I have a 1989 190E I6 with over 300k miles on it (mileage gauge stopped about 8 years ago a bit under 250k). Up until about two years ago it ran fine, before beginning to have trouble starting. Since then I have put in a new cold start valve, crank position sensor, distributor cap, and over voltage relay, along with numerous spark plugs, with some relief of the problem for a few starts after each replacement. About half a year ago it developed into a problem idling, before it stopped starting altogether. After the latter few parts being replaced on it, I left it to sit since then. I'm coming back to it now and I am still lost as to what could be causing the current problem.

Coming back to it today, it cranks when turning the key, the plugs spark, and the engine refuses to turn over without the help of an unhealthy amount of starter fluid sprayed into the air intake.

Any ideas on places to start poking around?