What to do when my M103 is giving me trouble: A beginners guide

***Disclaimer: Anything written from this point forward is from personal experience with M103 engines. Your experiences may be different and reflect otherwise from anything I’ve written. I am not responsible for anything that happens to you or the car as a result of reading what I’ve written. I’m not here to offer advice as to which brand is better. Wrench responsibly***

What do I do when my 300E / M103 engine is giving me trouble:  A beginner’s guide

Is your 300E giving you starting trouble? Does it have a rough idle? Misfiring at all? Fuel economy sucks? High Idle? Well keep reading and we’ll shed some light on what makes the engine in your car tick – some tune up advice, proper care and preventive maintenance. We’re going to try and start with the basics, from easiest to some of the more difficult jobs to do on the engine.

Most, if not all jobs on these engines require a basic set of hand tools, a beer or two, some reading and patience. Keep in mind one basic rule when it comes to the M103 engine we find in the 300E’s – if you take something apart, it goes back together ONE way. It’s very hard to ruin something on these robust engines. This is simply an outline of what you need to keep your car running in tip top shape, DO NOT expect your car to be running flawlessly if you don’t keep up on maintenance. These aren’t Fords or Chevy’s, expect to pay to play. There are a ton of parts on these cars that are extremely expensive and it would be more cost effective to get some original parts from a junkyard. Spend and approach your issues wisely and you won’t be spending a ton of money on your car!

We’ll start from the top – “My car runs rough. Misfires, bucks, sometimes it’ll stall, where do I start?”
            First and foremost, when was the last time you did a complete engine tune up on your car? If you can’t remember, guess where you’re going to start?

SPARK PLUGS

            We’ll start with the absolute basics – spark plugs. If the spark plugs you just bought have “Platinum” “Iridium”  “Titanium” etc, you bought the wrong spark plugs for the car. These cars are very sensitive to the type of spark plugs that are installed in them because most common spark plug is resistor type. The problem for our cars is that the spark plug wires have resistance built into the wires. When used in conjunction with resistor type spark plugs, you are running your engine with an inefficient burn. Over time this could develop into a misfire, fouled plugs, and more varnish in your engine. The correct plug is a copper core, non resistor plug.

Here’s a list of common spark plug replacements for your engine:

Denso T20EP-U

Bosch H9DCO

NGK BP6EFS

Spark plugs should be gapped:  .032" - .035”

DISTRIBUTOR CAP

             One of the most overlooked parts on the engine. It’s relatively simple to replace, with a few Allen screws around it. Make sure to mark your spark plug wires when you remove them as confusing the wires will result in a bad misfire when starting the car, and/or your car just won’t start. Moisture has a tendency to make its way to the inside of the cap and cause all sorts of havoc on your ignition system. When you remove your cap check the inside center portion of the cap for the “rotor button” if the button is missing or doesn’t feel like it is spring loaded, then you must replace your cap. Along the same portion, if you see a slightly white hazing in the cap (usually red-orange) than you have an arcing problem, which is basically you not getting a complete and powerful spark, and in most cases is the cause of your misfire, big or small. Along the interior rim of the cap, you’ll find 6 points where the spark transfers from the rotor to the cap, you’ll find every single point to have some slight discoloration, that’s where spark transfer occurs and is normal. If the points are crusty, it’s a good time to replace the cap. If it’s been on long enough to get that bad, give your car a favor and replace it. If you don’t have a new part handy, you can clean up the points with some sandpaper and bring out a new nice shiny finish. This will buy you some more time with the cap and is a good point for some diagnostics.

I’ve had many cars stop misfiring just by cleaning up the cap. They get so corroded over time that it’s a wonder how some of these cars stay running. I usually replace the caps every 15k -25k miles (depending on weather, dry climates will be able to take these further) and I’ll remove the cap and clean it up at around every 7,500 miles. Keeps everything in tip top shape and you can keep tabs on the integrity of your parts.

DISTRIBUTOR ROTOR

            You have to remove the distributor cap to get to this part. This part spins around inside the cap and directs electric flow to every point on your distributor cap. Just like the distributor cap, if it’s crusty and worn down, it’s overdue for replacement. These are usually black in color, if you find white hazing around the part; it’s got the same arcing problem as the cap. Replace the part. Otherwise, with the same interval as your distributor cap, clean the rotor and you should be good to go.

