A Guide to Spark Plugs for the 1992-1996 Jaguar XJS
Keep your classic Jaguar running smoothly with the correct spark plugs for its unique 4.0L or 6.0L engine.
- The V12 engine requires 12 spark plugs and the replacement job is significantly more difficult and time-consuming than on the 6-cylinder engine.
- NGK is the most recommended aftermarket brand by XJS owners for both reliability and performance.
- Always check and adjust the spark plug gap before installation; V12 models require a very specific 0.025" gap.
- Use anti-seize on plug threads to prevent them from breaking off in the aluminum cylinder head, a known and difficult problem.
Understanding Your XJS Engine: 4.0L vs. 6.0L
The 1992-1996 Jaguar XJS was available with two distinct engines: the 4.0L inline-six (I6) and the 6.0L V12. These engines have different spark plug requirements and replacement procedures. The I6 is a more conventional design, making spark plug changes relatively straightforward. The V12, however, is a more complex engine where the spark plugs are located deep in the 'V' valley, making replacement a much more involved job.
Symptoms of Failing Spark Plugs
Worn or fouled spark plugs can cause a range of noticeable issues. If you experience any of the following, it may be time for a replacement:
- Rough Idle: The engine may shake or vibrate more than usual when stopped.
- Engine Misfires: You might feel a hesitation or stumbling during acceleration.
- Poor Fuel Economy: If you're filling up the tank more often, inefficient combustion from bad plugs could be the cause.
- Difficulty Starting: A weak spark can make it harder for the engine to turn over.
- Check Engine Light: On later models, a misfire can trigger a warning light.
A Note on V12 Misfires
On V12 models, a misfire can be a serious issue. Unburnt fuel can be sent into the hot catalytic converter, which can lead to overheating and, in extreme cases, a fire. This is often related to the ignition system, so addressing misfires promptly is critical.
Choosing the Right Aftermarket Spark Plugs
While Jaguar originally specified Champion plugs for the US market and NGK for Canada, many owners and specialists now prefer NGK for all models due to consistent quality. Aftermarket brands offer a range of options, from basic copper to long-life iridium plugs.
Brand Quality Tiers
- Top Tier (Excellent Performance & Quality): NGK and Denso are widely regarded as top choices for the XJS. Many owners report smooth performance and reliability with NGK's V-Power or Iridium IX plugs. Denso is another high-quality option.
- Mid Tier (Good Value & Reliability): Bosch and Autolite are considered good quality alternatives. Some owners have used them with success and find them to be a good balance of price and performance.
- Standard/Economy Tier: Champion, the original equipment brand, is still available. However, some long-time Jaguar enthusiasts feel that modern Champion quality is not what it used to be. Motorcraft plugs may also fit but are less commonly discussed for the XJS.
Iridium vs. Copper Plugs
Iridium plugs offer a longer service life and can provide a more consistent spark over their lifespan. This is especially beneficial for the V12 engine, where changing the plugs is a labor-intensive job. Traditional copper plugs are less expensive but may need to be changed more frequently. Some owners prefer the performance of copper plugs, especially on older ignition systems.
| Brand Tier | Example Brands | Estimated Price Range |
|---|---|---|
| Top Tier | NGK, Denso | $8.00 - $21.65 |
| Mid Tier | Bosch, Autolite | $4.00 - $7.00 |
| Standard | Champion, Motorcraft | $3.23 - $6.00 |
Note: Prices are estimates per spark plug and can vary by retailer and specific part number.
Known Issues, Recalls, and Service Bulletins
There are no direct NHTSA recalls for the spark plugs themselves on the 1992-1996 XJS. However, several Technical Service Bulletins (TSBs) and related recalls are relevant:
- TSB 18-43 (V12): Addresses rough idle and misfires caused by spark plug wires leaking voltage. It recommends a new wire set with revised routing.
- TSB 12-54 (4.0L): For 1995 AJ16 engines, Jaguar specified a 'hotter' spark plug (Part No. EBC 8143) to improve performance and reduce fouling.
- TSB 04-17 (V12): Notes that Jaguar stopped installing temporary 'transit' spark plugs for the 1995 model year, so a PDI (Pre-Delivery Inspection) plug swap is not needed on these vehicles.
- EPA Recall (V12): In 2001, a recall was issued for 1994-1995 V12 models for failing oxygen sensors, which could cause emissions issues and running problems that might be mistaken for bad plugs.
Frequently Asked Questions (FAQ)
How often should I change the spark plugs on my XJS?
For standard copper plugs, a good interval is every 15,000 to 20,000 miles. For iridium or platinum plugs, you can often go much longer, up to 60,000 miles or more. However, given the age of these vehicles and the tendency for the V12 to foul plugs, more frequent checks are a good idea.
What is the correct spark plug gap for my XJS?
The correct gap is critical. For the V12 engine, the gap is very small, typically 0.025 inches (0.64mm). For the 4.0L I6, the gap is usually larger, around 0.035 inches. Always verify the specific requirement for your plug and engine, and check the gap on each new plug before installation.
Is changing the spark plugs a DIY job?
For the 4.0L inline-six, it is a very feasible DIY job for someone with basic tools. For the 6.0L V12, it is a much more difficult and time-consuming task. It often requires removing the air conditioning compressor, throttle pedestal, and other components to gain access. Many owners budget a full afternoon or 6-8 hours for the V12 job.
Do I need to use anti-seize on the new spark plugs?
Yes, it is highly recommended to apply a small amount of anti-seize compound to the threads of the new plugs. This prevents the steel plugs from seizing in the aluminum cylinder heads, which can cause the plug to break during future removal. However, some modern NGK plugs have a special coating and the manufacturer advises against using anti-seize. Always check the plug manufacturer's recommendation.
Technical Specifications
OEM Part Numbers: EBC8143 EBC4021
Spark Plug Gap (V12): 0.025 inches (0.64mm). Spark Plug Gap (I6): ~0.035 inches. Engine variants: 4.0L inline-6 (AJ6/AJ16), 6.0L V12.
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
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