EFI vs CIS - PeachParts Mercedes-Benz Forum

PanzerSD · #1 10-07-2009, 11:27 PM

Why, on the early 116 M117 engines did they use EFI injection systems, then later switched to the CIS? I thought EFI was a step FORWARD in fuel injection??

dhjenkins · #2 10-07-2009, 11:34 PM

Sometimes you figure out something that works better, but don't have enough bugs worked out to take it to market.

PanzerSD · #3 10-07-2009, 11:36 PM

yeah, Lost of things to go wrong on and EFI engine, I like the CIS, it's much more understandable

and ready for turbo!

Wodnek · #4 10-07-2009, 11:46 PM

The interesting thing is that all the european manuafacturers seemed to abandon djet at the same time. It does seem that kjet was a step back. My 73 450 SE had quite a bit more poke then my 80 450 SEL. Much bigger difference then the extra weight of the SEL could account for.
Was Djet just too expensive? Was it not up to the emerging emmission testing?

PanzerSD · #5 10-08-2009, 02:03 AM

It's more likely that the number and quality/cost of the many components were questionable. Germans don't give up without reason

the Emission numbers were also probably more consistant with the K-Jet, than D-Jet But I'm just speculating. I much prefer the K-Jet system. and the air cleaners look better

iwrock · #6 10-08-2009, 02:43 AM

Quote:

Originally Posted by PanzerSD

It's more likely that the number and quality/cost of the many components were questionable. Germans don't give up without reason

the Emission numbers were also probably more consistant with the K-Jet, than D-Jet But I'm just speculating. I much prefer the K-Jet system. and the air cleaners look better

I think you may be on the right track there...

From what I am reading about D-Jet vs CIS, you sound like your on the right track....

mbdoc · #7 10-08-2009, 08:16 AM

Early EFI systems were plagued with problems. MB even recalled all of the 1971-74 cars to replace fuel hoses. MANY MB's caught fire & burned, over 500 totaled.

Trigger points & the MAP sensor were terrible. So was the actual computer.
Plus the system wasn't sequential, so emissions were higher.

CIS was more stable, although not as smooth at idle.
CIS can't make emission requirements of todays systems.

babymog · #8 10-08-2009, 09:32 AM

The original move to the L-Jetronic system was for the power, efficiency, etc. that it offered and several manufacturers moved to it.

However, failure of the system left the car dead, and for that reason primarily some European manufacturers with reputations of durability and dependability including Volvo and Mercedes-Benz returned to the K-Jetronic / CIS system as it could have failures and still have a "limp-home" mode. The KE-Jetronic / CIS-E helped to make the CIS cleaner and more efficient until a more reliable electronic system was developed.

tjts1 · #9 10-08-2009, 01:18 PM

Volvo went from Djet, Ljet, Kjet (80-82) and then immediately dumped Kjet in favor of LH 1.0-3.1 from 83-93. By 89 they were using knock sensors with 10:1 CR on 87 gas. Those systems were perfectly reliable. I don't know why mercedes stuck with Kjet for 15 years right up to Motronic.

Mike D · #10 10-08-2009, 01:24 PM

Quote:

Originally Posted by tjts1

I don't know why mercedes stuck with Kjet for 15 years right up to Motronic.

Because it worked!

lkchris · #11 10-08-2009, 03:07 PM

EFI is simply a generic term for fuel injection and does not describe a particular system.

The earliest Bosch systems were D-Jetronic followed pretty much simultaneously by K-Jetronic (CIS) and L-Jetronic.

The advantage seen at the time by Mercedes in using K-Jetronic is that the car will still run following failure of system electronic components.

Nevertheless, L-Jetronic is the survivor, and nothing is fitted with anything else these days.

babymog · #12 10-08-2009, 03:40 PM

The knock sensors in the early cars altered ignition timing, not the fuel mixture.

In the early EFI systems, which ran the gamut from the VW squareback and '70s 264 et al Volvos, were prone to complete (walk-home) failures. The progression was then to the mechanical CIS/K-Jetronic for its reliability.

These days it is common to run a car 150,000miles or more with only the MAS cleaning and O2 Sensor replacement in the fuel system, some more. It's come a long way since Bosch Motronic etc.

I still like the mechanical injected diesels too, ...

Quote:

Originally Posted by tjts1

Volvo went from Djet, Ljet, Kjet (80-82) and then immediately dumped Kjet in favor of LH 1.0-3.1 from 83-93. By 89 they were using knock sensors with 10:1 CR on 87 gas. Those systems were perfectly reliable. I don't know why mercedes stuck with Kjet for 15 years right up to Motronic.

MTI · #13 10-08-2009, 03:19 PM

Early EFI units by various brand suffered from limited/poor electronics, however as technology rapidly improved, so did the system control and the ability to monitor and adjust the system to many more engine and environmental situations.

As a result, for those pre mid-90's Mercedes owners that are willing to convert their CIS systems to EFI, there's a whole lot of HP and fuel economy waiting to be tapped.

compu_85 · #14 10-12-2009, 06:04 PM

I prefer the way CIS drives when it's working properly... there's no waiting around for the computer to figure out what you want to do and if it might make a few too emissions. Though the CIS cars I'm used to driving are all lill 4 cyl VWs CIS drives way better then Motronic or even Didifart... errr I mean Digifant (VW's interpretation of Motronic in the late 80s). Same with the good ole mechanical injected diesels... though EDC is quite responsive it's not quite as good as yee olde springs and levers.

-Jason

GGR · #15 10-12-2009, 07:20 PM

D-Jet systems can be reliable even 40 years later. I just did 10.000 miles last summer around the US with a W111 Coupe equipped witha Djet 3.5. My only issue with the system has been the non original fuel pump relay that didn't like the heat of the desert. Problem sorted out on the spot by bypassing the relay with a bit of wire. The problem is the price of some elements when they go wrong: MAP sensors and trigger points are a scandal.

I am currently working on a euro 5.0 M117 that will go in that same car. I am replacing the Kjet system by Megasquirt. In this respect Djet elements come handy, starting with a 4.5 Djet intake manifold that I have grafted to the engine. It was designed for EFI from the beginning, so no strange air routing through the ruins of the Kjet system. Throttle body and intake ports diameter are the same as on the CIS system (later M117 engines went for bigger diameters though). D-jet injectors and rails are a straight fit and plenty available around. I will also be re-using the Djet fuel pump and air & water temp sensors. There were some clearance issues with the 4.5 plenum that were sorted out with a bit of grinding. But nothing compared with the work and cost involved with the fabrication of fuel rails as I have seen in other posts. In a few words, I keep the reliable and readily available D-jet stuff and I ditch out the problematic ones like the trigger points and MAP sensors as well as the "computer". Granted the look of the Djet style air cleaner is not as nice as the Kjet ones. But with the older style valve covers it will look stock in my W111 Coupe. Below are some pictures: