jmartins

Just finished my car's K-Jetronic injection and some questions once it's the first time I'm seeing one injection system like this working.

It´s a normal behavior the injectors squirts fuel as soon as the starter turn on?

There is a lot of fuel at oil pan, so I'm assuming that the injectors are squirting too much fuel while I'm cranking the engine and flooding it.

Is there a way to delivery less fuel while cranking?


Thank you very much!

jmartins

The Fuel pressure is about 19-20 PSI at 24 C.

Rich9928p

Hello,

There is an excellent book published by Bosch titled "Gasoline Fuel-Injection System K-Jetronic"

Do a web search on the term "K Jetronic gasoline injection system" and you will find it, someone has posted a scan of the book.

The book explains the theory of operation. This section answers your question:

The injection valves have no metering function themselves, and open of their own accord when the opening pressure of e.g. 3.5 bar is exceeded. They are fitted with a valve needle (Fig. 9) which oscillates (“chatters”) audibly at high frequency when fuel is injected. This results in excellent atomization of the fuel even with the smallest of injection quantities. When the engine is switched off, the injection valves close tightly when the pressure in the fuel-supply system drops below their opening pressure. This means that no more fuel can enter the intake passages once the engine has stopped.

So, when the engine stops running the fuel pressure should drop and the fuel injectors should close. No fuel should be injected when the engine isn't running. The fuel injectors could be clogged open or there is a problem where the fuel pressure in the lines is too high and the injectors are not closing.

The CIS fuel system must be kept clean and long periods of not running the engine can cause major problems. I've had the fuel distributors and WUR rebuilt on both my '79 CIS 928s. The fuel injectors need to be cleaned, and are very different from the newer electric injectors, CIS injectors are cleaned by flowing fluid in reverse direction. New CIS fuel injectors are less expensive than electric injectors, and the Bosch P/N 0-437-502-010 (for Mercedes) work nicely in the 928s.

Rich
'93 GTS Cover Girl
'87 S4
'79 5-speed rescue
'79 Euro 5-liter track beast

Jadz928

Depending on where the A/F screw is set will open or close injectors.

Turn A/F screw full CCW (injectors should not open at this setting)
Jumper fuel pump (should not here injectors)
Turn A/F screw CW until injectors open (audible 'squealing' sound)
Then turn screw CCW until injectors just close
Turn off fuel pump

This is your baseline A/F. If your system and control pressures are good, your 928 should start.

More tuning is required, but this should get you going. Good luck!

PS. Be careful not to dump too much gas into your oil. May want tl check and change it when tuning is complete.

jmartins

Thanks Rich for the information about the book. The injectors are Bosch Brand new and Mercedes Benz ones.

Jim, the A/F screw is the allen one located close to FD, right? How I should screw and unscrew it? It has one small coil, should I press and screw it?

Thank you very much!

Jadz928

Yes, that is the one... and that is how you do it... with a 4mm 'allen' type

jmartins

Thanks Jim!

Will try this tomorrow and let you know.