Project Volkswagen Jetta GTI

Wading into the fuel supply issues on our Mk2 GTI.

Before Volkswagen’s engineers secretly appropriated an Audi 80 GTE engine with Bosch K-jetronic fuel injection, nailed it into a stiffened Mk1 Golf shell, then lowered and uprated the suspension, nobody used the term hot hatch. But having convinced VW management that their skunk works project had legs early in 1975, the German company launched its new Golf GTI (Grand Tourer Injection) a year later in late 1976 and spawned a whole new genre that’s still going very strongly to this day.

Despite this, arguments rage about which car was the first hot hatch. According to some enthusiasts of the marque, the Simca 1100 Ti started the whole thing back in 1973. Others argue it was the Renault 5 Alpine in 1976. Even more ardent fans insist the Sunbeam Lotus was the first proper hot hatch as its Lotus engine gave it 150bhp. They’re all wrong. The phrase ‘hot hatch’ was invented for the Golf, so it wins. Now let that be an end to it. Except this isn’t a Golf, or even a hatchback. That;s because having invented the hot hatch market sector, VW then decided that for those for whom a traditional boot was simply a must-have, they’d slice said hatchback off behind the C-pillar and graft one on. Enter the VW Jetta, which is to all intents and purposes a Golf with a boot.

It was never going to be an easy job replicating the success of the firstgeneration Golf GTI. A tough act to follow it may have been, but with the secondgeneration Golf GTI Volkswagen hit the mark once again, with the top of the line model spawning not one, but two variations of the ever-popular GTI, one with eight valves and one with 16. The Mk2 Golf was produced between 1983 and 1992, with a total of 6.3 million being built across all markets. The GTI was the star of the show, building upon the Mk1 GTI’S hot hatch characteristics in a more refined package. The Jetta was right there beside it, along for the Mk2 ride, once again with a boot lid. With a 16-valve version of the tough and happy to rev 1.8-litre inlinefour engine, our Mk2 Jetta GTI should be wonderfully engaging to drive, offering a truly analogue driving experience which is both enjoyable and easy to live with.

This makes a good one an excellent choice for a first classic – and one you can easily use as a daily driver too. Indeed, ours was in regular use until about 20 years ago, when it was laid up and dry stored, although it still got run up to temperature once a year even then. But then the fuel pump sender fell off into the bottom of the fuel tank, so it hasn’t run now for many years. It came with a replacement used fuel tank and a secondhand pump/sender unit, but we’ll start with an assessment.

Bosch K-jetronic fuel injection

K-jet, as K-jetronic is generally known, is a mechanical fuel-injection system that was first used in 1973 on the Porsche 911T. Since then it’s been employed on a variety of European cars, including Ford, Volvo, Mercedes, Nissan, Renault and, of course the Volkswagen-audi Group. Funnily enough the last car to use it was another Porsche – the 1994 911 Turbo.

The system is also often referred to as CIS, which stands for Continuous Injection System, as unlike modern electronic injection systems, K-jet’s injectors supply a steady flow of atomised fuel onto the back of the engine’s inlet valves, where it then sits and waits for the valve to open before it can get into the combustion chamber. Let’s take a look at the major components of the system.

Fuel Pump

At the heart of the K-jetronic system is the electric fuel pump. K-jet needs upwards of 5 bar pressure to operate, and many cars employ more than one pump to achieve this. Our Jetta has two, one in the tank and another underneath the car. The pump (or pumps) maintain a constant pressure in the fuel supply line, and this pressurized fuel is essential for efficient atomization and injection into the combustion chambers.

Fuel Filter

The tiny particles of dirt often present in pump petrol would quickly destroy the metering head and fuel injectors, so K-jet systems use a fine mesh filter to ensure clean fuel to these close tolerance parts.

Accumulator

The fuel accumulator, or pressure damper, is a key component that helps stabilize and maintain the fuel pressure. It is connected to the fuel supply line and acts as a buffer, dampening pressure fluctuations caused by the pulsating action of the fuel pump. The accumulator ensures a consistent and stable fuel supply to the rest of the system, and it also maintains a residual pressure in the fuel system when the engine is switched off, which allows for easier hot starts and avoids fuel vapour locks.

Fuel Distributor

The fuel distributor is a pivotal component responsible for precisely metering and distributing the pressurized fuel to each injector. It comprises a mechanical plunger and a series of fuel passages. As the plunger moves, it regulates the amount of fuel flowing to the injectors based on the engine’s demand, which is determined by factors such as throttle position and

engine speed. The position of the plunger is controlled by the airflow meter, which is connected directly to it.

Butterfly Throttle Valve

Just like a similar device on a carburettor, this valve is connected to the throttle cable and controls the amount of air that can flow through the airflow meter.

