There are four engine choices in the X100 generation of Jaguar XK8. It started with the normally aspirated 3996cc V8 that offered 294hp at 6100rpm and 290lb ft of torque at 4250rpm. This had a bore and stroke of 86.0 x 86.0mm and was coupled to a five-speed ZF automatic gearbox.
For the 2003 model year, Jaguar introduced the 4.2-litre V8 as a replacement for the 4.0-litre unit. It had a bore and stoke of 96.0 x 90.3mm, giving a capacity of 4,196cc, which was enough to boost power to 305hp at 6,000rpm and torque to 310lb ft at 4,100rpm. A new six-speed automatic gearbox was used with this engine, which helped it reduce 0-60mph to 6.1 seconds from the 4.0’s 6.5 seconds.
Every XK model has a top speed limited to 155mph, but the XKRs see off 0-60mph in 5.2 seconds thanks to their extra supercharged power. Both supercharged V8s share the same bore and stroke as their normally-aspirated counterparts, but the Eaton blower ups power to 375hp at 6,150rpm in the 4.0-litre and 406hp in the 4.2. Torque comes in at 387lb ft for the 4.0-litre, while the 4.2 offers 408lb ft, though both forced induction engines deliver their peak torque at lower revs than the normally aspirated engines to give them an even more muscular feel.
For the original XKR, the five-speed gearbox was sourced from Mercedes to cope with the power, while the later model shared its six-speed unit with the naturally-aspirated model.
XKR's supercharged V8 offered huge grunt
Much of the XK8’s reputation for poor reliability centres around the engines. However, this is down to the Nikasil liners used in the early 4.0-litre V8s, both standard and supercharged, which would erode in certain circumstances. The engine was particularly prone to this damage if used for short trips and not allowed to thoroughly warm up while using high sulphur content fuel. Notable symptoms include poor starting, sluggish performance as the cylinders leak, and piston slap. Jaguar replaced a lot of Nikasil-lined engines under warranty and good faith, so look for a plate on the engine’s crankcase that says ‘Genuine Jaguar Exchange Product’. If there’s no such plate, it could be an original Nikasil engine under the bonnet that has been used regularly for longer drives, in which case you should have nothing to worry about on this front. Any Nikasil engine that has lasted until now is unlikely to suffer any problems in this area as modern fuel has a much lower sulphur content.
If you’re unsure, ask for a blow-by compression test which will show if the cylinders are leaking due to wear. An engine in good condition should register between nine and 19 litres of air per minute, while more than 30 litres of air per minute indicate a seriously worn motor.
The first AJ-V8s to come with steel liners in place of the Nikasil items will have an engine number beginning 0008181043 or higher. It must be remembered some XK8s were built using existing engine stock, so checking the engine number is the only way to be sure which engine is fitted.
However, even with replacement engines and earlier 4.2s that had steel liners as standard, the XK8 is notorious for letting go in an expensive fashion. These engines came with plastic timing chain tensioners that deteriorate and break up. In the worst case scenario, they will let go completely and ruin the engine.
The upper chain tensioners are the most prone to failure, even with the revised design introduced by Jaguar with the 4.2-litre. A fix is at hand, however, with steel tensioners that were used on the later 4.2s and many exchange unit 4.0s. If there is no history of an earlier engine’s tensioners being changed, budget for this in the price you pay, though it’s not an expensive job and can be done without removing the timing chains. The lower tensioners also break, but without ruining the engine. Replacing the lower tensioners is a wise precaution. Having all of the chains and tensioners replaced will cost around £1200 at an independent specialist.
All four of the XK’s V8 engines should run very smoothly and quietly. Any hesitation to throttle inputs is likely to be due to either a faulty throttle sensor or blocked throttle body. Replacing a sensor is simple, while blocked throttle bodies will need taking apart and cleaning, which is time-consuming but straightforward.
You won’t be able to check the water pump so readily, so budget for replacing this on early XK8s if there is no documented evidence of the working being carried out. Early one-piece water pumps have a plastic impeller that can fail and break up - worst-case scenario again resulting in a wrecked engine. A two-piece replacement water pump costs around £130 and has an impeller that won’t break up.
The other points to watch are sticky thermostats, which are easy and cheap to replace, and radiators corroding due to leaves and debris collecting in them. You should also listen for any rumbles from the XKR’s supercharger due to worn bearings. This isn’t a terminal problem, though, and relatively cheap to fix.
The ZF five-speed gearbox used on the 4.0-litre XK8 was supposed to be a sealed for life unit but it benefits from regular fluid changes every couple of years. It can also suffer from a steel retaining clip that wears through the steel surround and falls out, which causes the gears to slip or requires a few revs before Drive will engage.
With the Mercedes gearbox of 4.0-litre XKRs, the output shaft and planetary gear assemblies can let go, resulting in the gearbox seizing. A replacement secondhand ’box is around £600. The six-speed ZF gearbox used in all 4.2-litre XK8s is reliable and trouble-free.
Finally, the exhaust lasts well but many owners opt for a more open system that lets the V8 engine note become more noticeable. So long as this sort of exhaust has been properly installed it shouldn’t give any trouble.
Owner's View:
"I bought a noisier exhaust for the car from jaguarpartsdb.com which has given a slightly deeper rumble under acceleration without being offensive. Highly recommended as it was only £199 and I fitted it myself."
Dave Clarke