Jaguar XJR-S: Does it have Sequential Injection?

For enthusiasts and owners of the iconic Jaguar XJ-S, and particularly its high-performance derivative, the XJR-S, understanding the intricacies of their electronic fuel injection (EFI) systems is paramount. These sophisticated systems, while revolutionary for their time, can present unique challenges. One common question that often arises, especially concerning the more potent XJR-S models, is whether they benefit from the advanced technology of sequential fuel injection. Let's delve deep into the mechanics, history, and nuances of these fascinating Jaguar EFI setups.

Unravelling the Jaguar XJR-S Fuel Injection System

The core question that often sparks debate among Jaguar aficionados is the type of fuel injection system employed in the XJR-S. While the standard XJ-S models evolved through different EFI iterations, the XJR-S, a product of JaguarSport (a collaboration between Jaguar and Tom Walkinshaw Racing – TWR), often featured more specialised components derived from racing applications. Indeed, it is confirmed that the Jaguar XJR-S did feature sequential fuel injection.

According to reports from individuals deeply familiar with these vehicles, such as John Goodman, the XJR-S was equipped with a Zytek ignition and sequential fuel injection system. This was no mere off-the-shelf solution; it was, in essence, the very same injection system found in Jaguar's formidable Group C Le Mans race cars of the 1980s. Jaguar itself made a significant point of publicising this advanced feature, highlighting its racing pedigree and the performance benefits it brought to the road-going XJR-S. It's speculated that TWR might have had a surplus of these high-performance components, finding a perfect home for them in the exclusive XJR-S. This sophisticated system contributed to improved fuel economy compared to the earlier H.E. engines, defying initial scepticism.

The Evolution of XJ-S Fuel Injection: D-Jetronic vs. Digital P

Before the advent of the XJR-S's sequential system, the standard XJ-S models utilised two primary types of Lucas fuel injection systems, both based on Bosch designs. Understanding these earlier systems provides crucial context for the XJR-S's advancements.

Lucas D-Jetronic System (1976-1980 XJ-S, 1976-1981 Saloon)

The earliest XJ-S models (1976-1980) employed a Lucas system derived from the Bosch D-Jetronic. Key characteristics included:

• Triggering: Utilised a trigger board within the distributor, operated by a magnet in the rotor.
• Throttle Sensor: Featured a 'throttle switch' providing an on-off pulse as the throttle moved.
• Fuel Regulation: Fuel regulators maintained a constant pressure in the fuel rail.
• Oxygen Sensors: Lacked oxygen sensors, meaning no feedback control for mixture adjustment.

Lucas Digital P System (From 1980 XJ-S, H.E. Engine from July 1981)

From 1980 onwards, Jaguar transitioned to the Digital P system, also similar to Bosch designs. This system represented a significant leap forward:

• Triggering: Derived its triggering directly from ignition pulses, a simpler and often more reliable method.
• Throttle Sensor: Employed a 'throttle potentiometer' in the same location, providing a smoothly varying resistance as the throttle moved, offering more precise throttle position sensing.
• Fuel Regulation: Fuel rail pressure varied according to intake manifold vacuum, allowing for more dynamic fuel delivery.
• Oxygen Sensors: US-specification Digital P systems incorporated two oxygen sensors (one in each exhaust downpipe), enabling feedback control. Roger Bywater noted that the ECU could even control the mixture in each bank separately. Non-US models without strict emissions laws often omitted this feedback capability.

The improved fuelling accuracy of the Digital P EFI system was crucial in making the higher 10:1 compression flat-head engine viable, which first appeared in July/August 1980. This 300 bhp engine was considered the most lively version of the 5.3 V12. However, it had its fragilities, including marginal cooling and distributor build quality issues that could lead to excessive advance and piston failures, particularly during sustained high-speed driving on German Autobahns.

The H.E. (High-Efficiency) engine, arriving in July 1981 for both XJ-S and XJ12 saloon models, effectively marked the end of the D-Jetronic era.

