My moms old 300SEL (with the M103) that was once my car is now my "work car" again as she is getting a brand new SUV very soon. I have been using the car often. Its been reliable the past 4 years we have owned it, but recently (even in this dreafully hot weather we have had) until the engine is fully warmed to operating temperature, its "cold natured" meaning that the car will misfire (a dead miss) on initial acceleration (up to about 2500 rpms) every time you pull from a stop until it warms up. If I let it warm up for 10 minutes before I leave in the mornings, its fine when I take off. Even on these hot and muggy mornings (lows in the 70s) it does this. The car had new spark plugs put in about 4 months ago, a new EHA valve, and a new fuel pump about 2 months ago. We replaced the coil about a year ago. It kinda acted like this before the coil was replaced, but it did it all of the time. Maybe that is going bad again? I used to could take off in the car at any temperature and it would run smoothly. Now something is wrong since it will "chug" and misfire on initial acceleration until fully warmed up.

What else could it be? We didnt replace the spark plug wires, could that be it?

Also, when I hit certain bumps, there is like something is causing a "thump" that can be felt through the steering wheel, but its when the REAR tires hit the bump. You cant hear it, but you can feel it. It did this before, but stopped for a while when we put on some new tires, but its seemed to have come back a little. The muffler is getting loose and had came loose from a clamp, but the muffler shop put it back on today and it still does it. Both mufflers have holes rusted and its going to cost $601. to go back with the dual pipes, or $186. if I go universal (with a single pipe), which I likely will.

Thanks in advance!

Your cyclinders are probably being filled with oil, drips of gas, or coolant from the old headgasket giving way, some leaky valves, and letting it drip in. The warming up of the car is the engine burning all of that crap out and keeping it out while the car is running and warm.

86560SEL, when was the Oxygen sensor replaced last? I would attribute the acceleration hesistation and poor engine performance when cold to that.

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Fred Hoelzle

Cold start and warm-up operation is dependent on the OVP operating correctly.
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Dont know about that, not having to add any oil or coolant.

To be honest, I have no clue. I know that the check engine light will come on for about 30 seconds after the car is first started then goes off and dont come back on. Guess that could be it. Thanks.

Thanks. I think we replaced that awhile back, but could be wrong.

Skip the muffler shop $100 exhaust, you get what you pay for... pinch bent pipes, reverse flow mufflers, etc.

The factory exhaust really is not that expensive new from a dealer with good pricing (I paid just over $500 for mine on the 300SEL from MB Sugarland in Texas)

Or just search around for someone parting a car and buy either the resonator or rear muffler if one is bad on that car, then buy the other new. You may become lucky and buy both in good used shape for nothing. Used typically go for about $50/ea.

Check the cap and rotor, the 300SEL buzzes down the road and eats up the accessories more quickly than most vehicles.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Its easy spending your money but seriously, you can't diagnose this running problem without putting new wires, cap and rotor on. If you shop around you might be able to do the whole lot for $150. As for the exhaust, I second the suggestion to bargain with a dealer for the original setup. They are made better than aftermarket (no resonating noise and last longer). Last time I bought the center muffler and rear muffler for my DD (300e) I paid $450 at the dealer for the parts. Its an hour job for a good DIY, and it will last another ten years.

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88 300e
89 300e
93 400e
93 500e
77 308 GTB

Misfire.
To start with, I would remove the distributor cap and check that it is dry inside and then move onto checking the cold start injector was shutting off properly.
Ignition leads on M103 engines are usually good enough (our '90 300 is still on the original leads), but the insulation can crack. Run it in the dark to check that there is no arcing around the leads.

Exhaust.
If it had twin pipes from the factory, it won't like a single pipe in the system, unless you fit one large enough to match the original system's flow rate and there might not be space.

The thumping.
Check that the exhaust doesn't foul the body anywhere when you push it upwards.
Rear subframe mounting bushes are worth checking as well.

Would worn valve stem seals, a typical M103 engine problem, account for the cold running misfires?

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)

Just on a start up it will miss for a bit.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Surprising the fault is eluding detection with so much input .
If possible squirt a little starter fluid to check the cold idle stabilizes .

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Thanks... well the issue continues... I have just been letting it run for about 10 minutes before leaving in the mornings, so there has been no issue. One morning though I let it run for only 5 minutes and it gave me a terrible time on pulling out onto the main highway from my house. It actually stalled on me once.

Autozone has new plug wires for $102 and those are Bosch.... those have been the cheapest Bosch brand I have found, even on fastlane they are $115.

I think the plug wires are the only things we havent replaced and I think they are the originals, so I am sure they are ready for a change. May get some tomorrow and throw on there.... hopefully cure the issue, if not, oh well. lol.

Thanks again.

Elsewhere someone mentioned the rotor was causing a miss but that was only after warm-up, could reverse be true?
Regards
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