Airflow Meter

To achieve optimal combustion, the K-jetronic system relies on an airflow meter to measure the quantity of incoming air. The airflow meter takes the form of a lightweight steel disc that is positioned in the air intake stream. As air flows through the meter, the disc moves, and this mechanical motion is translated to the metering head’s plunger.

Fuel Injectors

The fuel injectors in the K-jetronic system are responsible for delivering precisely metered fuel directly into the intake ports of each cylinder. The amount of fuel injected is determined by the position of the plunger in the fuel distributor and the input from the airflow meter. The injectors operate at a constant pressure, ensuring consistent fuel atomization for efficient combustion. These injectors are really nothing more than a nozzle, with no ability to control the amount of fuel they deliver. They simply open at a given pressure, and close below it.

Warm-up Regulator

The warm-up regulator is a temperature-sensitive component designed to optimise fuel delivery during engine warm-up. The warm-up regulator adjusts the fuel pressure to provide a richer mixture while the engine temperature increases, ensuring smooth operation during the critical warm-up phase. Once the engine reaches the desired temperature, the regulator stabilizes the fuel pressure for normal driving conditions.

Cold Start Injector

During cold starts, the K-jetronic system employs a single cold start injector to add additional fuel into the inlet manifold. This injector operates independently of the fuel distributor and is activated only during cold engine starts. Once the engine warms up, the cold start injector is deactivated, and the regular fuel injectors deliver all the engine’s fuel needs.

Auxiliary Air Valve

The auxiliary air valve is crucial for regulating the amount of air entering the intake manifold. It opens and closes based on the engine’s temperature, allowing more air during cold starts to create a fast idle speed. As the engine warms up, the auxiliary air valve gradually closes, and the engine idles more slowly as a result.

Bosch K-jetronic mechanical fuel injection is generally a very reliable system that really only requires a new fuel filter every so often to keep it happy. As long as it gets this it can rack up many hundreds of thousands of miles without problems.

K-jet later became KE-JET, the E standing for electronic. However, the system remained largely mechanical, the difference being that the later system used a lambda sensor in the exhaust and an ECU to control fuel pressure, thus providing even more accurate mixture control.

1 We’re lucky to have this mobile lift in the Classics World workshop, so after loosening the wheel bolts we got the Jetta up in the air to better see what we’ve got.

2 This car hasn’t failed an MOT so there’s a chance the suspension bushes will still be serviceable. Although equally possible is that time will have ravaged them.

9 This front jacking point looks slightly suspect, although it held firm with the car’s weight on it. We’ll need to give it a good clean and then assess whether it needs any welding.

6 The rear of the nearside sill seam looks slightly crusty, but further poking failed to impact it, so a clean up and some antirust wax should do the trick here.

5 This bad boy exhaust box needs to return to the back pages of Max Power magazine, but as that’s closed we’ll probably sell it. Stainless steel it may be, pretty it ain’t.

4 Rear discs and calipers also provide the handbrake function. The cables seem free so will hopefully be OK with just a clean up. Springs and struts all look serviceable at this stage.

3 With reconditioned calipers coming in at around £70 a corner, our gut feeling is to buy a set of four, along with new discs and flexihoses at the same time. Spot the go-faster discs.

8 Unfortunately the same can’t be said for the offside, where a small hole appeared after some poking. However, even here the rusted area is very small and the repair should be easy enough.

7 Inside the rear wheelarch near that sill seam and some underseal has lifted, but the steel underneath still feels sound.

Fuel pump – an electric fuel pump provides around 5.5bar pressure for the K-jet system.

Accumulator – the accumulator (circled) smooths fuel pulses and maintains residual pressure in the fuel rails when the engine is turned off.

Fuel filter – clean fuel is essential for reliable K-jet operation so systems always include a large filter.

10 Time to get that fuel tank out. It’s plastic, but the three securing straps are steel. Thankfully the retaining bolts, although crusty looking, screw up into the car’s interior, so the threads live in a relatively benign environment and they all came undone relatively easily, and certainly without snapping.

11 The tank is also held in with these two screws on the filler neck assembly, which is all one moulding and part of the tank.

Ford V6 with K-jet – many European cars used K-jet, including this Ford Capri 2.8 Injection Special. You can see the metering head at the front on the right.

K-jet equipped Mercedes V8 with airflow meter arrowed at the front, metering head behind it.

13 A good hosing and scrubbing later and we could transfer this rubber section on to our replacement tank. It had been filthy, but was still in very good order.

12 With the tank out and lowered to the ground, we needed to clean all that accumulated mud from the filler area as it was clear our replacement tank was missing something.