Residual pressure being lost like others have said due to check valve failure or/and leaking cold start injector.
More test for you guys with D Jet.. more at the w116 forum here stickied at the top under Bosch D and K Jet Troubleshooting.
Main relay;
1)There should be 12 volts at pin # 24 of the ECU.
2)If not,check for 12 volts at pin # 24 of the relay.
3)If 12 volts is not present at the relay but it clicks,replace with a new relay.
4)If there is 12 volts at pin # 24 of the relay,repair
# 24 wire between relay and ECU.
5)If the relay does not click,inspect the wire from the main relay pin # 45 to ground.
6)If the ground is good and 12 volts at relay pin # 38
when the engine is cranked,replace the main relay.
Trigger points;
1)With the engine not running and ECU harness disconnected from the ECU,check the resistance between
ECU harness 12 and 21,then from 12 to 22.One should show a low resistance and the other should show an open circuit.
2)Rotate the engine 360 degrees and check the resistance.They should have an opposite reading now.
3)If not,perform the same test at the trigger points themselves.This will tell you if it a component or wiring fault.
Temperature sensor 1
1)With the engine not running and the ECU harness
disconnected from the ECU,connect an ohmmeter between
ECU harness pin # 1 and 13.At an ambient temperature
the reading should be 200 ohms.
2)If the resistance is well above 200 ohms,check resistance at the sensor itself to determine if it is
a component or wiring fault.
3)Check resistance of all the terminals to chassis ground.There should always be an open circuit.
Temperature sensor 2
1)With the engine not running and the ECU harness
disconnected from the ECU,check the resistance between harness terminal 23 and ground.The resistance should be 2000 ohms at ambient temperature.
2)If the engine is near operating temperature,the resistance should be below 1000 ohms.
Throttle switch
1)Test the throttle switch with the key on,engine off.
2)Slowly open the throttle,and as it opens the injectors should alternately click.You should hear
20 evenly spaced clicks.
3)If this is not met,place a 0.016 in.(0.4mm)feeler
gauge between the throttle stop and throttle stop screw.Connect a voltmeter to terminal 17 of the throttle switch.The voltmeter should show voltage while the ignition key is in the on position.Remove the feeler gauge and the voltmeter should show no voltage now.
4)If this fails,loosen the screws and rotate the switch until it meets these requirements.
5)If they can not be met and voltage is always present no matter where you position,replace the throttle switch.
6)If the voltmeter reads no voltage,ensure voltage is
being supplied to the switch.If so,replace the throttle switch.
7)If no clicks are heard while performing test 1 and 2,check wire numbers 20,17,14 and 9 for continuity end
for end from the ECU to throttle switch.
Pressure sensor
1)With the engine not running and the ECU harness
disconnected from ECU,check the resistance from ECU
harness terminal # 7 to 15.Should be 90 ohms and then check the resistance between # 8 and 10.Should be
350 ohms.If this is not met,test the sensor itself.
2)on occasion,the sensor resistance will check out,but will not hold vacumm.Using a hand held vacumm pump perform this test again (step #1)
Injector circuit
1)With the engine not running and the ECU harness
disconnected from ECU,check resistance of ECU harness
pins # 3,4,5,and 6.All should have less than 25 ohms.
2)If the resistance is greatly higher or lower than
25 ohms,test the injector itself.
3)If the injectors pass,repair the wiring harness.
