Cis injection question
 

Cis 1976 911. What is the current thought on new injectors. Refurbished or new. If new are there quality issues on some brands or suppliers.
Thanks lyndon

You can try to clean them by using an injection cleaner additive in the fuel at first try. The problem on these for CIS - they're mechanical valves opening with a pretty wide pressure range from 2,5-3,6bar / 35-52psi when new as Bosch/Porsche specified in the tech spec book for the 911. Problem is: They worn out mechanically over time. A refurbishment, as we know it from the Carrera injectors, is afaik not possible.
If the cleanup with the additive won't work, I personnaly would buy new ones from Bosch as I did when I worked my CIS. IIRC the average mileage lifetime of them is about 100.000mls... Adrian Streather mentioned in his book "Porsche 911 SC essential companion" on how to backflush them. Several threads already discussed this here as well. Never did it personally, the old injectors laying in the garage waiting for a try... Might work, might not. Don't know.
But again - it's always worth a try before buying new ones, no question.

Further reading:
https://forums.pelicanparts.com/porsche-911-technical-forum/399562-backflush-cis-injectors.html
https://forums.pelicanparts.com/porsche-911-technical-forum/403449-how-backflush-cis-injector.html

Many more, even videos:
https://www.google.com/search?q=Bosch+cis+injector+cleaning+backflush

The YT Channel "KlassikATS" has some of the best Porsche and CIS videos in the net: https://www.youtube.com/@klassikats, especially the video series on the 2,7 CIS engine: https://www.youtube.com/playlist?list=PLkWFofpgz2OgF9FdliKnlzuY0pygqRkMj. The basics are pretty the same to the 2.7l and 3.0l SC engines.

Thomas

Correct me if I’m wrong.

The Injection systems prior CIS. They were time injection by way of the pump and timing. Pump raised pressure until the injector opens and sprays into the open valve.

The CIS injectors don’t have a valve, they are wide open. Hence “Injection System”.

Small bit to clarify. The valve in the CIS injector is to prevent dripping. So wide open when engines are running.

I think this might help you decide whether to buy new or clean.
It's a great video, pretty much all you need to know...

PorscheDude, your questions answered perfectly here as well.


https://www.youtube.com/watch?v=YZ4Zx6SPlMU
Bosch K-Jetronic Performance: Why Change Fuel Injectors?
Mercedessource


I just finished watching it.

Phil

Good explanation. I like the test rig. My thoughts are buy new. Hard to backflow the filter.

Not fully correct, the CIS injectors are closed when engine is not running! System prior to CIS was D-Jetronic (D for german word "Druck" = pressure)
Further reading: https://jetronic.org/index.php/en/d-jetronic/51-history

Otherwise they would continue spraying after shutting down and flooding the engine until fuel accumulator and fuel lines are drained. Opening pressure of them is between 2,5 and 3,6bars as you can see in the picture.

https://i.imgur.com/AY5W6KX.png

When shutting down the engine the pressure falls under 2bars which is high enough to keep the fuel pressurized for at least half an hour to prevent building bubbles of boiling fuel due to engine heat which otherwise prevent successful hot start.

Refer to the Bosch Technical Documentation about CIS:
https://files.bmwclassic.nl/E21/K-Jet/Bosch_K-Jetronic_web.pdf

Excerpts:

https://i.imgur.com/EvAhHYA.png

https://i.imgur.com/FndCjze.png

You're correct in the fact that once the valves opened by the pressure they remain open as long as the pressure is high enough which produces a continuous fuel spray in front of the intake valve of every cylinder, that's why the name comes: CIS = Continuous Injection System. The fuel amount is controlled by the fuel distributor with its moving plunger by the air flow sensor plate and lever.

Thomas

I swapped in the Bosch units, job was pretty easy and (in relative terms) the cost was minor. based on the improvement in the car's running in the meantime, it was undoubtedly overdue.

Schulisco, most excellent. I’ve never seen a cross section view of the injector.

L Sjoberg, glad you got through your problem.

Here is the internals of one of mine, minus the valve needle. The odds of cleaning one successfully are pretty slim.
http://forums.pelicanparts.com/uploa...1717778993.jpg
I did pick up one of the pressure testers so I could check for drips, spray pattern, and opening pressure.
Any that fail are replaced with new ones.
http://forums.pelicanparts.com/uploa...1717779113.jpg

Keith, since it appears you will drive air cooled 911's for the foreseeable future, I highly recommend that you buy the book, "Bosch Fuel Injection and Engine Management" by Charles Probst. It's the best single book about how the Bosch CIS and Motronic systems work, how to troubleshoot, and how to improve them.