The title says it all. I had this car dialed in pretty good. I'll try adjusting the valves this weekend. Any ideas as to why the car would idle rough after getting a new transmission? Thanks

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'81 300SD

Was the torque converter new? Ive seen bad converters cause that.


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Check the mounts?

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

There's some way they can adjust the shift point of the car, maybe that has something to do with it, the bowden cable. Maybe that was tweaked a certain way previously for your old transmission and now it needs to be tweaked to work with your new transmission. But it's like when you hit a certain speed (or RPM) it will make the car shift.

Also there's a transmission vac switch or something that might need to be replaced.

Good luck.

Do a valve adjustment first and then come back to us.

Motor mounts may have been weak and broke while lowering transmission and engine to access the bell housing bolts.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thanks for all of the replies. The torque converter that came with the transmission is also rebuilt. I replaced the motor mounts about a year and a half ago, but the shop changed the transmission mount which might be effecting overall balance of the drive train. I'll adjust the valves tomorrow, but I've always stayed on top of that and the car was idling well before the new transmission, so I'm not expecting that much difference. I have a new rack damper bolt on the way.

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'81 300SD

If it smooths out when you rev it, check the motor mounts. If it gets worse the higher the RPM, check the torque converter.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

It smooths out when I put the car into Park or Neutral, and when I let my foot off the brake. The ride on the highway is much smoother than it used to be. Before the swap, I would get an annoying amount of engine vibration and a low end growl between 2700 and 3100 RPM, and then the engine would get quiet and smooth when I hit 3200 and above.

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'81 300SD

I stay away from shops as much as possible ,in your case take it up their and make them figure it out .You brought it to them idleing fine ,this puts the burden on them not you to get it straightened out.If they installed the transmission correctly that should include having everything working perfectly , should be a warrenty on the paperwork ,you paid to have it installed and the transmission either is setup to the engine incorrectly or theyve got a bad new part on the trans.Have them chase it down . Soon as you start replacing parts they have no obligation and might void any help from them to get it figured out.Is it a shop that does only transmissions?
Could have been out of balance from the factory ,this video demonstrates this.Ask for the manufacturer of the TC and their warrenty.Soon as you approach them with their is a problem ,stop driving it ,their response will be ,what have you done or tried ,nothing should be your response, your the customer and you paid them ,save a headache .https://www.youtube.com/watch?v=nk6YqloTNWw

I have this same problem right now. It's very odd to me. Lemme know if u have a diagnosis or solution

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Which problem are you referring to, the problem I have now with idle, or the problem that I used to have at high RPM?

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'81 300SD

It's a transmission rebuild shop that specializes in Mercedes and BMW transmissions. Most of their transactions are with dealers and garages. They probably only take one walk in every week or two. They give a 2 year/unlimited mile warranty with their work. I'm confident that they'll do what needs to be done to get it right, unfortunately they are 28 miles away from me.

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'81 300SD

Sounds a lot like motor mounts to me. It is very possible they got broken when they disconnected the trans mount and tilted the trans back. What brand mounts did you have on the engine?

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

The mounts are Lemfoerder that I put on the car about a year and a half ago. I have my suspicions as to their authenticity since I didn't see Lemfoerder written on the mounts, could have been Chinese knock offs put in a Lemfoerder box. Bought them off of Ebay.

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'81 300SD