420SEL Engine Problems
 

This is a 1991 420SEL just turned 90,000 miles. Last week I noticed a slight skip in the engine which has gotten worse to the point that it felt like it was running on 6 cyl. I changed the Distributor cap thinking it might help but made no difference. Now it wont even fire at all. Any suggestions? I've heard that once they reach 90 to 100 thousand miles that you should change the timing chain, is this usually the case?

420 problems
 

Changing the timing chain at 90-100k miles is intended to fix it before it breaks.
If the engine ran before you replaced the cap I would recheck that job (maybe a missing rotor?)
There are many things that can cause a miss. Plugs (fouled or broken ),plug connectors (burned open), ign. wires (leaking), intake leaks,fuel injectors,fuel distributor and so on.
Good luck Mike

420 Engine Problems
 

Thanks for the reply, it did run before but just enough to get me home, like running on 6 cyl. I did put cap and rotor on but haven't checked the coil yet.
Are there any sensors that could give me this problem?

My 84' 500SEL developed misses like yours once in a while and I have been able to fix the problem each time with a bottle of Redline fuel system cleaner. I am guessing that, in my case, dirt flakes off somewhere in the system and clog the injector(s). I have found that Chevron gas make it worse for some reason.

many causes, go basic checking, fuel system, electrical system, thats basic and can be helpful in eliminating unwanted expences.

Do check the rest of the electrical connections,the plug wires develop more resistance with age,you must do the basic first and pulling the plugs for inspection on up to the conduction from the cap to them,If this is good we now will have a starting place to help you diagnose your troubles.JNT