SPARK PLUG WIRES

            I’ll be honest here; I’ve never replaced a set of Mercedes original spark plug wires. They’re extremely robust and some of the best wires around. I’m not saying they’re invincible, but they’re pretty darn close. The wires themselves are very low resistance, but the boot ends themselves have roughly 1k ohm resistance built into them. Pull out your ohmmeter and you should get a spec from 800ohm to 1.3k ohm for a good set of wires. You can easily check the spark plug wires for arcing on a dark night and a little bit of misted water. If it looks like a small lightning storm on your engine, you’ve found part of your misfiring.  MAKE SURE YOU PULL THE WIRES OFF OF THE PLUGS BY THE ENDS, NOT THE WIRE. If you yank on the wire, you risk pulling the wire out of the end! It’s happened to me and it’s not fun. Don’t bother with any aftermarket wire saying you’ll get increased fuel economy, etc they’re full of it. I’ve never found any noticeable difference in an aftermarket set of wires vs. an OEM set of Bosch wires.

OKAY!

So you’ve gotten this far, what you’ve done so far was a very basic maintenance check on your M103 engine. More often than not, it’s simple maintenance that most owners neglect in ownership of their cars. Now we’ll move on to some less basic and slightly more involved parts of your ignition system.

OVP RELAY (Overvoltage Protection Relay)

All of your engines vital components are protected by this little relay. Do some more research elsewhere on this part as it can cause a multitude of problems from hard starting, stalling, rough idle, etc. I’d be typing out a novel trying to explain the function and theory of this particular part. Essentially, if you have the old style (single 10amp blade fuse) on top, replace it immediately with the updated 2 fuse design. They’re more reliable and will save you headaches down the road. A bad OVP relay will still let the car start, but it’ll run rough and may stall.

FUEL FILTER

Come on now, if you’ve never replaced the darn thing then do it ---- 15-20k mile service intervals. They don’t last forever and they’re very easy to forget about. A poor fuel supply is enough to cripple the car. It’s cheap, it’s a little smelly, you might get a little buzz from doing the work under the car, but it’s easy.

AIR FILTER

I’ve never experienced an air filter crippling any of my cars before, even when completely covered in leaves, debris and bugs. Replacing it is good and cheap insurance. If you’ve never done it, chances are you might be getting a more efficient burn and better fuel economy! Pays for itself! Don’t bother with aftermarket air filters, the factory filter is the BEST cold air intake we can get for these cars.

OXYGEN SENSOR

Another overlooked part on the cars. Some cars didn’t come equipped with a check engine light, so most people never think to look, is the O2 sensor. They have 100k mile service interval. Replace it and don’t worry about it for another 100k miles. Otherwise, you’re burning more fuel than you really need to so its another part that pays for itself over time. It’s connected under the passenger side carpet and gets fed out through a grommet on the trans tunnel.

NOW, I’ve had it happen to me twice (on the same car no less). I guess over time the sensor wire might feed itself out of the grommet and give itself a lot of slack to be waving around as you drive your car. There’s a problem with this as it’s able to touch the spinning driveshaft. I’ve had these sensors SHORT out on the driveshaft and make the car completely inoperable. It would buck and run extremely rich. Misfire like crazy, etc. A short in the O2 sensor sends a variety of signals to the engine computer and everything else just goes terrible. Crawl under the car and make sure the O2 sensor wire is nowhere near the driveshaft. Easy, preventable maintenance.

LAMBDA ADJUSTMENT (Air/Fuel Ratio mixture)

This only applies to cars with completely functioning O2 sensors. If your O2 sensor is suspect, replace it and THEN can you attempt to adjust your lambda. This is the last this you can do to improve fuel economy and drivability with M103 engines. I’ve been getting an amazing 26-27 miles to the gallon on a car with proper tune and maintenance in check. Checking lambda is best done with a multimeter that can read duty cycle in %. If you don't have a duty cycle meter, but you do have an old dwell meter, you can substitute the dwell meter for the duty cycle meter when setting the lambda adjustment. You simply look for mid scale on the dwell meter. All a dwell meter is, is a duty cycle meter marked up in degrees instead of %.

           

       

            The “screwdriver” in the above is simply a 3mm allen wrench. It doesn’t have to be long, just long enough to engage the screw so you’ll be able adjust lambda. Adjustment can be done with the air cleaner in place, but can only be done if the anti tamper ball in the adjustment tower has been removed. Take off your air cleaner and see if you have the ball still in the adjustment tower. If the ball was in there, I would cover my work area and use a dremel to saw just underneath the ball so I could remove it. It’s cleaner than breaking it off like some shops do.

Monitoring adjustment is done at the X11 diagnostic connector on the driver side fender well. The signal provided at pin 3 of X11 is called the "lambda on/off ratio" signal. It is convenient to use pin 2 of the same connector as a ground reference. Make sure your multimeter is set to duty cycle % and adjust the lambda until the % bounces on/off at around 44-49% duty cycle. Get it to just under 50% that’ll be the optimal running range for that engine. You get the best fuel economy, power, and emissions at this range.

