CIS K-Jetronic: Injection timing? - PeachParts Mercedes-Benz Forum

Benz Mondi · #1 03-27-2017, 05:46 PM

This is the fuel injection system used throughout most of the mid-80"s to early 90's cars. Since it's mechanical and has no electronic "trigger" at the injectors and no timing mechanism in the fuel distributor to fire the injectors, how does the system know when to squirt the fuel into the cylinder? Thanks.

Adriel · #2 03-27-2017, 06:00 PM

This why K-Jetronic is Constant Injection System: fuel constantly injected behind the intake valve and the intake valve basically handles when fuel enters the combustion chamber.

Am not able to on the mobile to get you the film, but in about a couple of hours can post a link. Or just search YouTube yourself. ;p

Frank Reiner · #3 03-27-2017, 06:01 PM

Quote:

Originally Posted by Benz Mondi

This is the fuel injection system used throughout most of the mid-80"s to early 90's cars. Since it's mechanical and has no electronic "trigger" at the injectors and no timing mechanism in the fuel distributor to fire the injectors, how does the system know when to squirt the fuel into the cylinder? Thanks.

The answer is in the name of the system: CONTINUOUS Injection System (CIS). It is a constant flow system, not a timed or pulsed type.

Diseasel300 · #4 03-27-2017, 06:16 PM

Elaborating on what others have already said above, the injection is continuous on all cylinders at the same time all the time. The fuel pressure and volume control how much fuel is being injected at any given time determined by the metering plate in the intake stream ahead of the throttle plate.

The injectors pulse based on the fuel pressure similar to a diesel injector, but at a much lower pressure. The idea being that they "buzz" and create a "fog" of fuel at the intake port. When the port opens, the "fog" is sucked in and mixed with the intake air, then compressed and burned like any other "port-injection" fuel-injection engine.

It's a clever system, but very complicated and difficult to troubleshoot if you aren't familiar with it. When it works properly, it works GREAT. When it has a problem - people have sold and scrapped cars after getting fed up trying to fix them!

Adriel · #5 03-27-2017, 09:18 PM

Here is the film that was going to post: https://www.youtube.com/watch?v=a4fJAfXYxWk.

Over all, simpler, more robust than the previous D-Jetronic, with about the same efficiency, if not more efficient (D-Jetronic can easily run too rich due to a sensor/sensing value out of specification, especially once got older and parts wear, especially the M.P.S. diaphragm). My '71 Volkswagen Squareback still has her D-Jetronic and love it more, but even those oft got converted to carburetors.

Here is what Tram said when first purchased the 280TE:
"D- Jetronic wasn't as accurate as you think, plus very prone to breakdown. In the early days of what we now call L- Jetronic, VW called it "AFC Fuel Injection" for "Air Flow Control". It was far more efficient with less moving parts such as trigger points and the MAP sensor to fail, but air flow meters- being electronic- are still prone to problems, and expensive.

The K- Jet (or CIS later CIS-E) system is just as efficient as L Jet, but without the electronic issues. About the only things that go really wrong is the plunger sticks, the O2 sensor goes out of spec, or the differential pressure regulator starts leaking.

It's a really great system. There is a later system called "Motronic" that adds a pair of magnetic crank sensors that while efficient, is adding more electronics again.

What you need to remember about K-Jet is- like everything else- "don't overthink it"!"

Hope this helps.

Mike Murrell · #6 03-28-2017, 07:39 AM

Mine is 26 yrs. old and I've never done a thing to it. That may be why it works.

Adriel · #7 03-28-2017, 11:41 AM

Quote:

Originally Posted by Mike Murrell

Mine is 26 yrs. old and I've never done a thing to it. That may be why it works.

Mine been gong since September 1984, but did sit for almost 10 years. With it running not 100 percent, have some parts to rebuild the fuel distributor and make sure fuel is going in the right places.

Right now, need to focus on other things, like filing taxes (today's project).

t walgamuth · #8 05-23-2017, 06:52 AM

My advice parallels Mike's, don't touch your fuel injection until you are certain all the other systems are working correctly. Its very unlikely to be the problem and its easy to Screw it up.

Adriel · #9 05-23-2017, 11:11 AM

Quote:

Originally Posted by t walgamuth

My advice parallels Mike's, don't touch your fuel injection until you are certain all the other systems are working correctly. Its very unlikely to be the problem and its easy to Screw it up.

Okay, Tom.

Saw something very interesting: flushing the distributor using a separate tank filled with carb cleaner and A.T.F. because of sitting. Thought that had validity and be sure nothing was sticking before tearing apart and finding nothing wrong. What do you think?

Frank Reiner · #10 05-23-2017, 11:33 AM

Quote:

Originally Posted by Adriel

Okay, Tom.

Saw something very interesting: flushing the distributor using a separate tank filled with carb cleaner and A.T.F. because of sitting. Thought that had validity and be sure nothing was sticking before tearing apart and finding nothing wrong. What do you think?

A:

Perhaps you could alert all of us as to the source of what you "saw", so that no one will ever rely on that source.

Bringing a strong, non-petroleum based solvent into contact with the diaphragms in the fuel distributor is a spectacularly bad idea.
The solvents in "carb cleaners" are intended for metal cleaning, and will attack any rubber, whether natural or synthetic.
A more appropriate solvent would be kerosene, or "paint thinner/mineral spirits".

Adriel · #11 05-23-2017, 12:05 PM

Quote:

Originally Posted by Frank Reiner

A:

Perhaps you could alert all of us as to the source of what you "saw", so that no one will ever rely on that source.

Bringing a strong, non-petroleum based solvent into contact with the diaphragms in the fuel distributor is a spectacularly bad idea.
The solvents in "carb cleaners" are intended for metal cleaning, and will attack any rubber, whether natural or synthetic.
A more appropriate solvent would be kerosene, or "paint thinner/mineral spirits".

Mr. Reiner, thank you so very much for the help!

It was the Source, but on a 6,3, so maybe different.

Brake cleaner fall into that also, right? I keep that way from rubber just based on what it does to my skin (you try and avoid, but still happens).

Or diesel oil, right? Kerosene is #1 diesel, thus my thought of why.