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Based on the number of questions we get about Bosch D-Jetronic Fuel Injection, many owners (as well as many professional mechanics) find its operation and maintenance arcane and mystifying. In fact, it's really not all that complicated, and problems can usually be analyzed with no more than a pressure gauge, a multimeter and a little common sense.
D-Jet is inherently a high-performance system, and it's picky about everything else about your motor being right if it's to work correctly. A prime factor in its operation is the level of vacuum in the intake manifold, so it's sensitive to vacuum leaks, compression and valve adjustment deficiencies, or ignition trouble. There's no Lambda sensor to provide corrective feedback based on the composition of exhaust gases and no anti-knock function. It's strictly a fuel management system, and does nothing to control ignition at all.
Bear this in mind, and don't be overly hasty to jump on fuel injection as the cause when there's a problem.
Fuel is supplied to the injectors at a constant, regulated pressure. Each injector contains a small solenoid which snaps the flow fully on or shuts it fully off. The ECU's job is to determine how much fuel is needed and then control how long the injectors are snapped on during each piston stroke (even at full throttle, the injectors are on only a small fraction of the time). The ECU will vary this period in accordance with the following inputs:
Two or three more components (depending on year model) are used to aid cold starting, acting much like an automatic choke would on a carburetor. These are not controlled by the ECU, nor do they provide signals to it. They are the Auxiliary Air Valve (the silver metal cannister below Temp Sensor #2), the Cold Start Injector (centrally located on the intake manifold), and, in later cars, its enabling element, the Thermo Time Switch.
The Fuel Pressure Regulator, on top of the fuel "rail" supplying the injectors, maintains the injector supply at a nominal 30 PSI (adjustable by turning the nut on top). It does this by returning "excess" fuel to the tank -- a partial obstruction in the return line will result in pressure to the injectors rising over the first several minutes of operation (one possible cause of good initial starting followed by increasing excessive richness). For modified, high-performance motors requiring more fuel, adjusting the regulator for higher fuel pressure is the principle way to get it.
Fuel pressure is readily measured by inserting a gauge between the fuel rail and the Cold Start Injector.
NOTE: Due to the pressures involved, all fuel hoses must be rated for fuel injection. Also, use only FI-type clamps -- regular hose clamps will cut into the hose material and cause potentially dangerous leakage in time.
Injectors and Trigger Points:
If only one plug indicates a fuel problem, the associated injector may be at fault. First, please see our article on D-Jet Injector Seals in the Archive.
If all the plugs show a problem, it's not likely that all four injectors are going bad at once. Carefully check the wiring, including the grounds grouped together at the rear of the intake manifold. Check for problems in the fuse box -- the ECU senses analog voltages; if there are voltage supply problems to begin with, generalized problems will be the result.
It is possible to check injectors for spray pattern and flow rate, but I won't encourage you to do it at home. If troubleshooting the rest of the system leaves you with buggy spraying as the only remaining possible failure, have the injectors checked by a professional.
Under the cover is a circuit board with 22 segments, across which three wipers are swept as the throttle butterfly shaft moves. One segment and one wiper make contact and signal the ECU when the throttle is closed. Another segment and wiper make contact to signal wide open throttle. The middle wiper sweeps the remaining 20 segments as the throttle is opened. Each time it "climbs" a segment, it signals the ECU to fire all injectors one additional time. This is, essentially, what D-Jet uses in place of a carburetor's accelerator pump.
Check the adjustment of the switch on its shaft and the condition of its contacts by turning on the ignition (don't start the motor) and manually operating the throttle through its full range. You should be able to count exactly 20 clicks from the injectors as you slowly open the throttle.
It's worth noting here that vacuum leaks, depending just where they are, can cause a D-Jet to run either lean or rich. This is not true for most other FI systems and not true for any carbureted system I know of, so even experienced mechanics are sometimes sorely baffled by this phenomenon.
The simplest mechanical check of a the Pressure Sensor is to unhook its connecting hose from the manifold and suck on it. No air should leak through the diaphragm in the sensor (of course, there are more graceful ways of applying vacuum, if you're both hesitant to do that and want to spend more money).
Temp Sensors #2 and #1:
Temp Sensor #1 causes the mixture to richen when the intake air is cold. A failure of this sensor or its wiring will affect performance and economy, but not major problems.
This is the province of the Cold Start Injector (aka "the fifth injector" or Cold Start Valve). It receives power from the starter solenoid only when the motor is being cranked over. In earlier models, the power is routed through a third FI relay activated by the ECU. In actual practice, the Cold Start Injector simply fires for a few seconds whenever the motor is cranked, regardless of temperature.
In later cars, the third relay is absent and power is supplied directly from the solenoid. The circuit is completed by a ground routed through the Thermo Time Switch, which senses coolant temperature. If the coolant is hotter than 95 degrees F., the Thermo Time Switch opens and no ground is supplied. If you've ever flooded a warm motor by pumping the gas out of turn, this should make perfect sense to you.
If hard cold starting is the main trouble, check that the Cold Start Injector is receiving power while cranking. Check that the other pin on its connector is grounded (on later models, only when the coolant is cold). If that doesn't reveal the trouble, remove the injector, hold it tip-down in a glass jar and see if it sprays when a helper cranks the motor.
The fifth injector does have full regulated fuel pressure applied to it, warm or cold, and fuel leakage through it can cause rich running and poor fuel economy, although not usually enough to cause serious driveability problems. As long as you have the injector out and pointing into a jar, turn on the ignition and don't crank the motor. No fuel should drip.
A cold motor needs to idle slightly fast until the coolant and oil temperatures come up, requiring a little additional air and fuel. On a carbureted motor, this is usually accomplished by holding the throttle slightly open on a choke-operated cam. The D-Jet uses an Auxiliary Air Valve instead, which simply allows a small amount of air to bypass the throttle butterfly when cold, gradually reducing the amount as the coolant warms up. To the rest of the system, this is identical to holding the throttle slightly open; the ECU automatically supplies the needed matching fuel as dictated by pressure and temp senders.
There's nothing electrical about the Aux Air Valve on a Volvo D-Jet (unlike on air-cooled Porsches). Test it by unhooking the connecting hose from the intake manifold and drawing air through it. With a cold motor, air should flow through readily; when warm, it should be much harder (it's OK if you can get a little bit through).
Always begin by checking all connections (including grounds) and by verifying that there are no fuse problems. All resistance checks must be conducted with the ignition off.
Common sense will go a long way towards helping you troubleshoot EFI problems efficiently. If nothing fuel-related seems to be getting power, check fuses and relays (a manual with a wiring diagram specific to your model is more than helpful in this). If the car is tough to start but runs well when warm, look into the components that affect Cold Starting. Note that if a wire falls off a temp sensor, the ECU will go towards rich; high resistance equals cold temps, so an open circuit must mean you're in Siberia at the height of winter -- the ECU's trying to help you out.
Obviously, I can't cover every problem or symptom you might experience with your D-Jet car, but I hope I've given you a basic understanding of just what makes the thing squirt. If you understand this much, you know more than most pros to whom you might entrust your car (unless they're specialists in early Bosch EFI; the later systems work quite differently). Give troubleshooting it yourself a shot -- you'll get a lot of satisfaction from figuring out what's wrong, to say nothing of potentially saving quite a bit of money.