EHA (Electro-Hydraulic Actuator)

If your car smells like gas, it's most likely coming from the EHA on the back of the distributor. It also can cause long starting, rough idling, hesitation from a stop or sudden acceleration. It has two small green orings that, over time, harden and cause fuel leaks and cause all sorts of havoc on your sense of smell and the running/starting condition of your car. The EHA makes last second pressure adjustments to the upper and lower chambers of your fuel distributor. It works together with your Lambda to keep everything together in your fuel system. Check out your EHA AFTER you're done checking and adjusting everything else. They rarely go bad, but when they do, you'll often smell it. Either the O-rings have started leaking, or the EHA has internal leakage that you'll be able to see externally.

The EHA is mounted on the rearward side of the fuel distributor assembly. You have to take the air cleaner off to gain access to it. Removal is done with a torx bit (CAREFUL, washer's can fall from screws and you're screwed) and adjustment is done internally with SMALL 1/8" increment turns. Clockwise richens the mixture, and counter-clockwise leans everything out.  Precise adjustment is a real pain in terms of watching duty cycle and a CIS fuel pressure test kit - so small increments works ;)  Often times, enriching the fuel mixture a tad will significantly help with off idle acceleration. There's a small flat blade screw that's blocking the 2mm Allen adjustment key - make sure before adjusting you make note of where the EHA was originally, in case you go too far with your adjustments.

After adjusting the EHA, I recommend you recheck your Lambda and fine tune from there.

FUEL PUMP RELAY

            This only applies to the early M103’s as the later cars used a MAS, rather than a separate relay for the fuel pump. If your car is stalling out while driving, doesn’t start, or has trouble starting (you should always hear the fuel pump prime with the key in #2 position) than chances are your fuel pump relay has failed or is in the process of failing. It’s located next to the OVP relay on early cars.

To test, fuel pumps jumper sockets 87 & 30. If your fuel pump turns on, you’ll have a bad relay on your hands. A good used relay is under $40 on ebay. Or you can open the relay up and check for cold solder joints. I’ve resoldered a few relays and they’re working fine to this day.

CRANK POSITION SENSOR (CPS)

When these fail, your car will not start or run. If the sensor dies when the engine is running, your engine will stop running. If your car starts fine when cold, but doesn’t restart when warm (have to let the engine cool down to restart) than in most cases your CPS is failing. The CPS usually doesn’t have any effect on how rough the engine runs. It’s usually ON or OFF with the CPS. The difference in resistance with temperature is great enough that it sends incorrect signals to your EZL. Resistance values should be in the range of 650 to 1200 ohms. Lower than 650 and you’ve got a dead sensor. Resistance can be measured at the EZL end of the CPS wire.

IGNITION COIL
            The ignition coil is mounted on the driver side fender well and can be exposed to the elements if you don’t have a splash shield installed on your car. It’s also very rare for these to fail, but when they do, they’ll often give you a NO START situation. They do last a very long time. I haven’t found any specific ignition coil test procedures, as I’ve always had a spare junkyard one around to throw on for diagnostics. I don’t believe I’ve ever come across a bad ignition coil amongst the 20+ cars I’ve had my hands on. 

FUEL INJECTORS

            These really should be replaced at 100k intervals if you have no means of properly cleaning them (pop tester). They get gummed up over time and instead of a nice cone spray pattern, they dribble and can leak down into the cylinders when the engine is off. Symptoms of leaking down or gummed up injectors include very hard starts, long cranking when starting, running rough, misfiring, running rich or under some conditions, lean running. If you’ve never replaced your injectors in the lifetime of the car and it seems to be giving you any of the above symptoms after you’ve gone through the rest of the car, it would be a good idea and excellent preventive maintenance for your motor. You just might gain a few MPG’s back in the process. Now would also be a great time to replace the fuel injector seals. Apply a small dab of white lithium grease or oil to make installation easier and so you don’t rip the seals.
            Take a look at your fuel injectors. If they are silver, they’re steel and original. Replace them. If they’ve ever been replaced, they’re going to be brass / copper in color.

What do I do when my 300E has idle issues?

When it comes to high idle on these motors, you have a plethora of vacuum and some electrical components that could cause you to have an erratic or high idle. Again, like above, I’ll try to order these in a plan of attack when it comes to problems.

Firstly, you’ll want to have your engine at operating temperature for this test. Vacuum leaks should be more pronounced when doing this simple test, spraying a little starting fluid or brake cleaner around vacuum components with the engine running. If you have a vacuum leak, the engine will run completely differently when it sucks in the starting fluid. It’ll “rev up” and you’ll more than likely have found your vacuum leak and your high idle. Celebrate! Just be careful when you're doing this and shoot in short bursts. Nobody wants you to blow up.

