Fuel or ignition problem

I hope you can help with a problem on a 1998 VW Sharan 2.0 petrol manual, with approximately 90,000 miles on the clock. There is no service history. The Sharan starts fine, but there is intermittent power, which is only noticeable when building up speed. It pulls back, there is a ‘popping’ noise that seems to come from the engine, then it goes back to normal power. There is no smoke or backfire from the exhaust, although there’s a light blow from the exhaust system somewhere under the middle of the car. It does seem to use a bit of water, as the header tank needs topping up every few days, but there is no water in the oil.

We have fitted new spark plugs and ignition leads. Steve Wooldridge The symptom of ‘popping’ is generally a sign of a weak mixture or incorrect timing. As the previous history of the vehicle is unknown, I would start by checking the camshaft timing. If the camshaft belt has been recently changed it is possible that the timing may be one tooth out on either the camshaft or the auxiliary shaft which drives the distributor. I would also check that the distributor has not been moved or dislodged.

Once the valve and ignition timing have been confirmed, I would then check for coolant loss. This could be quite significant. It is possible that coolant is leaking across the cylinderhead gasket or the inlet manifold gasket. Not all head gasket problems will result in coolant entering the oil ways.

Finally, I would fix the exhaust leak, which can upset the Lambda readings and the fuel air mixture, and inspect the inlet manifold for any vacuum leaks. Cleaning the air mass sensor and throttle body would also be a good idea.