Resolving Mercedes M103 idle issues

5/20/2018 Resolving Mercedes M103 idle issues

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What to do when a M103 engine is giving trouble

Most, if not all jobs on these engines require basic tools, some reading and patience. One basic rulwhen it comes to the M103 engine is that if you take something apart, it goes back together ONEway. Its very hard to ruin something on these engines, but they do require maintenance. Many paare expensive and it would be more cost effective to get some original parts from a junkyard.

Wellstart from the top My car runs rough. Misfires, bucks, sometimes itll stall, whedo I start?When was the last time you did a complete engine tune up on your car? If you cant

remember, guess where youre going to start?

SPARK PLUGS

If the spark plugs you just bought arePlatinumorIridium etc, youvebought the wrong ones.These cars are very sensitive to the type of spark plugs used: most common spark plugs are resistotype. The problem for M103s is that the spark plug wires have resistance built into the wires and ifused with resistor type spark plugs, you are running your engine with an inefficient burn. Over timethis can develop into a misfire, fouled plugs, and more varnish in your engine. The correct plug is a

copper core, non-resistor plug such as Denso T20EP-U, Bosch H9DCO, or NGK BP6EFS. Sparkplugs should be gapped .032" - .035.

DISTRIBUTOR CAPOften overlooked, itssimple to replace with a few Allen screws around it. Moisture has a tendencymake its way to the inside of the cap and cause all sorts of problems. Remove the cap and check t

button; if it is missing or doesnt feel spring loaded, then replace the cap. If you see a slightly whhazing in the cap (usually red-orange) than you have an arcing problem, meaning an incomplete aweak spark, and in most cases causes a misfire. The interior rim of the cap has 6 points where thespark transfers from the rotor to the cap. Each will have some slight normal discoloration wherespark transfer occurs, but if the points are crusty, its a good time to replace the cap. If you dont

have a new part handy, you can clean up the points with some sandpaper and bring out a new niceshiny finish. This buys some more time with the cap and is a good point for some diagnostics.

Many misfires can be sorted just by cleaning the cap. Replace it every 15k -25k miles (depending oweather, dry climates will be able to take these further) and remove the cap and clean it every 7,5miles. Keeps everything in tip top shape and you can keep tabs on the integrity of your parts.

DISTRIBUTOR ROTOR ARMThis part spins inside the cap and directs electric flow to every point on your distributor cap. If itscrusty and worn down, its overdue for replacement. These are usually black in color: if you find

white hazing around the part, it has the same arcing problem as the cap. Replace it. Otherwise, cleat the same time as your distributor cap.

SPARK PLUG WIRESMB spark plug wires are extremely robust and some of the best wires around. The wires themselveare very low resistance, but the boot ends have roughly 1k ohm resistance built in: a good set ofwires should measure 800ohm to 1.3k ohm. Check the spark plug wires for arcing on a dark nigwith a little bit of misted water. If it looks like a small lightning storm on your engine, youvefoundpart of your misfiring. MAKE SURE YOU PULL THE WIRES OFF OF THE PLUGS BY THE ENDS, NOTTHE WIREor you risk pulling the wire out of the end! Ive never found noticeable difference in anaftermarket set of wires vs. an OEM set of Bosch wires.


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OVP RELAY (Overvoltage Protection Relay)

All of your engines vital components are protected by this little relay. Do some more researchelsewhere on this part as it can cause a multitude of problems from hard starting, stalling, rough idetc. Id be typing outa 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 tupdated 2 fuse design. Theyre more reliable and will save you headaches down the road. A bad OVrelay will still let the car start, but itll run rough and may stall.

FUEL FILTERCome on now, if youve never replaced the darn thing then do it - 15-20k mile service intervals. Thdont last forever and theyre very easy to forget about. A poor fuel supply is enough to cripple the

car. Its cheap, its a little smelly, you might get a little buzz from doing the work under the car, buits easy.

AIR FILTERReplacing it is good and cheap insurance. If youve never done it, chances are you might be gettingmore efficient burn and better fuel economy! Dont bother with aftermarket air-filters, the factoryfilter is the BEST cold air intake we can get for these cars.

OXYGEN SENSORAnother overlooked part on the cars. Some cars didnt come equipped with a check engine light, so

most people never think to look at the O2 sensor. They have 100k mile service interval. Replace itand dont worry about it for another 100k miles. Otherwise, youre burning more fuel than you realneed to so its another part that pays for itself over time. Its connected under the passenger sidecarpet and gets fed out through a grommet on the transmission tunnel.