Here's a comparative overview of the two primary XJ-S EFI systems:

Electronic Control Units (ECUs): The Brains of the Operation

The Electronic Control Unit (ECU) is the heart of any EFI system, interpreting sensor inputs and dictating fuel delivery. Over the XJ-S's production run, several ECU models were used, each offering improvements:

• D-Jetronic ECUs (Type 3CU): Examples include LRZ103 and LRZ113.
• Early Digital P ECUs (Type 6CU): Catalyst versions: DAC 2597, DAC 3586. Non-catalyst: DAC 2596, DAC 3062. These early 6CUs triggered from the coil negative terminal and had internal protection for high voltage spikes.
• Later Digital P ECUs (Type 16CU): Catalyst versions: DAC 4118, DAC 4585, DAC 6335, DAC 6337 (low compression, presumably 11.5:1). Non-catalyst: DAC 4119, DAC 4478, DAC 4586, DAC 6336 (high compression, presumably 12.5:1). There was also a low compression non-cat version, DAC 6338. The 16CU is generally considered superior to the 6CU.
• Facelift Model ECUs (Lucas 26CU): Introduced for the 1992 model year, the 26CU superseded the 16CU, bringing significant advancements: improved starting, more efficient warm-up, diagnostic fault data storage, low quiescent current drain, EPROM socket for modifications, JDS diagnostics serial link, fuel used output, enhanced software, and improved limp home capability. Crucially, the 26CU delivered three injections per engine revolution, a notable upgrade from the 16CU's one injection per revolution (though it reduced to one per revolution after a set number of injections). This allowed for much finer control of fuelling.

ECU Interchangeability: While 6CU and 16CU ECUs are 'pin-compatible' and can be swapped, it's vital that the car's other specifications, such as engine compression, match. For instance, installing an ECU from a 12.5:1 compression car into an 11.5:1 model could lead to dangerously lean fuelling at full load. The 26CU is not interchangeable with the 6CU/16CU due to fundamental differences in its operation and signal processing, particularly its adaptation for the cleaner signal from the Marelli ignition system.

Fuel Injectors: Lifeline of the Engine

Fuel injectors are crucial for precise fuel delivery. In the XJ-S, the wiring harness was typically set up to operate injectors in four sets of three, which were then combined within the ECU to form two sets of six. In early D-Jetronic systems, each set of six included three injectors from each bank. However, with the later Digital P systems (especially those with oxygen sensors), each group comprised all six injectors from a single bank. This allowed the ECU to control the mixture of each bank independently, leveraging the oxygen sensor feedback.

Common Injector Problems and Troubleshooting:

• Plugged or Bad Injectors: Can cause rough running or misfires in a specific cylinder. A misfire leads the oxygen sensor to detect excess oxygen, causing the ECU to richen the mixture for all cylinders, making the whole engine run poorly.
• Testing: Windings should measure around 2.4 ohms at room temperature and not be shorted to the injector body.
• Cleaning: Fuel injector cleaners added to the tank can help. Professional flushing, either in-situ or off the engine, is more effective.
• Leaking Injectors: If an injector jams open or leaks, the trip computer (which calculates fuel usage based on intended flow) will remain inaccurate while actual fuel economy plummets.
• Seals and Noise: Injectors are mounted with two rubber rings. Replacing these can eliminate vacuum leaks and reduce injector noise. These are standard Bosch items, available at most auto parts stores.
• Replacement: Individual injector replacement is acceptable; replacing the entire set is often unnecessary and costly. Note that D-Jetronic and Digital P injectors have different flow rates and are not interchangeable. Later models (1992 onwards) received smaller, lighter injectors with a lower-mounted fuel rail for improved fuelling control at small pulse widths.

Throttle Potentiometer: Smooth Response or Stumbles?

The throttle potentiometer (or throttle position sensor, TPS) is located near the throttle pulley on top of the engine. It provides a continuously varying resistance signal to the ECU, indicating throttle position. A faulty pot can cause erratic throttle response, similar to a bad accelerator pump (which the XJ-S doesn't have). If it breaks up, the EFI computer gets confused, leading to stumbles during throttle changes, even if it runs fine at constant throttle.

