hi everyone dynamo here from qld. I recently inherited a rather neglected mechanically 1998 S40 sedan. the car is in really good condition inside but was told by letter it had an issue with the idle speed. I have since then replaced idle speed control unit, plugs, air and oil with filters and new coolant and new fuel filter. the engine still runs ok but still runs rough then almost dies. when it loses idle and starts to run really rough then I hear a loud click and it runs smooth. apart from this issue the car has come up really nice and I don,t want to sell it. i checked for vaccum leaks and checked all pip[es for cracks. nothing. can any suggest what else to look for as I,m rather stumped. p.s also replaced the manifold gasket as well. i can,t work out what or where the click comes from but when it does happen the engine is very quiet and smooth. map sensor is working good. help!!
Is it hard to start? Fuel pressure regulator could be a culprit. when I was looking to by one 3 seemed to have this problem. Pull the vacuum side hose and see if there is fuel in it.
The only thing I can think of that clicks is the A/C compressor clutch when it switches on/off. Does the problem persist when the A/C is switched off?
Has it stored any codes? Perhaps get yourself a scanner or find someone who does and try pulling them. The loud click could be the AC clutch as James said. The AC switching on will open the idle valve and bring the revs up which will effectively mask the idle problem. I have limited experience with whiteblocks as I've only had my V70 a year but do these have an etm or cable throttle? If cable it may require the tps to be adjusted or replaced. If it has an etm it may also need to be serviced or replaced.