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Discussion Starter #1
So I just bought this two days ago, and I've put 250 miles on it since. Everything was running great... now I have an issue that came out of nowhere.

First the coolant temp gauge started bouncing between 40 and 80c... then between 80 and 120, and then sitting at idle it was sitting right at 120. So I let it sit for a few... start it back up and its sitting at 120. Drive it and it drops to 80 while driving, creeping back up to 120 by the time I get home.

1. I haven't had the car long, so is that normal?

Simultaneously, It feels like a misfire is consistent... at really low RPMs (like 15) you can really feel it. at 30 you can kind of tell its there but its more drivable.

Both of these just started the last 3 miles or so... any idea what it could be?

(extra info- The guy I bought it from said this happened about 2k miles ago. he disclosed itg when I bought it- He has receipts for it, but had the coilpack replaced and at the time they noticed a small water pump leak, so they replaced the water pump too, and the belt. It was driving perfectly find when I first got it, so what could have caused these symptoms to return? planning on having the rear diff and trans fluid plus oil change done in about an hour, then looking at doing the rest of the 100k (plugs and stuff) even though I'm a little short.... but the temp and the coil pack seem completely unrelated??)

Thanks!
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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temp probe fell out of its holder?

do a search, top right and replace it where it belongs!
 

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Discussion Starter #3
Where is the coolant temp probe? I did searches and found a lot about the external temp probe but not the coolant temp.
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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the external temp probe when not in its holder will bounce those temps around
 

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Discussion Starter #7
So the ampient thermo will bounce the coolent temp?

Just drove it to the shop... When I started it bounced between 60 and 80 once or twice but then stabilized at 80, Never went over about 82. No misfire. Misfire only happens when temp gets over 100k. Sounds like a thermostat issue doesnt it? The thermostats stuck closed.

I'll replace the thermostat if the ambient thermo doesnt fix it. Does that sound right?
 

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Premium Member 2002 SLK230K
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If the PO replaced the water pump the system could be air locked, i would replace the t stat then bleed the system.
 

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I just replaced my thermostat. Super easy on this car and now my temp gauge stays just north of 80. Prior to that, it would bounce around mostly lower and I didn't know if it was normal. I bought the car in January, so it just recently would go over 80. Drives much better now and engine seems to be smoother.

ref
 

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Discussion Starter #10
Just ordered a new thermo. Went ahead with an OEM- didn't decide to go hotter or colder or other brand.... I'll get working on some of my other fixes while that ships and then I'll throw it in and see if that fixes it. I'll probably just do a full drain and fill after I get the new thermo in there. That should make sure theres no airlock.
 

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I hate to say it but it sounds like what happened when I had a cracked head gasket...it would sometimes be great, fouled a couple plugs though, didn't always run great and would spike at 120.
 
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