The Jaguar XJS V12 engine: 5.3, HE and 6.0 — design, performance, common problems

The V12 was the only engine in the XJ-S for the first eight years of production. This article is about the engine alone: how it is built, how the versions differ and which faults come up most often. The full history of the model is in Jaguar XJS 1975–1996, and the choice between the V12 and the six-cylinder in the Jaguar XJS buying guide.

Where does the Jaguar XJS V12 come from?

Jaguar introduced the V12 in 1971 in the Series 3 E-Type; a year later it went into the XJ12 saloon, and from September 1975 it was the core engine of the XJ-S. It was built until 1997. According to production tables, 161,583 cars with the single-cam V12 were made in that time, and the last engine was built on 17 April 1997. From 1983 the XJ-S was also offered with a straight-six, but the V12 stayed in the range until the end of production.

How is the Jaguar V12 built?

It is a 60-degree V12 with an aluminium-alloy block and heads, one camshaft per cylinder head and two valves per cylinder. The 5.3 displaces 5,344 cc with a 90 mm bore and 70 mm stroke.

From the start of XJ-S production the engine was fuel-injected, with a Bosch D-Jetronic system adapted by Lucas (other V12 Jaguars used four carburettors until April 1975). Lucas/Bosch P-digital injection arrived in 1979, and Marelli digital ignition from late 1988. For an owner this matters in practice: ancillary parts and their replacements vary by model year.

What did the 1981 HE heads change?

The HE (High Efficiency) heads of July 1981 reduced fuel consumption and raised power thanks to combustion chambers designed around an idea by Swiss engineer Michael May. According to the Jaguar archive, engineers led by Jim Randle and Trevor Crisp spent five years developing the concept. The results:

• the compression ratio rose from 9.0:1 to 12.5:1,
• output in European specification: 299 bhp at 5,000 rpm and 318 lb-ft (about 431 Nm) at 3,000 rpm,
• a higher-current ignition amplifier (8 A instead of 5 A) and a twin-coil arrangement,
• a taller 2.88:1 final drive, which helped economy and top speed.

Jaguar quoted more than 20 mpg out of town and a top speed of 155 mph for the XJ-S HE. Practical Classics estimates that the HE used 10–20% less fuel than the earlier version.

How does the 6.0 V12 differ from the 5.3?

The 6.0 V12 has a longer stroke (78.5 mm instead of 70 mm) and a displacement of about 5,994 cc. It first appeared in the sporting XJR-S prepared by JaguarSport, and arrived in the standard XJS in May 1993. According to Ateupwithmotor, the standard 6.0 produced 308 hp DIN and 479 Nm, and was the first XJS V12 to get a four-speed automatic, the GM 4L80E. Earlier V12s used a three-speed automatic — a Borg-Warner until 1977, then the GM 400.

US-market versions, with catalytic converters and different calibration, had less power — worth checking in the paperwork of any car imported from the US. For example, the 1995 Jaguar XJS 6.0 V12 in the MB Classics archive was originally delivered to the US market and came to Poland via Germany.

What are the common problems with the XJS V12?

Most V12 problems come from neglected maintenance rather than design faults. Buying guides — including Practical Classics and the AutoClassics guide quoted by Motorious — list the same points:

Cooling

The cooling system is the most important thing to check. The coolant must be changed every two years, otherwise the radiator silts up and the aluminium engine overheats. According to the guides, brown coolant in the header tank is a reason to walk away. A screeching viscous fan coupling is a common but easy fix.

Ignition and spark plugs

The two rearmost spark plugs are hidden by plumbing, and according to Practical Classics changing them takes hours of dismantling. In many cars nobody has changed them, so they are often damaged. Look for evidence in the service history that all twelve have been replaced.

Oil, timing chain and exhaust

Practical Classics recommends oil changes at least every 3,000 miles. A rattle from the front of the engine may mean a loose timing chain, and according to Motorious exhaust gaskets often fail at around 50,000 miles.

Gearbox and mountings

The GM automatics are durable, but a knock from the gearbox mounting often means the spring in its support has collapsed. Older cars sometimes have a harsh first-to-second change.

During an inspection, also look into the vee of the engine: the rubber fuel and coolant hoses should be supple, with no cracks or signs of leaks.

How do you check a V12 before buying?

The most useful step is to compare the paperwork with what you see under the bonnet and on the test drive. Before the viewing, ask for invoices and check:

• the dates of coolant changes (ideally every two years) and any radiator replacement or cleaning,
• replacement of all twelve spark plugs, including the two rear ones,
• how often the oil was changed (Practical Classics suggests at least every 3,000 miles),
• the market specification: according to production tables the US-spec V12 HE had 263 hp rather than the 299 bhp of European cars.

On site, look at the colour of the coolant in the header tank, listen to the engine from cold (a rattle at the front) and watch the temperature gauge throughout the test drive, including when standing in traffic. A wider checklist for the whole car — body, suspension and interior — is in the Jaguar XJS buying guide.

What does it cost to run an XJS V12?

Labour has a big influence on V12 costs. Access to the ancillaries is difficult, so jobs that are simple on a six-cylinder engine (plugs, hoses, gaskets) take longer on the V12 — the rear spark plugs are a good example. Motorious rated the XJS at 2 out of 5 for running costs and 2 out of 5 for DIY friendliness.

Parts availability is good. Jaguar Classic Parts supplies spares, accessories and consumables for the XJS, and the factory catalogue covers more than 22,000 part numbers. Specialist suppliers in the UK add to that.

Fuel is a fixed cost worth working out before you buy: even by Jaguar's own figures, the V12 HE used about 14 l/100 km (just over 20 mpg) in gentle out-of-town driving.

V12 or six cylinders — which engine should you choose?

It depends on what you expect from the car. According to buying guides, the V12 is the version most sought after by collectors, while the 3.6/4.0 straight-six is cheaper to run and simpler to maintain. A comparison of both from a buyer's point of view, along with the differences between the coupé, XJ-SC and convertible, is in the Jaguar XJS buying guide. For how the Jaguar V12 compares with the Mercedes SL's V8, see Jaguar XJS vs Mercedes SL.

More model guides are on the blog, and the cars I have for sale now are in the MB Classics showroom.