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Spark Plugs

Recommended service intervals:

M104 every 45,000 mi or 4 years
M111 every 45,000 mi or 4 years
M119, M120 every 60,000 mi or 4years
M156, M275 every 60,000 mi or 5years
M112, M113 motors every 100,000 mi or 5 years
M272, M273 motors every 91,000 mi or 5 years

Some years have slightly different intervals. Check your owner’s manual for the best information, or call us, and we will confirm it.

It helps to lubricate the threads with anti-seize as you don't want to gall the spark plug bores on removal or installation. You can also use light oil if you prefer but anti-seize is a better thermal conductor and is preferable in the interface. Galling occurs when the aluminum cylinder head threads seize and stick to the plated steel spark plug threads. Many spark plug manufacturers specify installing plugs dry as the anti-seize can increase the torque and cause thread damage. This is probably not the case but you should decrease the torque to compensate if desired, approx 20%. Just be careful the anti-seize doesn't get on the electrode as it is contains metal particles and is conductive and the spark will find the lowest resistance path to ground. Avoid the use of air tools until you are sure the spark plug is moving freely, and use a torque wench to finally install. The correct torque is important as it controls the rate of heat transfer, causing the plug to run hot if too loose. A hot plug can cause pre-ignition, occuting when the fuel air mixture ignites before the optimum piston position. This can cause peak cylinder pressures to occur before the piston is at TDC and if this occurs over time the engine pistons can be eroded and damaged. If the plug is too tight the threads may be damaged and hard to remove later. If the plug cannot be inserted and tightened all the way finger tight the threads may need to be chased with a thread chaser. This has to be done carefully by lubricating with penetrating oil if you are cutting carbon, and by greasing the recesses in the tap if you are removing metal, as it will catch the shavings from falling into the cylinders. Fortunately, overheating plugs and galled threads are extremely rare on a Mercedes if you follow the tips in this section.       

Replacing plugs on the M112 and M113 engines is much easier with the special tool to remove the wires without damaging them. This 17mm wrench was designed to adjust valves back before hydraulic lifters were common. This tool (110 589 01 01 00) is now more commonly used as a spark wire removal tool. It gives you the proper fork like opening and leverage to make a difficult removal job easy. Hose pliers and boot removal tools are other less preferable options but do work. If you do these a lot you may want to order one. Reproductions of this tool are available but the original design is open on both ends and twists in different directions, which helps when doing the back cylinders. The copies I have seen are not quite the same, although they may work well. You may also want to consider a spark plug installer. It's an all rubber tool that allows you to insert and finger tighten plugs. Cross threading is not something you want on a back cylinder where you have barely enough room for a hand. There are many different deigns including the all rubber one from Mac tools which works well. Be aware the most common source of ignition problems on these two engines are the spark plug wires. At the dealer the price of one spark plug wire is over $50. When you have 16 on a M113 this gets expensive quickly. Many times to accommodate a budget only a few cylinders are done at a time. Iterating repairs and diagnosis time can get even more expensive. We can sell an OEM wire set for a M113 for around $400 while our labor to install is also substantially less than the dealer. Replacement of spark plug wires isn't that common but if you need to do several cylinders consider doing a set. The labor to replace the plugs and the wires is substantially less than doing them separately, so if your dealer is using menu pricing, make sure you ask for them to adjust for the labor overlap.  

One other issue on The M112 and M113 engine is that the oil baffles at the top of the valve covers seep oil, and changing the valve cover gaskets alone won't stop the oil seep. The reason it appears in this section is that if usually occurs after 5 years on most of these vehicles the plugs also need to be replaced. If you can replace the plugs and reseal the valve covers the labor is reduced substantially. The seep can cover the coils and wires in oil and dirt, and although it can be cleaned up, it’s something that can be avoided if you inspect the valve covers for the need or resealing at the same interval the plugs are replaced. The oil baffles on the valve covers are resealed with engine sealant 003 989 98 20 10, the same RTV type sealant that is used for most Mercedes engine assemblies that have no gaskets. This is an outstanding sealant that forms a very strong bond when fully cured, has a 10min work time, and skins over quickly so it can be put into service within 30min. It takes awhile to fully cure but that is unimportant to its sealing qualities. Be aware it is critical that the surfaces to be bonded be cleaned of all old adhesive and dirt or any oil.  Any oil and the finished assembly will leak. It is a labor intensive repair but it doesn't have to cost $1000. I recently resealed two valve covers both baffles and replaced the plugs on a CLK 430 and the total repair was approximately $800. The dealer estimate was $925 just for the valve cover gaskets. You could pay as much as $925 and would be amazed that in many cases the valve covers go in the spray cabinet washer the gaskets get replaced and the baffles are never touched. Technically, the oil baffle reseal is not part of the valve cover gasket operation so it may not always be done. It is an additional operation and is the most time consuming part of the job if done correctly. If you are in need of this repair make sure you do both the baffle and the gasket reseal, and if you are near 5 years or 100k miles replace the plugs also. The labor overlap is considerable so it’s not unreasonable to ask if this was considered when preparing the estimate. At Foreign MotorWorks you can be sure all labor overlaps will be considered when preparing any estimate, so you don't double pay labor charges. For example if I need to remove the coils to remove valve covers or the plugs and I am doing both the total labor needs to be reduced to reflect the reality that the coils were only removed once. It is common practice for the labor amounts to be summed and no overlap considered.      
 

