What to do when my M103 is giving me trouble: A rough 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***
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 (I still think dealers supply these, but it's been awhile since I've gotten them.)
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 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/Beru copper 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.
![Image]()
For a more detailed explanation of the OVP relay go here: http://artisanexcite.blogspot.com/2013/12/the-ovp-relay-and-you.html
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 %.
![Image]()
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.
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.
![Image]()
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. They’re not easily cleaned like electronic injectors or diesel injectors and they’re service life is really determined by their inability to be serviced by being disassembled like other injectors. I've heard of places to clean the CIS injectors, but I wouldn't trust it. 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.
***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***
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 (I still think dealers supply these, but it's been awhile since I've gotten them.)
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 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/Beru copper 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.
For a more detailed explanation of the OVP relay go here: http://artisanexcite.blogspot.com/2013/12/the-ovp-relay-and-you.html
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.
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. They’re not easily cleaned like electronic injectors or diesel injectors and they’re service life is really determined by their inability to be serviced by being disassembled like other injectors. I've heard of places to clean the CIS injectors, but I wouldn't trust it. 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.
WzNWw~~60_35.JPG)
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. 
