I've been told that Bosch spark plugs may be the best option for this car. It is idling rough and I thought I would start with new plugs. Which particular model Bosch spark plugs might be the best to use? Also, I have found the following brands of spark plug wires for this car, any ideas which brand might be a good choice for a 1991 560SEL?

Karlyn

Beru

Standard

NGK

URO

Walker

Bremi

Autoparts Premium

AI generated:
For a 1991 Mercedes-Benz 560SEL (W126 with the M117.968 engine), here are the current best/recommended spark plugs and wires as of 2025:

### Spark Plugs (OEM-spec and best upgrades)

| Recommendation | Part Number | Notes |
|----------------|------------------------------|-----------------------------------------------------------------------|
| Factory OEM | Bosch F8DC4 (or Mercedes 003 159 94 03) | Copper core, non-resistor, 14mm thread, heat range 8 |
| Best upgrade #1| NGK BP6EF (or BPR6EF with resistor if you have radio noise issues) | Very popular, slightly colder than stock, excellent performance |
| Best upgrade #2| Bosch FR8DCX or FR7DCX | Copper, slightly better heat range control |
| Long-life option | Bosch Platinum +4 or NGK Iridium IX | Not necessary on this engine, but will last longer (some report slightly less smooth idle) |

Most W126 560SEL owners today run **NGK BP6EF** or stick with the original Bosch F8DC4. Gap them to **0.032" (0.8 mm)** – do NOT go wider.

### Spark Plug Wires (Ignition Cables)

| Recommendation | Part Number / Set | Notes |
|----------------------|---------------------------------|-----------------------------------------------------------------------|
| Top choice (OEM) | Mercedes-Benz/Berlin (A1261500119 full set) | Genuine Mercedes, made by Beru or Bosch, ~$280–$350 |
| Best aftermarket | Beru (Germany) 0 901 356 607 set | Essentially the same as OEM, ~$180–$220 |
| Excellent & cheaper | Bremi (Germany) 126 150 00 18 set | Very high quality, made in Germany, ~$140–$180 |
| Good budget option | Karlyn/STI (uses Beru ends) | Decent quality, ~$100–$130 |
| Avoid | Cheap no-name eBay/Amazon sets | Often wrong resistance, cause misfires and radio interference |

**Strong recommendation**: Go with the **Beru** or **Bremi** wire set. Both are German-made, use the correct metal shield ends (very important on the M117 for RFI suppression), and have the proper resistance (~1–2 kΩ per wire).

### Quick shopping list (2025 prices approx.)

- 8× NGK BP6EF plugs → ~$32–40 USD
- Beru or Bremi wire set → ~160–220 USD
- New Beru or Mercedes distributor cap + rotor while you're in there → ~120–180 USD (highly recommended at this age)

Torque plugs to ~20–22 ft-lbs (do NOT overtighten – the M117 heads are easy to strip). Use a small amount of anti-seize on the threads.

This combination is what most active W126 560SEL owners (R129 500SL/600SL too) run in 2025 and will give you the smoothest idle and best performance the M117 is capable of.

I found a lot of the AI information to be incorrect, because is based on prior incorrect advise given.

Your car came with spark plugs BOSCH W9DCO from the factory, and is best you keep it that way. Not to be confused with WR9DCO, which are resistor plugs. Unfortunately, they are not manufactured, but from time to time they do show up on EBay.
As an alternative to W9DCO, Mercedes recommended BERU 14-9 DUO, or CHAMPION N 12 YCC. I may have a set of BERU plugs, if interested.

I am running BOSCH spark plug wires or KARLYN.

__________________
1991 420SEL (Cal), 1991 560SEC (Fed)

NGK on the wires.

X2 on original non-resistor plugs.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I'm just curious, what's the advantage of using NGK wires vs. Bosch?

__________________
1991 420SEL (Cal), 1991 560SEC (Fed)

Just personal preference, I've always been satisfied with NGK products. Honestly, I might use Bosch for an OE look of course with everything nicely placed in the loom and the lettering similarly indexed.����

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I just got a great deal on a package of ten bosch copper spark plugs for $22. They should arrive by mid-week so I'm going to give them a go, if it isn't the plugs, then I will turn my attention to fuel delivery/fuel injection and perhaps a new set of spark plug wires.

If it isn't the plugs? Did you pull the plugs and test them or check their condition? You might be better off doing some troubleshooting before you end up just parting the car out because you failed to notice a vacuum leak from a deteriorated hose.

__________________

1990 190E 3.0L

Since it's running roughly, do a compression test whilst the plugs are removed.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Euro and NA models use different plugs. JFYI