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Getting the firing order wrong after a cap or wire change is one of the quickest ways to make a healthy R107 run terribly. Here is the order, how the cylinders are numbered, and how I keep the wires straight.
Firing order of the M116 and M117 V8s
1-5-4-8-6-3-7-2
This is the commonly published order for the Mercedes M116 and M117 V8s. Confirm it against the factory manual or the label on your cap if anything looks different.
How cylinders are numbered
Mercedes numbers the V8’s cylinders 1 to 4 on the right bank (passenger side on a left-hand-drive car), front to back, and 5 to 8 on the left bank, front to back. Cylinder 1 is at the front of the right bank. This is different from many American V8s, so do not use a domestic diagram.
Distributor rotation
The distributor sits at the front of the engine. Check the rotation direction before you route anything: watch the rotor while someone bumps the starter (ignition coil disconnected), or look for an arrow on the distributor body or cap. Then follow the firing order in that direction from the terminal for cylinder 1.
Plug wire routing tips
- Change one wire at a time, so each new wire goes to the same terminal as the old one.
- Keep wires in their original holders and separators. Wires for consecutive cylinders running side by side can cause crossfire.
- Mark the cap terminal for cylinder 1 with a paint pen.
- Keep wires away from the exhaust manifolds.
Six-cylinder M110 order
The 280SL’s M110 inline six fires 1-5-3-6-2-4, with cylinder 1 at the front.
Quick answers
- Why does my V8 run rough after new wires? Usually two wires are swapped. Compare to the order above and check cylinder numbering.
- Is the order on the intake manifold? Some engines have it cast or labeled; trust the manual if it differs.
Related reading
- Tune-up: cap, rotor, wires and timing
- Spark plugs
- Where is TDC for cylinder 1? Turn the engine clockwise by the crank bolt until the timing mark lines up with the pointer and the rotor points at the cap terminal for cylinder 1.
- Does the firing order differ between the 3.8 and 5.6? The commonly published order is the same for M116 and M117 V8s.
- Can wrong wire routing damage the engine? Severe crossfire can cause backfiring and very poor running, so fix it right away.
Before you start
Have the new cap, rotor and wires laid out, a paint pen, and a clean rag. Work in daylight, one wire at a time, and start the engine before you put the air cleaner back on so you can listen for misfires.
