Spark Plugs?

[sc_rufctr]

I changed the park plugs on my daily August 2018 and I think it's time again. The engine occasionally misfires when fully warmed up and the rest of the ignition system is relatively new. It's a 1992 W140 S420 with close to 100,000 miles but it uses old school ignition wires, distributor caps/rotors and copper core non restive plugs. MB recommends changing the plugs every 30,000 miles.

So is it worth going to Platinum or Iridium cored plugs? There are three things that give me pause about this idea...

1. Anything with an exotic core is expensive. (Three or four times the price!)
2. The longer a spark plug is left in place undisturbed the more risky it becomes to remove IMO.
3. I've heard it's best to use the recommended plugs in the old MB's because they don't like a spark plugs with a restive cores.

I've done a bit of reading on the Mercedes sites and there's mixed info at best. Some people have used them with no problems and are happy with their decision. Others tell nightmare stories of burned up distributor caps and failing ignition leads.

FWIW: I change the plugs every 2 years in my 911 SC. I drive it regularly so I'm not concerned about millage so much.

Thoughts?

(Pic from the last time I changed them - This is the left hand bank of the 4.2 litre V8)

[asphaltgambler]

You may have either high secondary resistance in one of the wires or there's a low resistance leak to ground. I would try very light mist spray of water on the wires while the engine is running, starting from the plugs working back to the cap.

Also the wire / plug end could be voltage leaking onto the well sleeve, there may be visual clues when examining that

[svandamme]

30 k miles, i'de replaced em long before interval wise.. preempt the misfires and ensure smooth removal.

i'm with asphalt on the wires, but replacing plugs don't hurt to try first if they are over 20k miles.


What kind, well, I figure a broken primary cat or secondary cat or engine costs way way more then the plugs.. and so far i've found plugs for my cayman online at considerably better prices then most sites have em.. I get bosch at 8 euro a pop.. I mean what's 48 euro every 2 years for a Cayman ? peanuts really
I used them
Heavy duty yttrium-enhanced center electrode with copper core .. But I have no idea bout older other cars so other then "don't let price make the decision" i got nothing

[Fast Freddy 944]

If an engine is running like crap after a tune up, internal engine wear might be an issue. Time for a new car? Maybe..

[svandamme]

well, you could just have a look why first, if it has ODB2 . .. at least have a look at the short and long trim states.. it will tell you what the computer does to compensate.

V8 .. 2 banks.. all it needs is one faulting o2 sensor on one side and yer engine will run out of whack..


but you need to be sure of the spark first. That sets it all off and is primary
o2 sensors

I would think external titbits are more likely the culprit then internal wear at his mileage..
IF all the tidbits checkout and it still runs rough ,you'de almost have to see it with a compression/leak test , but that's really last thing to look for.

[sc_rufctr]

Originally Posted by Fast Freddy 944

If an engine is running like crap after a tune up, internal engine wear might be an issue. Time for a new car? Maybe..

Right now if I start it, it runs perfectly. The problem only happens when it's fully warmed up.

But yes I'd love an excuse to buy a new car.

[sc_rufctr]

Originally Posted by svandamme

well, you could just have a look why first, if it has ODB2 . .. at least have a look at the short and long trim states.. it will tell you what the computer does to compensate.

V8 .. 2 banks.. all it needs is one faulting o2 sensor on one side and yer engine will run out of whack..


but you need to be sure of the spark first. That sets it all off and is primary
o2 sensors

I would think external titbits are more likely the culprit then internal wear at his mileage..
IF all the tidbits checkout and it still runs rough ,you'de almost have to see it with a compression/leak test , but that's really last thing to look for.

It's a 1992 so only OBDB1... Blink codes

[Fast Freddy 944]

Originally Posted by sc_rufctr

Right now if I start it, it runs perfectly. The problem only happens when it's fully warmed up.

But yes I'd love an excuse to buy a new car.

Pull out the spark plugs for each cylinder, are they oily? Are they turning white? Oil tells me that the piston rings and valve seals are worn out, time for a rebuild. White plugs tell me that its running extremely lean, as if there an air leak. If its crapping out and no mechanical issues, you could have a bad coil pack, bad spark plug, faulty spark plug wire. Connector issues, bad engine managment computer. etc.

[svandamme]

Originally Posted by sc_rufctr

It's a 1992 so only OBDB1... Blink codes

right, missed the year.
and probably only 1 o2 sensor downstream?
might be worth testing it's resistance

[Nostril Cheese]

Its an MBZ, so its probably a resistance issue. Check for cracked leads, frayed connectors, worn rotor/cap etc..

Use the factory recommended plugs. Platinum and iridium plugs can cause issues with Bosch if not specified for use. The only real advantage is increasing the interval between changes.