Troubleshooting and Repair:

• Testing: The resistance across the pot should vary smoothly as the pulley is rotated. Intermittent readings indicate a problem.
• Repair: Some owners have successfully repaired faulty units by carefully opening them, cleaning contacts, or even modifying the wiper arm to contact an unworn surface. Another creative solution involves using a rear window defogger repair kit to improve conductivity.
• Alternatives: Given the original Bourns part's unreliability and cost, some have adapted TPS units from other vehicles, such as certain Ford models, with custom couplings. Another ingenious solution suggested by Roger Bywater is using a 5K wire-wound potentiometer on a bracket, connected to the throttle spindle.
• Adjustment: After replacement, the throttle pot needs adjustment. With ignition on and the throttle at idle stop, connect a digital voltmeter to the red and yellow leads. Loosen the mounting screws and rotate the pot until the voltage reads between 0.32-0.36V.

Oxygen Sensors: The Key to Mixture Control

For US-spec Digital P systems and later, oxygen sensors are critical for the EFI's closed-loop operation, allowing the ECU to adjust the fuel mixture for optimal emissions and efficiency. The dash indicator typically signals replacement at around 30,000 miles, but sensors can fail sooner.

Testing and Replacement:

• Testing: A digital voltmeter (never analog!) can test a sensor. Apply a propane torch flame to the tip; voltage should rise to 0.8-0.9V quickly. Remove the flame, and it should drop drastically. Slow response indicates a faulty sensor.
• Replacement: Generic Bosch sensors are perfectly adequate and significantly cheaper than Jaguar-branded ones. Ensure the correct thread. For three-wire (heated) sensors, use a heated replacement for faster warm-up.

Temperature and Vacuum Sensors: Critical Inputs

Various sensors provide crucial data to the ECU, influencing fuel delivery:

• Air Temperature Sensor: Provides minor trim to fuel schedules. (Detailed data not provided in source).
• Water Temperature Sensor: Also provides a 'minor' trim, but a failure can lead to major issues, such as extreme richness if the ECU believes the engine is freezing cold. These are known for intermittent failures. Bosch parts are recommended over Lucas.
• D-Jetronic Vacuum Sensor: An external cylindrical unit. Prone to internal leaks, which cause the engine to run rich, especially at part throttle. Testing involves sucking on the hose; no hissing and a sustained vacuum indicate a good seal. Replacement units are hard to find.
• Digital P Vacuum Sensor: Located inside the ECU in the boot, requiring a vacuum line to run the length of the car. Later models incorporated a vacuum reservoir to eliminate resonance.

Common Fuel Injection System Issues and Troubleshooting

EFI Grounding Problems

Poor grounding can wreak havoc on the EFI system, potentially blowing ECUs or causing extremely rich running conditions. Always check the fuel injection harness grounds, especially after radiator replacement or any work in the engine compartment. An intermittent ground on an ECU connector pin can also cause starting issues.

Idle Stumble & Hesitation

• Intake Manifold Leaks: A common cause of idle stumble. While an overall leak might cause a high idle, a leak near a specific cylinder can cause that cylinder to run lean, leading to misfires and potentially serious damage (like dropped valve seats). WD-40 can be used to locate leaks.
• H.E. Idle Stumble: A characteristic of mid-80s H.E. engines, often normal but disconcerting. Some report minor improvement by disconnecting and plugging the vacuum line to the right-side fuel pressure regulator, increasing fuel flow back to the tank.
• Hesitation: Can sometimes be linked to a weak battery or a partially blocked vacuum line to the ECU.

Rich Running Conditions

If your engine is running overly rich, check the fuel pressure regulators mounted on the fuel rail. If they are leaking fuel into the manifold via their vacuum lines, they need replacement. Also, older 3.6 AJ6 engines with Lucas 8CU EFI can run rich due to engine wear causing lower manifold vacuum, which the ECU misinterprets as higher air intake. AJ6 Engineering offers an ECU modification for this.

Cold Start Challenges

For cold start issues, some suggest removing the thermo-time switch or the cold start relay, depending on the specific model and symptoms. A faulty water temperature sensor can also cause overly rich starting conditions.

