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300 SEL misfire on start up

2140 Views 12 Replies 8 Participants Last post by  RSA560
Hi all,

I bought my first Merc today... a 1988 W126 300 SEL.

As as I start the engine from cold, it misfires for a few seconds and then runs perfectly smoothly and does not misfire at all after that.

I noticed that the misfire only lasts as long as the oil pressure gauge is rising. As soon as it rises to its normal level, the misfire goes away.

All this only takes a few seconds... may be 8 or 10 seconds.

It could be just a coincidence about the oil pressure gauge and misfire syncronising.

Please let me know why this could be
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Seems 8-10 seconds is a bit long for the oil pressure to build up, especially if the engine is cold.

Mine usually coughs a little bit when you start it, but as long as it starts every time and runs well after a few seconds, I'd say it's nothing to worry about.
More information is needed – miles and recent work done, if you know – if you don’t and have no service records do a tune-up including new cap and rotor.
My 1991 300SE does it also.

You turn the key, wait some seconds and then it starts but with some rumble, likt it isn't running on all his 6 cylinders. When I put it in neutral and give it a little rev it's gone and runs smooth ... Just the startups are a bit messy.

Don't know what it could be.

Mine has now 119.000 km's on it. Not very much...

ps: sorry for the hijack :$
mines the same, I did unplug my cold start valve and it got better, but once the car is warm it runs smooth.
Sounds a bit like a general problem for the 3L 6-in line.

No fix fir this little problem?
try this

I dont know if this little issue is common only to the 6 cylinder engine... The last 2 posts seem to be from owners of 560s.

Try this and tell us what happens. It has helped mine a great deal.

Once you remove the air cleaner housing, you will have full view of the throttle linkage.

Look out for the micro switch which gets pressed when the throttle is in the fully off or idle position.

Open the throttle by either working the linkage by hand or getting someone to press the pedal but release it gently.

Now see if the switch is depressed. If not, adjust the linkage so that it is depressed once the throttle is released.

Fit the air filter back and try starting it.

Do not press the throttle when starting the engine.

Mine starts much quicker than before and doesnt seem to miss any more.

Try this and let us all know the results.
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it's most likely worn leaky injectors, what happens is the fuel system loses pressure and on start up the system has to regain pressure do to dripping injectors, in the morning take out the plugs and sniff them. That's why when you rev it it disappears. I thought it was the cold start valve on my car but it's the injectors.
Thanks, I will try it and report back. :thumbsup:
reving didnt help

on mine reving didnt help. It always took the same amount of time to start running smooth. Just a few seconds. The few times i used it after the linkage adjustment, it has run smooth from the start.
Guys my 300 did it from when I bought it at 169 000kms - cold and semi-hot starts. If started within 10 min when hot it would start-up smooth. Then at 220 000kms I noticed oil in the coolant - then had the head gasket replaced along with the valve-guides.....
problem solved, hot and cold!!!
My 560 does exactly the same, in fact if I don't step on the gas during a cold start, it will stall. If it sits for more than half hr after warm-up, it would start up by itself but run rough for 4/5 seconds - drive's me nuts!!!
I'm thinking valve guides to be the culprit???
Forgot too mention that the 500 SE is 100% smooth - cold/semi-hot/hot - never misses a beat and never requires a prod on the gas! Must be the lower mileage, and hence unworn valve guides???
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