SL/R129: SL320 Rough Idle
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Newbie
Joined: Aug 2024
Posts: 2
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1995 SL320
SL320 Rough Idle
There are several threads on this but haven't found a solution yet. Hoping for some collective knowledge. 1995 SL320 with 130K miles. When cold, everything is fine. When driving, everything is fine (cold or warm). When the car warms up, the idle is a bit low (~700 rpm in park, ~500 in d so about 100 rpm below spec). Doesn't change with AC load. It has been this way for about a year but not like that for the prior 8 years I've owned it. It recently developed an occasional miss which makes the low idle a real problem at a stop light. Here's what I've done and have yet to find the problem:
1) Replaced plugs (Bosch F8DC4 same as what I removed)
2) Replaced coils (original ones were in and installed Bosch)
3) Replaced plug wires and HTs
4) Replaced engine harness (about 6 years ago)
5) Replaced MTA/throttle body last week with one manufactured 2004 (amazing that mine worked at all given there was no insulation)
6) Swapped engine module, idle/cruise control module, MAF, resonance flap from my other '95 SL320 that runs flawlessly
7) Replaced rubber vacuum connectors & tested the all the vacuum lines under the hood
8) Removed & cleaned EGR metal line and confirmed EGR works
Codes: none when I had just the low idle speed. Now with the occasional misfire, I have 21 and 22 blinks on socket 4 of the 38 pin connector. Those are coil related codes for cylinders 1&6 and for cylinders 2&5. I put the old coils back in - no change. Yesterday I drove longer than normal (maybe 20 minutes after it warmed up) and the check engine light came on and the car began running very rough and no power. Checked codes again and now have a the prior codes plus a 47 blink code on socket 4. My code documentation doesn't even show 47 blinks! Any help with this problem would be greatly appreciated.
1) Replaced plugs (Bosch F8DC4 same as what I removed)
2) Replaced coils (original ones were in and installed Bosch)
3) Replaced plug wires and HTs
4) Replaced engine harness (about 6 years ago)
5) Replaced MTA/throttle body last week with one manufactured 2004 (amazing that mine worked at all given there was no insulation)
6) Swapped engine module, idle/cruise control module, MAF, resonance flap from my other '95 SL320 that runs flawlessly
7) Replaced rubber vacuum connectors & tested the all the vacuum lines under the hood
8) Removed & cleaned EGR metal line and confirmed EGR works
Codes: none when I had just the low idle speed. Now with the occasional misfire, I have 21 and 22 blinks on socket 4 of the 38 pin connector. Those are coil related codes for cylinders 1&6 and for cylinders 2&5. I put the old coils back in - no change. Yesterday I drove longer than normal (maybe 20 minutes after it warmed up) and the check engine light came on and the car began running very rough and no power. Checked codes again and now have a the prior codes plus a 47 blink code on socket 4. My code documentation doesn't even show 47 blinks! Any help with this problem would be greatly appreciated.
Junior Member
Joined: May 2009
Posts: 25
Likes: 3
From: Montreal, Canada
2017 GLC43 AMG
If you also have hard start issues when the engine is warm you may want to have a look at this thread. https://mbworld.org/forums/sl-class-...hot-start.html
