Just picked up this 84 300D this past week. Had to do some minor stuff to it, put in a windshield, etc....It's been cool down here in Va last couple of days, today temps only got up to 55 or 60....I noticed a few days ago, when you crank this one up, it runs real rough upon startup and only will stay running if you hold down the gas, like the idle speed is set way too low..She won't even stay running unless you hold the throttle open a tad...After about 3-5 minutes of holding throttle open, she will idle like normal around 500 rpm...It didn't do this when I first got the car as it was warmer then.....Any ideas????

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Looking for another Mercedes 300D or SD as a project vehicle..

Have you gone through the basics of valve adjustment, cam timing, and IP timing?

No, actually have not yet....What gets me is that up until the temp dropped some the car started fine and still does after a little warmup time...First diesel I've seen that needed warmup time....Once I get moved and where I'll have my garage to work in will go through and check out the IP timing, etc. This car was originally from NJ where it is normally colder than here in Va...Should the IP timing be different for colder weather??? My first thought was cold fuel, wondering if the diesel could be jelling, haven't drained the tank or anything, it has a half tank of diesel in it....Not sure how old it is though, car had neen sitting for about 8 months before I got it....

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Looking for another Mercedes 300D or SD as a project vehicle..

Well, then, you can stop all of your musings regarding the issue(s) until you perform those tasks.

My SD starts at 12F. in one second and idles without any roughness. It's a little sluggish at that temperature when it needs to move the vehicle........to be expected.

My 85 300d has the same problem. Today I put in an Eight degree offset key to correct the problem. Just got her back together tonight. Its fourty degrees out and she started right up. Still had to hold the pedal for a couple of minutes though. IP timing is my next move until the compression tester I ordered gets here.

Goodluck

Do you plug it in at that temp? Just cus it will start doesnt mean you should

.........well.......I'd have plugged it in but the extension cord running into the lobby of the Marriott doesn't look too cool...........

One night last winter (about -10F in Green Bay) I had an extension cord running from the balcony of my Residence Inn hotel room (2nd floor) to my 300D in the parking lot.

Yeah.......BTDT, but, usually it's in a Super 8 or similar where you can get away with it..........stayed out in Pittsburgh one real cold night and ran the wire down from the second floor window..........got down to near zero..........glad I did it.

the anal engineer oops i mean my father takes and puts his car out of the heated garage on purpose on below zero nights just to see My motto is it started must be ok doesnt impress him No wonder there is such an array of digital amp and volt metters all over the boat and swithches so i know the condition of all 6 *D batteries @@@@ Engineers

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commercial fisherman diesel in my blood in my boats trucks and Dear old Dad has had me drivin them since i got my first license in 1968
1986 300 SDL 427654
1999 Chevy Crew Cab Dually 225423
1986 300 SDL 287000 Dad's
1987 190 Turbo 158000 Mom's (my inheritance)