Hi, just joined up here, so a newbie. Does anyone know of a good web page with an overview and pictorial diagrams of the mercedes hfm injection system for my car so i can study it.
I just purchased a 1993 300e 124.028 engine is 104.942HFM with hfm fuel injection system and it's having idle speed regulation issues(fluctuation between 0 and 2000 rpm). the just off throttle idle position with load is fine, such as putting into gear and driving off..no hesitation or misfiring with plenty of pulling power., it's just the unloaded idle that is a problem..seems like a vacuum leak to me but visual doesn't reveal any and i am unfamiliar with this system and i like to know the system before replacing parts. An overall block diagram with descriptions would be helpful. Thanks in advance.

Well you have a 93 so watch for the biodegradable wire insulation on the upper and lower wiring harnesses. Coil/coils are always an issue because no one wants to pay 60-80 dollars for them.

im not sure if you have an idle control valve,my 89 has one,it controls idlemairmspeed,if its clogged it will cause a popping in the plenum and a hesitation,with eratic idle,mine is located under fuel distributor,its a big silver can looking thing,with 2 black plastic pipes coming out of it,clean it with carb cleaner,then test it,you can leave it off with the pipes off then look inside it,turn key over as to start the car but not fully,the valve should freely open then close.

Thanks, Yes I am learning about the crumbling wire harness problems, is there a way to determine if the harness has been updated by a part number or some way other than cutting into the sheathing?
I can't get it into my garage until later the week to do any serious work on it yet. right now, i'm just trying to familiarize myself with the HFM system.
I did find info on this forum on reading codes with an led. After clearing the codes, a 4 comes back on pin 8. I think this is the maf? so, i think that's going to be my first point of attack so far. But i would also like to know if it has the dreaded wiring harness because from what i read, i think it would be foolish to spend a lot of time on diagnosing if that hasn't been changed first.

Thanks for your suggestion, What i know so far is this is not CIS, and therefore doesn't have the fuel distributor and idle control valve. i would be more familiar with that CIS but unfortunately this HFM with the hot wire(maf) and "drive by wire" throttle actuator is a new learning for me. I'm starting to understand it though thanks to the wealth of info on this forum.
My goal here is so that once i can put the car into my garage and work on it(it's too hot here in arizona to work on it in my driveway) i will have gained enough knowledge about it so that i can diagnose intelligently without throwing parts at it.. I really appreciate all the help i've been getting so far. it's been tremendous.

There's no idle control per se on this car, idle is controlled by the electronic throttle valve.

If it runs fine under load, it is unlikely you have wiring harness problems. You can sometimes determine the age of the harness by locating the part number tag. It is near the ECU, in the electronics compartment behind the battery. Remove the plastic shield to expose the ECU, ABS contrller, and OVP relay.

For this car, step 1 in diagnostics is to build the code reader. Search for the "Arthur Dalton code reader" for the parts list at Radio Shack - about 10 bucks total. Pull the error codes, clear the error codes, drive it, and see what comes back. That's the starting point.

Could be a vacuum leak, though this engine control system can compensate for vacuum leaks, to a degree. The plastic intake manifold has been known to leak, and there are some vacuum lines underneath the intake.

I've seen fluctuating idle but otherwise good running on this engine from an old O2 sensor. How does it run when in open loop (e.g. from a cold start)?

heck the map sensor, the back thing connected to the air filter box bya flex rubber hose

You can check the date on the engine wiring harness. White tag locate behind the battery.
Mine said F.D. 16/12/2005

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