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Registered
2005 Deville
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164 Posts
Discussion Starter #1
OK this isn't for a Cadillac and if it's not allowed I'll understand. My dad's 86 Buick Century T Type 3.8 ( 110,000 miles) idles rough in drive but smooth in reverse and neutral. Rough on acceleration smooth at constant speed. Roughness worse when engine warmed up. Has new plugs and wires. Any ideas?
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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86,781 Posts
Re: Rough Idle in Drive Smooth in Reverse

'86? :hmm: Did it have a distributorless ignition in '86? If so, maybe a bad coil or ICM.

P.S.
I'll move this to the lounge for lack of a better place.
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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19,260 Posts
Re: Rough Idle in Drive Smooth in Reverse

OK this isn't for a Cadillac and if it's not allowed I'll understand. My dad's 86 Buick Century T Type 3.8 ( 110,000 miles) idles rough in drive but smooth in reverse and neutral. Rough on acceleration smooth at constant speed. Roughness worse when engine warmed up. Has new plugs and wires. Any ideas?
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check the motor mounts -

the engine will lift when in drive and accelerating -

could be a broken/chaffed wire - or bad/loose ground -
 

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Registered
2005 Deville
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164 Posts
Discussion Starter #4
Re: Rough Idle in Drive Smooth in Reverse

Thanks for the replies. I put the car in drive with the brakes and pressed the accelerator and the motor only rose an inch which I believe is normal so the mounts should be OK? The 3.8 has an ICM and it has been replaced a couple of times in the past so it may be time to change it again. Could a bad EGR valve do this?
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
Joined
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86,781 Posts
Re: Rough Idle in Drive Smooth in Reverse

If the EGR pintle valve is not seating and leaking, it will cause a rough idle. Open it enough and it will cause a stall. Can't hurt to pull it and clean it.
 

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Registered
Cadillac CT6 (2019)
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2,280 Posts
Re: Rough Idle in Drive Smooth in Reverse

Maybe a transmission mount; the difference between drive and reverse would be opposite torque post-transmission.
 
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