Absolutely stumped, Random misfires
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Newbie
Joined: May 2025
Posts: 4
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Mars Red 2002 Slk32, 2002 C32
Absolutely stumped, Random misfires
Recently picked up a 2002 c32 for parts but I’m attempting to resurrect it. The car starts to randomly misfire and then goes into safe/limp mode and no matter how hard you push the pedal, it’s like barely responsive. CEL reads P0300, P0301, P0302, P0303. No other CEL pops up. When you reset the codes, it runs okay up until you get to 3k rpm then misfires bad and the car starts to aggressively shake and won’t run well until you stop and get the codes erased. Fair to mention, I’ve done a compresión test on this 206k mile engine and it’s pretty good. 149,150,152,150,151,149. Fuel relay has also been replaced along with various other things. Last owner, my buddy just gave up on it after throwing way too much money at it. Only thing that hasn’t really been replaced is the crank position sensor and the camshaft sensor. I doubt it’s that but maybe? Also when it starts to misfire and you come to a stop, you can hear the serpentine belt squeezing and very aggressive engine shake. Also since I own a 2002 Slk32, I notice the supercharger doesn’t even engage on the c32. I’ve been lurking here for a while but finally took the guts to post because not even star diagnosing mechanics have been able to tell me anything. I’ve done a full check on ignition and injector wiring and all looks good. Is there a k40 relay in this car?
Me/last owner have replaced:
- Coil packs
- Brand new correctly gapped Spark plugs
- New Catalytic Convertor
- New Intercooler Pump
- New Genuine Bosch Injectors
- Newer Throttle body
- etc
Me/last owner have replaced:
- Coil packs
- Brand new correctly gapped Spark plugs
- New Catalytic Convertor
- New Intercooler Pump
- New Genuine Bosch Injectors
- Newer Throttle body
- etc
Last edited by 1KaramQ; May 26, 2025 at 02:56 PM.
Thread Starter
Newbie
Joined: May 2025
Posts: 4
Likes: 0
Mars Red 2002 Slk32, 2002 C32
it’s the first thing I checked, there is definitely fuel pressure going all the way down. I definitely should attempt to log when it starts to bog and misfire. Part of me thinks it’s just some small sensor or a huge vacuum leak but I’ve checked quite literally everything vacuum that is visible
Junior Member
Joined: Sep 2009
Posts: 17
Likes: 0
SLK 230 R170
i had this issue two different times before, cant remember the codes though. I first tried to clean the throttle body of all the oil and gunk (it solved similar issue before), then tried a used throttle body, then a brand new OEM Bosch throtle body (brand new AMG TB was rediclously expensive), but the problem kept recurring. I also will get the ESP malfunction error at times.
Finally it was the ECU.
First ECU was bad because water had entered due to a clogged gutter next to ECU box ( i live in a very rainy climate). - installed a used ECU (cloned by a local ecu shop) -> solved the issue for many years.
After many years, the same issue came up, and this time the same ECU was repaired (if not mistaken, leaking capacitor) -> problem solved for now
Finally it was the ECU.
First ECU was bad because water had entered due to a clogged gutter next to ECU box ( i live in a very rainy climate). - installed a used ECU (cloned by a local ecu shop) -> solved the issue for many years.
After many years, the same issue came up, and this time the same ECU was repaired (if not mistaken, leaking capacitor) -> problem solved for now
Member
Joined: Jun 2018
Posts: 249
Likes: 44
From: San Antonio, TX
2005 C55 AMG
Interesting problem. I'd start with cleaning the MAF (probably not the problem, but very low cost), then try the crank and cam position sensors. At least they're not expensive either - around $30 each. I hate the parts canon approach, but with minimal codes thrown it does eliminate things. They do go bad just with age.- usually resulting in a long crank/hot no start). I'd also suggest some contact cleaner on all the connectors (MAF, crank and cam sensors, injectors, coils, etc.)
The other things I'd look into are:
- fuel filter (I know you said you have good pressure, but if it's a non-OEM filter it could mess things up since the OEM filter also had the pressure regulator inside it.)