On plugs in older engines before 1990, it is not uncommon to find the incorrect spark plugs in vehicles with distributor type ignition systems. If you plan on upgrading sparkplugs you need to carefully inspect or replace all the secondary electronic parts or the end result may be carbon tracking, current leakage, and misfiring.  It is not practical or beneficial in many cases to do this. Use the OEM plug recommended for the motor. Some high energy plugs may last longer, but often have larger gaps and need more voltage to fire. If your coil can’t provide this voltage or the spark plug wiring insulation is poor, that voltage will find another path to ground. As a result the car won’t idle smoothly, making them a poor choice in this case. With sparkplugs only use the OEM parts or the results may be less than desirable even if the plugs are made of superior materials. Beyond issues of gap are subtle variations of heat range and resistance that make the choice of the correct spark plug best left to the design engineer. It's best to keep to the OEM recommendations, as there is little evidence to suggest that changing spark plugs from the OEM specification would have any benefits.  
Use caution when replacing plugs on Mercedes motors produced between 1991-1196 as the insulation on the engine wiring harness can be brittle. On a M104 engine this is very common but many of these have already been repaired by replacing the engine harness. Check the insulation on any of the exposed wires in the engine harness on the engine. The heat causes the insulation to crack and flake off. If the insulation is cracked don't touch anything unless you are committed to replacing at engine wiring harness because it may not start after replacing the plugs, or may misfire or backfire into the intake which can damage the throttle valve.   
 
A common cause for misfires is a no spark or weak spark condition. For anyone interested in diagnosing a weak spark condition consider this. In a vacuum the voltage required to jump a gap is high, but the voltage drops with increasing pressure reaches a minimum and sharply increases again. This is called a Paschen’s law. If you use an ordinary spark plug to diagnose a no-spark condition you may mistakenly think the spark is normal and present, but when in the high pressure environment of an engine cylinder the spark may be absent.  Use a high energy spark tester; these have very large gaps to compensate for the decrease in the needed voltage at atmospheric pressure.  The reason for this is that in a high vacuum ionization of a gas cannot assist spark generation. Gas presence is sparse in a vacuum, therefore this ionized conductor not being present results in higher energy sparks.  At high pressure ionization of a gas in inhibited by electrostatic forces caused by crowding of the ions in the gap. That’s why at lower pressures like atmospheric where an ionizable medium air is present but its concentration is still small that minimum spark energy occurs. With this mind it also helps to explain why some misfires only occur under load, because this is where cylinder pressures are highest and the needed spark energy is also the highest. Weak plug wires may not show up at idle but may turn on the CEL under hard acceleration. A high energy spark tester simulates the higher load condition where the misfire occurs. Most OBD II code scanners can also be helpful locating the problem cylinder if the vehicle was built after 1997. Beyond this ignition scopes provide the best window into cylinder ignition condition and compression, but they are seldom needed on OBD II vehicles because the on board testing is so comprehensive. Misfire recognition is very sensitive, and in case where a leaking valve may be suspected, a relative compression check can be performed electronically on some engines with a scan tool which can monitor starter draw current.  The relative current draw for a leaking cylinder will be low, that in many cases is much faster than a conventional compression check which can be performed after if needed.  If the problem can be uncovered quickly with the correct equipment the corresponding diagnosis cost to you will be the lowest. If you are doing the repairs yourself or at a shop without diagnostic technicians consider the cost of replacing unneeded components, as it may be better to have it professionally diagnosed by a specialist before replacing any parts. You can then choose to repair it yourself if desired.      

 
 
 

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