Passing Emissions Tests: A Few Tricks of the Trade

If your classic Jaguar is struggling to pass emissions tests, and a full repair isn't immediately feasible, a few temporary measures have been suggested:

• Alcohol in Fuel: Adding a gallon of denatured alcohol or 99% isopropanol (12% concentration) to the fuel tank can create a very clean-burning mixture, helping to reduce CO emissions. However, this is hard on older rubber fuel system components and should only be used as a last resort for the test itself.
• Diverter Valve Rewiring: Some mechanics suggest rewiring the vacuum lines so the diverter valve continuously pumps air into the exhaust manifold, which can lower emissions enough to pass.
• CO Analyser: Tools like the 'Gunson's Gastester' allow DIY monitoring of CO emissions to help fine-tune adjustments before a test.

Frequently Asked Questions (FAQs)

What exactly is sequential fuel injection, and why is it better?

Sequential fuel injection (SFI) means each injector sprays fuel just before its corresponding intake valve opens, timing the fuel delivery precisely with the engine's combustion cycle. This contrasts with earlier 'batch fire' systems (like D-Jetronic and Digital P) where injectors might fire in groups, or all at once, regardless of individual cylinder timing. SFI offers superior fuel economy, reduced emissions, and improved throttle response because it provides the exact amount of fuel needed by each cylinder at the precise moment, minimising fuel waste and optimising combustion.

Can I upgrade my XJ-S's ECU to a newer version?

While 6CU and 16CU ECUs are 'pin-compatible' and can often be swapped (assuming matching engine compression), the later Lucas 26CU (from 1992 facelift models) is not directly interchangeable with earlier units. The 26CU operates differently and is designed for the Marelli ignition system, featuring less filtering in its signal input circuitry. Upgrading to the 26CU would require significant modifications. For earlier engines, specialists like AJ6 Engineering can modify existing ECUs or provide enhanced programming for 16CU units to improve performance and address specific issues.

My XJ-S has a rough idle when warm. Is this normal?

For mid-80s H.E. engines, a slight 'stumble' at warm idle can be considered somewhat normal, even if disconcerting. However, it's crucial to rule out underlying issues like intake manifold vacuum leaks, which can lean out individual cylinders and lead to serious engine damage. Fuel injector cleaning or replacement, and oxygen sensor checks, should also be performed. If no specific fault is found, some owners have reported minor improvements by manipulating the right-hand fuel pressure regulator's vacuum line.

Why are oxygen sensors so important in the Digital P system?

Oxygen sensors are vital for the Digital P system's 'closed-loop' operation, particularly in US-spec vehicles. They monitor the oxygen content in the exhaust gases, providing feedback to the ECU. This allows the ECU to continuously adjust the fuel mixture towards the stoichiometric (ideal) air/fuel ratio, optimising combustion for lower emissions and better fuel economy. Without functional oxygen sensors, the system operates in 'open-loop' mode, relying on pre-programmed maps, which is less efficient and adaptable.

What's the best way to test my fuel injectors?

You can perform a basic test by checking the winding resistance (around 2.4 ohms). For a more thorough test, professional shops can flush and test injectors off the engine, observing their spray pattern and flow. For the adventurous DIYer, unbolting the injectors from the manifold (while still connected to fuel and electrical lines), and momentarily 'zapping' them with 12 volts (briefly!) can allow you to observe their spray pattern into small jars, checking for consistency and dribbling. Always ensure proper ventilation and no ignition sources during this procedure.

Conclusion

The Jaguar XJR-S, with its advanced Zytek sequential fuel injection system, represents a pinnacle of performance engineering for its era, drawing directly from Jaguar's racing success. While its predecessors, the D-Jetronic and Digital P systems, laid the groundwork for electronic fuel management, the XJR-S pushed the boundaries further. Understanding the nuances of these systems, from the evolution of ECUs and injector grouping to the critical role of various sensors, is essential for any owner. Troubleshooting common issues like grounding problems, throttle potentiometer failures, or idle stumbles requires patience and a methodical approach. By maintaining these sophisticated components, you ensure your classic Jaguar continues to deliver the smooth, powerful performance it was designed for, keeping the spirit of these magnificent machines alive on the road.

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