised to get a new sensor tho!
isn't the oxygen sensor the Lambda sensor? Was probably the earth from the cable that wasn't attached to the inlet earth plate.
Actually... my mistake... it was the knock sensor causing the misfire... shorting out.