Over time the sensor wire can feed itself out of the grommet and the slack waves around as youdrive your car, allowing it to touch the driveshaft. The sensors can SHORT out on the driveshaft anmake the car completely inoperable. It would buck and run extremely rich, misfire like crazy, etc. Ashort in the O2 sensor sends spurious signals to the engine computer and confuses it. Crawl underthe car and ensure the O2 sensor wire is away from the driveshaft. Easy, preventable maintenance


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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 attempt to adjust your lambda. This is the last thing you can do to improve fueeconomy and drivability with M103 engines. Ive been getting an amazing 26-27 miles to the gallonon a car with proper tune and maintenance in check. Checking lambda is best done with a multimethat can read duty cycle in %. If you don't have a duty cycle meter, but you do have an old dwellmeter, you can substitute the dwell meter for the duty cycle meter when setting the lambdaadjustment. You simply look for mid-scale on the dwell meter. All a dwell meter is, is a duty cyclemeter marked up in degrees instead of %.

The screwdriver in the above is simply a 3mm Allen wrench. It doesnt have to be long, just longenough to engage the screw so youll be able adjust lambda. Adjustment can be done with the aircleaner in place, but can only be done if the anti-tamper ball in the adjustment tower has beenremoved. Take off your air cleaner and see if you have the ball still in the adjustment tower. If theball was in there, I would cover my work area and use a Dremel to saw just underneath the ball socould remove it. Its 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 pinof the same connector as a ground reference. Make sure your multimeter is set to duty cycle % anadjust the lambda until the % bounces on/off at around 44-49% duty cycle. Get it to just under 50thatll be the optimal running range for that engine. You get the best fuel economy, power, andemissions 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 alscan cause long starting, rough idling, hesitation from a stop or sudden acceleration. It has two sma


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green o-rings that, over time, harden and cause fuel leaks and cause all sorts of havoc on your senof smell and the running/starting condition of your car. The EHA makes last second pressureadjustments to the upper and lower chambers of your fuel distributor. It works together with yourLambda to keep everything together in your fuel system. Check out your EHA AFTER you're donechecking 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 seeexternally.The EHA is mounted on the rearward side of the fuel distributor assembly. You have to take the aircleaner off to gain access to it. Removal is done with a Torx bit (CAREFUL, washers can fall fromscrews and you're screwed) and adjustment is doneinternallywith SMALL 1/8" increment turns.Clockwise richens the mixture, and counter-clockwise leans everything out. Precise adjustment is areal pain in terms of watching duty cycle and a CIS fuel pressure test kit - so small increments worOften times, enriching the fuel mixture a tad will significantly help with off idle acceleration. There'small flat blade screw that's blocking the 2mm Allen adjustment key - make sure before adjusting ymake 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 RELAYThis only applies to the early M103s as the later cars used a MAS, rather than a separate relay forthe fuel pump. If your car is stalling out while driving, doesnt start, or has trouble starting (you

should always hear the fuel pump prime with the key in #2 position) than chances are your fuelpump relay has failed or is in the process of failing. Its located next to the OVP relay on early cars.


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To test, fuel pumps jumper sockets 87 & 30. If your fuel pump turns on, youll have a bad relay on your hands. A goodused relay is under $40 on ebay. Or you can open the relay up and check for cold solder joints. Ive resoldered a fewrelays and theyre 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, yourengine will stop running. If your car starts fine when cold, but doesnt restart when warm (have to the engine cool down to restart) than in most cases your CPS is failing. The CPS usually doesnt haany effect on how rough the engine runs. Its usually ON or OFF with the CPS. The difference inresistance with temperature is great enough that it sends incorrect signals to your EZL. Resistancvalues should be in the range of 650 to 1200 ohms.Lower than 650 and youve got a deadsensor. Resistance can be measured at the EZL end of the CPS wire.

IGNITION COILThe ignition coil is mounted on the driver side fender well and can be exposed to the elements if yodont have a splash shield installed on your car. Its also very rare for these to fail, but when they dtheyll often give you aNO STARTsituation. They do last a very long time. I havent found anyspecific ignition coil test procedures, as Ive always had a spare junkyard one around to throw on fodiagnostics. I dont believe Ive ever come across a bad ignition coil amongst the 20+ cars Ive hadmy hands on.


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FUEL INJECTORSThese should be replaced at 100k intervals. Theyre not easily cleaned like electronic or dieselinjectors and service life is really determined by their inability to be serviced by being disassembledlike other injectors. They get gummed up over time and instead of a nice cone spray pattern, theydribble and can leak down into the cylinders when the engine is off. Symptoms of leaking down orgummed up injectors include very hard starts, long cranking when starting, running rough, misfirinrunning rich or under some conditions, lean running. If youve never replaced your injectors in thelifetime of the car and it seems to be giving you any of the above symptoms after youve gonethrough the rest of the car, it would be a good idea and excellent preventive maintenance for yourmotor. You just might gain a few MPGs back in the process. Now would also be a great time toreplace the fuel injector seals. Apply a small dab of white lithium grease or oil to make installationeasier and so you dont rip the seals.Take a look at your fuel injectors. If they are silver, theyre original. Replace them. If theyve everbeen replaced, theyre going to be brass / copper in color.