If not, well you’ve got some work to do.

FLOOR MATS

            Yes, floor mats. You’d never realize that every time you get in your car you keep pushing the floor mats against the accelerator. Push it enough and your accelerator will never return to “idle” position. So please, check your floor mats. I’ve always secured mine with a small safety pin to the carpets. Works great and you’ll never have to worry about them.

THROTTLE LINKAGES

It’s an old car; make sure your throttle linkages aren’t hanging up. If you’re in a climate with drastic temperature changes, your roads get salt on them, or very sandy dry climates please check and lubricate your throttle linkages. Standard MB procedure is to use trans fluid to lubricate the ball and cup’s of the linkages. I’ve always done that, followed up with white lithium grease and then covered the entire ball/cup with grease/Vaseline to keep moisture out. My linkages are always happy. It’s good practice to do this every 15k miles or so. A simple check up is all you need to keep your linkages moving freely.

IDLE CONTROL VALVE

            Pretty self explanatory, the computers on the car control this to raise/lower idle when you start the car, have AC on, etc. Over time, just simply having the engine running, these can gum up. There’s a small valve inside, that when gummed up, can’t slide and will often be stuck giving you a high idle. The opposite can be said for when it gets stuck giving you a low idle. You can remove the valve and soak it in carb/throttle body or brake cleaner to remove the gunk and give you a decently working valve again. Give the valve a shake with some cleaner in it as well; you should be able to shake the small valve inside to clear out some more gunk. I’ve done this to a few valves and the crud that comes out of them is pretty foul. I’ve gotten perfectly working idle control valve by doing this.

            You can also give the valve a quick test: connect center pin to ground and positive to pin 3 and change the positive to connect to pin 1, the valve inside the IACV should move. If it doesn't move, immerse the IACV in some carb/throttle body or brake cleaner. If that doesn't help, it's time to change the IACV.

THROTTLE MICROSWITCH

            This switch is attached to the throttle linkages and is used by the “ECU” to determine off throttle conditions and bring your engine to “idle”. If malfunctioning, you’ll have erratic idle, a surging condition, etc. To test the microswitch, disconnect the cables and use a multimeter to test for continuity between pin 1 and pin 3, the circuit should be closed when the switch is depressed.

VACUUM COMPONENTS

            There are a multitude of vacuum components in the engine bay that are VERY neglected by owners. On every single M103 I’ve owned, I’ve made it a point to replace every single vacuum piece in the engine bay that I could handle. Usually costs about ~ $120 to get all the vacuum pieces including the air meter boot (which always has cracks in it) from AutohausAZ or similar distributors. These parts are rubber folks – you find me a piece of rubber that hasn’t become rock solid and brittle with thousands of heat cycles and vibrations. They WILL fail, so replace them. If you’re reaching 100k+ miles with these parts never replaced, you’re going to start running into issues. You’ll be chasing a pipe dream trying to figure something out.

For those with automatic transmissions, there’s a vacuum line that leads to the modulator on the side of the transmission. These have a tendency to leak and give you poor shifting and high idles. Playing with pieces is not a good way to inspect vacuum lines, REMOVE them and bend them all around to check for cracking.

AIR FLOW POTENTIOMETER

            Don’t let the name scare you. It’s a relatively simple part that MUST be in correct spec in order for your car to run properly. Otherwise you’ll have surging idle, very high idle, erratic idle, etc. I’ve have a car sit at 2k rpm no matter what because the AFP was misadjusted. You need to back probe the top two pins (1 and 2) and install the electrical connector. With a digital multimeter hooked up to read DC volts, start the car and let it get up to a warm idle. With the engine at idle adjust the potentiometer body by pivoting it slightly until 0.70v (plus or minus 0.10v) is obtained. If you’re unable to get that reading, then you’re going to have to replace the potentiometer.

Again, this is just BASIC stuff. If your problems still persist after checking and testing the components I’ve outlined here, then you have deeper problems. Do your research! Most issues have been covered extensively around the internet.

Folks, this is a basic run down of issues I’ve found in M103 300E ownership. Issues that can be carried over to MANY CIS-E fuel injected Mercedes cars. I’m not saying what I’ve written here is law, what I’m saying here is go over what I’ve written here and USUALLY you’ll have your problem solved. Do some more research on these topics because they’re very extensively discussed on multiple online forums. Basic maintenance and preventive maintenance is a problem solver for most of these robust engines. They don’t often fail, but when they do, don’t make an engineer somewhere in Germany cry.

Allen