Engine Repair - The alternative to engine replacement

If you look at the factory exhaust manifold – common to all V12s – you can see how short the exhaust gas runner is for cylinders 3, 4, 9 & 10 and how close it is to the entry cone of the primary catalyst.

Occasionally during the life of the V12 engine, usually due to a defective or aged ignition coil, there can be a slight misfire on one of the cylinders. As the V12 is so smooth anyway – 11 cylinders will carry and mask the misfire – a driver probably won’t even notice. However, the misfire transfers unburnt fuel for each cycle (when there was not complete combustion in the cylinder) directly onto the face of the primary catalyst. The primary catalyst is usually running at 480C (900F) so of course, the fuel automatically ignites upon contact with the catalyst.

In the Aston Martin V12s, the primary catalyst is made of a ceramic material. The uncontrolled ignition of the fuel upon the face of the primary catalyst starts to nibble away and erode the ceramic material of the catalyst brick. The dust and debris caused by this erosion action congregate in the catalyst cone collection area which, as you can see from looking at the OEM exhaust manifold, is very close to the exhaust outlet port for cylinders 3,4,9 & 10.

So how does this debris impact the engine? To pass emissions tests it is common industry practise for inert gas (NO2) recycling to be part of a modern engine’s design. In Aston Martin’s case internal gas recycling is used. A large degree of overlap is set between the exhaust and the inlet cam so the engine will for a very brief time suck back up the inert gas from the exhaust into the cylinder. This inert gas in the case of this problem can also contain the catalyst debris from the misfire.

Some of these ceramic particles will simply be mashed onto the piston but some will get mashed down the side of the piston ring and scuff the bore, the piston and the piston ring. This is when the real trouble starts.

As the oil lubricates away into the sump it will contain what is now effectively ceramic grinding paste. The oil with this paste now spreads around the engine and can fail the crank and cam journals and bearings causing irreversible damage to this componentry and necessitating an engine rebuild.

This is why we at Bamford Rose strongly recommends primary catalyst removal and ECU recalibration as insurance against this problem as the V12 cars age (see Phase 1 and Phase 2 V12 engine upgrades for further details).

A common problem affecting all V12 engines is that of Positive Crankcase Ventilation (PCV) valve failure. The PCV system removes unwanted gases from the crankcase.

The PCV system consists of a tube system, a one-way valve and the intake manifold (a vacuum source). The unwanted gases, called “blow-by”, are gases from the combustion chamber which have leaked past the piston rings into the crankcase.
If left unchecked this additional gas would cause increased pressure in the crankcase that would cause a big parasitic power loss. This is because the rotating crankshaft and moving pistons and connecting rods would be less efficient in a highly pressured crankcase. However, these gases cannot simply be vented back into the air as they are full of pollutants and, as a result of entering the crankcase, are further compromised by being combined with the oil that is sprayed into the crank crankcase to lubricate the engine.
These gases need to be exited from the crankcase and recirculated back into the inlet manifold to be sent back into the combustion chamber to be properly combusted. The PCV one-way valve regulates this process and prevents oil from being sent back with the recirculated gas.

The Aston V12 PCVs are beak type valves, within a rubber pipe, which over time are prone to becoming brittle and failing. When the valve fails it stays open allowing the oil to exit with the gases back into the manifold. In the worst case, 0.5 litres of oil can be lost every 1000 miles. There are 5.5 litres difference between the maximum and minimum oil levels for the V12 and this problem can consume the oil much faster than normal. Left unchecked if your V12 is doing the mileage and consumes the oil reserve the rear big ends of cylinder 6 and 12 which are the last in the oil feed circuit can seize up (the oil can also contribute to other problems see our section on V12 Exhaust Catalyst Failure in our Engine Repair section). So another reason to check your oil regularly!

The PCV valves should always be checked at service. The easy way to check this is by looking at air filter because if the filter has staining from engine oil you know the PCV valve has failed. This is because the circuit goes from the engine to inlet manifold pre throttle so if PCV valves have failed the oil that has been let through with the gases will gravitate into the airbox and be absorbed by the air filter. Unfortunately, the replacement of these valves is not as simple a job as you might think because to get to the valves, to replace them, the entire inlet manifold has to be removed – which is a very labour intensive task.

The dreaded V12 tick. The tick happens at idle when you will hear the engine exhibiting a metallic cyclic ticking sound. The sound is typically very hard to locate specifically and is often mistakenly attributed variously to: top end; valve train; or the crank.

The actual cause of the tick is the pistons fit within the cylinder liners. The noise is a result of the cylinder liners being worn into an ovoid shape. Thus at top dead centre (TDC), of the movement of the piston, the circular cylinder head is no longer an exact fit to the lining and the piston can rattle inside the liner – resulting in the tick.

The tick also can be exacerbated by small end bearing noise – the small end may prematurely wear because of the excessive movement of the piston at the top of its movement adding to the noise. In addition, the ovoid liner also results in excess oil consumption which can compound some of the issues further.

This issue impacts small numbers of earlier models of the DB9 (and small numbers of DB7 and Vanquish) with Cosworth built engines (later engines don’t seem to be suspectable). The issue is sporadic, impacting a few of the many early cars and also seemingly not linked to the cars usage. This is why it is important to check before purchase because the solution to this problem is, unfortunately, an engine rebuild that would typically cost £15-20,000.

To check for the tick you need to develop the heat above the piston as this is the best way to then hear any potential tick. To do this rev the engine at 2000-3000 rpm for 3 to 5 minutes then drop to idle. This should warm the engine and if the engine is going to develop a tick you should be able to hear this distinctive noise then.