Tapping engines

October 28, 2014 - 10:10 am

With even the latest GTi-6s now reaching 15 years old, many cars are now exhibiting a tapping noise from the engine. This noise rises as the revs rise and can often sound like a sticky tappet. However more often than not the cause is more sinister;

1. Bent Valves following a Cambelt snap
Because of the GTi-6’s XU10J4RS appetite for snapped timing belts, a common cause of the tapping is in fact down to bent inlet or exhaust valves in the cylinder head. This occurs when the cambelt snaps and the pistons hit the valves as the XU10J4RS is an ‘interference’ engine.
The garage doing the repair may visually inspect the valves and decide they are fine to reuse without checking them in a lathe or similar. They can be very slightly bent, and that is all it takes to make the engine tap. These bent valves won’t show up on a leak
down or compression test as the double valve springs are very strong, the only way to diagnose is by someone with experience with these engines or by removing the cylinder head again and inspecting them.

2. Damaged valve guides following a cambelt snap
Another potential cause of tapping can be the bronze Valve guides that may get slightly damaged when the belt snaps and the valves hit the pistons. These are an interference fit in the head so need need changing by a machine shop.

3. Valve to valve contact
The third cause could be when timing up the cylinder head, the camshafts have been turned independently. When this occurs it is possible for exhaust valves to touch inlets as they overlap slightly. This ‘touch’ is enough to bend them and a few engines have become ‘tappers’ this way.

4. Crankshaft pulley slippage
A common problem that occurs with the crankshaft pulley is that due to its construction, the timing hole used to set the engine timing can slip round from its original position. This occurs through hundreds of heat cycles, torque stress and age as the bonded outer ring of the pulley moves in relation to the inner ring. Since the timing hole is on the outer part of the pulley its position can move. Thus if the pulley has slipped and is used to set the timing on the engine, after a cambelt change for example, the timing can be out and piston/valve contact can occur. It is essential to use a known good pulley to time the engine. A triple check would be to check the pistons are at half height in the cylinders and as a rough guide the woodruff key is pointing to 9pm (if you visualise the engine as upright not leaning back in the engine bay). Many people use a lighter solid crank pulley to eliminate this problem and reduce some of the rotating mass.

There are cases of ‘tappers’ dropping a valve after operating fine for thousand of miles. The valves may be weakened from being bent/impact with pistons and decide to snap, decimating the engine, often requiring a new engine block.

Some people decide to replace the whole engine to save on costs but there is no guarantee, the engine you’re buying is in better condition than the one being replaced. Generally speaking the best way to solve the tapping is to replace all the valves and guides in the cylinder head and have it replaced by a competent mechanic.

5. Loose spark plug

To end on lighter note, not all causes of tapping are expensive to fix – it may be caused by something as innocent as loose spark plugs. Simply removing the coilpacks and re-torquing the plugs can stop the noise.