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Pelican Technical Article:

Spark Plug Replacement

Ken Freund

Time:

1 hour1 hr

Tab:

$20

Talent:

*

Tools:

13/16" socket, ratchet, extensions & flex coupling

Applicable Models:

Mercedes-Benz 450SL (1973-80)

Parts Required:

8 spark plugs, dab of anti-seize compound

Hot Tip:

Work on cool engine

Performance Gain:

Smooth running, easy starting, better mileage

Complementary Modification:

New spark plug wires, distributor cap and rotor (ignition points - early models)

Spark plugs should be replaced at intervals specified in the maintenance schedule. They also should be replaced when they become fouled due to engine problems such as excessive oil consumption and/or fuel-injection problems such as an excessively rich fuel mixture. On Mercedes V-8 engines, the cylinder heads are made of aluminum. Therefore be very careful when removing and especially when installing spark plugs, to avoid damaging or cross-threading the threads.

NOTES: Firing Order is 1-5-4-8-6-3-7-2, but Mercedes' cylinder numbering system is different than some other manufacturer's practices. The cylinders on the right bank (passenger side on USA models) are number from 1 to 4 going from front to back. The cylinders on the left (USA driver's) side are numbered from 5 through 8 from front to back. Therefore cylinder 1 is nearest to the oil filler cap. Recommended spark plug gap is 0.032 to 0.035 inch. Be sure to install spark plugs of the correct part number, reach and heat range for the application.

To ease reassembly and avoid installing the wrong plug wires onto the spark plugs, either remove and replace the spark plugs one at a time, or mark the wires to indicate their location.

If you have an air compressor available, use an air blow gun (wear approved safety goggles) to expel any dirt around spark plugs prior to removal. If not, spray some carburetor cleaner (wear approved safety goggles) with a tube around spark plugs to dislodge dirt.

Using a 13/16 inch spark plug socket and 3/8-inch drive ratchet, loosen and unscrew the spark plugs. On the right side, remove the three small screws holding the coolant tank in place and lift it aside for spark plug access. It should not be necessary to disconnect the hoses to do this. Reinstall the tank when finished.

Upon removal, visually inspect the spark plugs for wear, damage and deposits.

Whether you intend to reinstall the used plugs or install new ones, check and adjust their gaps as needed. Compare new plugs with the old ones to verify that the thread length (reach) is the same.

Steel spark plugs in aluminum heads can lead to electrolytic corrosion and stuck spark plugs, particularly in damp climates. Apply a small dab of anti-seize lube to the threads before installation.

Slip a short length of rubber hose over the insulator of the spark plug to make it easier to install the plug and start the threads. Do not attempt to force a spark plug into the cylinder-head threads. Thread the plugs in as far as possible by hand before using a wrench to tighten them. Tighten with a torque wrench to the recommended torque (18-21 ft-lb). If you don't have a torque wrench available, tighten the spark plug finger tight until the gasket reaches the cylinder head, then tighten about 1/2-2/3 turn more with a ratchet.

Install the spark plug wire connector and boot onto the spark plug until it is fully seated.

Repeat the procedure for each cylinder.

Note the date and mileage and enter the spark plug replacement into the vehicle's service logs.

Remove the spark plug wire boots by wiggling and twisting as you pull.
Figure 1

Remove the spark plug wire boots by wiggling and twisting as you pull. If they are difficult to remove, use a special tool to grip them without damaging the boots.

Spark plugs are close to the exhaust manifolds.
Figure 2

Spark plugs are close to the exhaust manifolds. Always work on a cool engine.

Use a 13/16-inch spark plug socket with an extension and ratchet.
Figure 3

Use a 13/16-inch spark plug socket with an extension and ratchet. To reach some plugs, a longer extension and swivel socket may be needed.

Slip a length of 3/8-inch ID rubber hose over the spark plug insulator to make it easy to remove without dropping.
Figure 4

Slip a length of 3/8-inch ID rubber hose over the spark plug insulator to make it easy to remove without dropping.

Remove the spark plugs and inspect each one.
Figure 5

Remove the spark plugs and inspect each one. Keep the old ones in order of which cylinder they came out of for reference if a problem is noted.

Measuring spark plug gap with a wire gauge.
Figure 6

Measuring spark plug gap with a wire gauge.

Spark plug gaps can be adjusted with a tool such as this, bend only the external ground electrode.
Figure 7

Spark plug gaps can be adjusted with a tool such as this, bend only the external ground electrode.

Gaps on new spark plugs should also be checked.
Figure 8

Gaps on new spark plugs should also be checked.

Adjust gaps if needed.
Figure 9

Adjust gaps if needed.

If the new lugs have a threaded cap like this, remove it so the wire boot terminals will make contact.
Figure 10

If the new lugs have a threaded cap like this, remove it so the wire boot terminals will make contact.

Install new spark plugs; slip a length of hose over the plugs and carefully start them by hand to ensure they are not cross threaded.
Figure 11

Install new spark plugs; slip a length of hose over the plugs and carefully start them by hand to ensure they are not cross threaded.

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Comments and Suggestions:
Phil Comments: Thank you...This was a great help.
Is the NGK BPR5EGP - 7082 plug OK to use in my 1974 Mercedes 450SL ? It is a platinum alloy spark plug.
Thank again.
Best, Phil
September 24, 2016
  Followup from the Pelican Staff: Any of these should be OK:

http://www.pelicanparts.com/cgi-bin/supertech/catalog.cgi?action=frameset&return-url=/cgi-bin/supertech/catalog.cgi%3Faction%3Dframeback%26page%3D1694&catalog-url=http%3A%2F%2Fwww.pelicanparts.com%2Fcatalog%2FSuperCat%2F3447%2FMBZ_3447_BASTUN_pg1.htm %3Futm_source%3DSuperTech%23item0 - Nick at Pelican Parts
 
Crank Mankateau Comments: Should I remove the air filter housing to replace the left side spark plugs, Access is limited? It is a 350 SL 1984 and there are many obstructions, the AC and the Air Intake pipe attached to the air filter housing??
Michael
August 14, 2016
  Followup from the Pelican Staff: Yes, get the duct out of the way. Leave the A/C lines alone. - Nick at Pelican Parts  
oldman Comments: the rubber hose made it a breeze to put back the plugs.
September 22, 2015
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
 

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Page last updated: Fri 12/9/2016 02:30:44 AM