Thank you in advance for any replies to my query. I am driving a 1984 500sec. As a bit of background, prior to last Friday she had been running exceptionally well. New idle control Unit, air slide valve and transmission have provided both a very smooth idler and runner. The only recent issue has been a stall at a red light several moths ago that was remedied with a new fuel pump relay that I was storing in my glovebox (the malfunctioning relay I had given a second lease on life by re-soldering).

When I went to start her last Friday she initially exhibited the signs of a failing starter; a whirring noise but no engagement. This problem has been fairly common in colder weather over the past few years. Only my procrastination, parsimonious nature and dream of relocating to a more gentle climate have prevented its removal and replacement. Generally a second or third attempt results in ignition and I am upon the road. On this particular day, however, when the starter ultimately performed its duty I was confronted with an idle that was very weak and spotty. Pressing the accelerator provided little if any power. Just enough power was present to limp her to my local mechanic.

My mechanic’s initial diagnosis was that of a failing catalytic converter. It was his intention to free up the exhaust and verify this presumption. As I recall I could not achieve any significant RPM. I disconnected the air slide valve connection to see if that would increase the RPM but it produced no change. I left and awaited the catalytic converter test to shed some light on the matter.

I returned to the garage the following evening (much more balmy day than the previous) as I had some papers in the vehicle that I needed. My mechanic was busy so I simply grabbed my keys and proceeded to the car. Curiosity prompted me to attempt to start her. She fired right up and idled perfectly. I took her for an extended drive and any hint of the previous day’s misconduct was absent. I added some injector cleaner to the almost empty gas tank and drove some more. My mechanic had not yet touched the car. I drove her again this evening (colder again) and while the starter did again misbehave, once ignition was achieved everything was running perfectly.

What could have been the cause of my vehicle’s Jekyl and Hyde behaviour? Could it still be a clogged catalyst that managed to free itself of obstruction? Spark plugs? (I am using platinum but intend to return to the stock at the next change) Ignition coil? Ignition control unit? Throttle sensor? Bad gas? Clogged injector? Intermittent fuel pump (filter is new)? OVP?

Any possible suggestions as to the cause of this recent unpleasantness would be greatly appreciated. Cheers

Interesting.
I would start with a new, fresh tank of gas and maybe a new fuelfilter.
A catalytic converter does not " return from the dead ".

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2007 C 230 Sport.

Thank you. I shall cross the catalyst from my list of potential suspects. This is a bit of a relief as the dealer quoted me $3,240.00 plus tax.

Not to pick nits, but a plugged cat can be intermittent if the guts are broken up. A piece could plug the exit and cause symptoms, then later change position and allow the engine to run. It'd probably rattle a good bit of this were the case.

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1989 300 SEL that mostly works, but needs TLC