What do I do when my 300E has idle issues

High idle on these motors can be caused by any of many problems with vacuum and some electricacomponents.Again, like above, Ill try to order these in a plan of attack when it comes to problems

Firstly, youll want to have your engine at operating temperature for this test. Vacuum leaks shouldbe more pronounced when doing this simple test, spraying a little starting fluid or brake cleaneraround vacuum components with the engine running. If you have a vacuum leak, the engine will rucompletely differently when it sucks in the starting fluid. Itll rev up and youll more than likely hafound your vacuum leak and your high idle. Celebrate! Just be careful when you're doing this andshoot in short bursts. Nobody wants you to blow up.

FLOOR MATSEvery time you get in the car you push the floor mats against the accelerator. Do it enough and theaccelerator will never return to idle position. So check your floor mats.

THROTTLE LINKAGES

Its an old car; make sure thethrottle linkages arent hanging up. If youre in a climate with drastictemperature changes, your roads get salt on them, or very sandy dry climates, check and lubricateyour throttle linkages. Standard MB procedure is to use trans fluid to lubricate the ball and cups ofthe linkages. Ive always done that, followed up with white lithium grease and then covered theentire ball/cup with grease/Vaseline to keep moisture out. Its good practice to do this every 15kmiles or so. A simple check-up is all you need to keep your linkages moving freely.


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IDLE CONTROL VALVEThe computers control this to raise/lower idle when you start the car, have AC on, etc. Over time,

just by having the engine running, these can gum up. Theres a small valve inside, that whengummed up, cant slide and will often stick giving a high idle. The opposite can be said for when itgets stuck giving you a low idle. You can remove the valve and soak it in carb/throttle body or brakcleaner to remove the gunk and give you a decently working valve again. Give the valve a shake wsome cleaner in it as well; you should be able to shake the small valve inside to clear out some mogunk. Ive done this to a few valves and the crud that comes out of them is pretty foul.

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

THROTTLE MICROSWITCHThis switch is attached to the throttle linkages and is used by the ECU to determine off throttleconditions and bring your engine to idle. If malfunctioning, youll have erratic idle, a surgingcondition, etc. To test the microswitch, disconnect the cables and use a multimeter to test forcontinuity between pin 1 and pin 3, the circuit should be closed when the switch is depressed.


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VACUUM COMPONENTSThere are a multitude of vacuum components in the engine bay that are VERY neglected by ownerOn every single M103 Ive owned, Ive made it a point to replace every single vacuum piece in theengine bay that I could handle. Usually costs about ~ $120 to get all the vacuum pieces including tair meter boot (which always has cracks in it) from AutohausAZ or similar distributors. These partsare rubber folksyou find me a piece of rubber that hasnt become rock solid and brittle withthousands of heat cycles and vibrations. They WILL fail, so replace them. If youre reaching 100k+miles with these parts never replaced, youre going to start running into issues. Youll be chasing apipe dream trying to figure something out.

For those with automatic transmissions, theres a vacuum line that leads to the modulator on the sof the transmission. These have a tendency to leak and give you poor shifting and high idles. Playinwith pieces is not a good way to inspect vacuum lines, REMOVE them and bend them all around tocheck for cracking.

AIR FLOW POTENTIOMETER

Dont let the name scare you. Its a relatively simple part that MUST be in correct spec in order for

your car to run properly. Otherwise youll have surging idle, very high idle, erratic idle, etc. Ive hava car sit at 2k rpm no matter what because the AFP was misadjusted. You need to back probe thetop two pins (1 and 2) and install the electrical connector. With a digital multimeter hooked up toread DC volts, start the car and let it get up to a warm idle. With the engine at idle adjust thepotentiometer body by pivoting it slightly until 0.70v (plus or minus 0.10v) is obtained. If youreunable to get that reading, then youre going to have to replace the potentiometer.

This is just BASIC stuff. If problems persist after checking and testing the componentsoutlined here, then you have deeper problems. Do your research! Most issues have beecovered extensively around the internet.

Issues can be carried over to MANY CIS-E fuel injected Mercedes cars. Basic maintenan

and preventive maintenance is a problem solver for most of these robust engines. Theydont often fail, but when they do, dont make an engineer somewhere in Germany cry.

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Resolving Mercedes M103 idle issues