- MAF brand. Make sure it's OEM or Bosch. I've heard of aftermarket MAFs making the engine run poorly.
- Rubber gasket(s) where the MAF mounts to the intake. If they're hard and leaking it could throw off the mix.
- Clear codes after fixing things.
I really hope it's not the ECU - that can get expensive. I had an Audi S5 (V8) that had a high pressure fuel pump go out and it took the ECU with it. $2000 later... (NOT at the dealer - that's doing my own install and sending it out). Interestingly, once I got the ECU back and figured out that it was one of the 2 high pressure pumps, I unplugged the bad one and the car ran great. Up to 3000 rpm where it would cut out. Sound eerily familiar? Fortunately, there are no high pressure pumps on either the C32 or the C55. They are a common failure item on every car I've had that has them (also BMW N54 335i - THAT one would intermittently fail. Usually while racing someone).
The other things I'd look into are:
- fuel filter (I know you said you have good pressure, but if it's a non-OEM filter it could mess things up since the OEM filter also had the pressure regulator inside it.)
- MAF brand. Make sure it's OEM or Bosch. I've heard of aftermarket MAFs making the engine run poorly.
- Rubber gasket(s) where the MAF mounts to the intake. If they're hard and leaking it could throw off the mix.
- Clear codes after fixing things.
I really hope it's not the ECU - that can get expensive. I had an Audi S5 (V8) that had a high pressure fuel pump go out and it took the ECU with it. $2000 later... (NOT at the dealer - that's doing my own install and sending it out). Interestingly, once I got the ECU back and figured out that it was one of the 2 high pressure pumps, I unplugged the bad one and the car ran great. Up to 3000 rpm where it would cut out. Sound eerily familiar? Fortunately, there are no high pressure pumps on either the C32 or the C55. They are a common failure item on every car I've had that has them (also BMW N54 335i - THAT one would intermittently fail. Usually while racing someone).
Last edited by Spooky55; Jul 10, 2025 at 09:00 PM.
Member
Joined: Jun 2018
Posts: 249
Likes: 44
From: San Antonio, TX
2005 C55 AMG
I'd still proactively replace the crank/cam sensors, clean contacts, make sure the fuel filter is OEM, and clear codes after each "fix."
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Newbie
Joined: Apr 2023
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2007 C230
Me too
Recently picked up a 2002 c32 for parts but I’m attempting to resurrect it. The car starts to randomly misfire and then goes into safe/limp mode and no matter how hard you push the pedal, it’s like barely responsive. CEL reads P0300, P0301, P0302, P0303. No other CEL pops up. When you reset the codes, it runs okay up until you get to 3k rpm then misfires bad and the car starts to aggressively shake and won’t run well until you stop and get the codes erased. Fair to mention, I’ve done a compresión test on this 206k mile engine and it’s pretty good. 149,150,152,150,151,149. Fuel relay has also been replaced along with various other things. Last owner, my buddy just gave up on it after throwing way too much money at it. Only thing that hasn’t really been replaced is the crank position sensor and the camshaft sensor. I doubt it’s that but maybe? Also when it starts to misfire and you come to a stop, you can hear the serpentine belt squeezing and very aggressive engine shake. Also since I own a 2002 Slk32, I notice the supercharger doesn’t even engage on the c32. I’ve been lurking here for a while but finally took the guts to post because not even star diagnosing mechanics have been able to tell me anything. I’ve done a full check on ignition and injector wiring and all looks good. Is there a k40 relay in this car?
Me/last owner have replaced:
- Coil packs
- Brand new correctly gapped Spark plugs
- New Catalytic Convertor
- New Intercooler Pump
- New Genuine Bosch Injectors
- Newer Throttle body
- etc
Me/last owner have replaced:
- Coil packs
- Brand new correctly gapped Spark plugs
- New Catalytic Convertor
- New Intercooler Pump
- New Genuine Bosch Injectors
- Newer Throttle body
- etc
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