W204 backfires while parked?
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2014 C300 4MATIC [W204]
W204 backfires while parked?
Hey All, I’ve had this problem for a while now and need assistance diagnosing.
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**Car**
2014 Mercedes C300 (facelift W204 w/ M276 engine) – flex fuel vehicle
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**Problem**
Car backfires while parked after driving. Or at least it seems like a backfire. I hear a loud-ish bang/pop and smoke/exhaust from the hood. I pop the hood and the smoke seems to come from everywhere – no specific spot I can identify – possible the driver’s side.
Used to happen when I came back to the car and hit unlock on the fob, then eventually started happening on its own around an hour after parked and locked. It’s not always consistent and thought I had solved the problem, but after changing the battery it’s back happening on its own.
After this happens the car doesn’t start as smooth, probably because it’s been knocked out of timing or something. Car works perfectly fine when running and no codes shown. Had been using E85 ethanol previously, but running the standard 91 premium in it since issues arose.
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**Circumstances**
Running errands around town – get the car warm by driving around 20-40 mins. Park and lock the car. Might make a couple of stops before leaving it for ~1hr when I will hear the backfire.
I've heard the car do a "settling" sound after parked. It sounds similar to hitting the unlock on the fob, when the systems prime, but this is after it's parked and sitting for a bit.
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**Background**
It’s a flex fuel vehicle, so during the warmer months I used E85 ethanol, since it seemed to idle slightly better and give a touch of increased performance. I had an unscheduled trip and left E85 in the tank at the airport, which got cold over the 5-7 days I was away. When I got back it drove home fine (~1.5 hrs) until the turn into our neighborhood and it started to misfire. I forgot I had E85 in the tank which gets slushy when below freezing.
I took it to a mechanic after telling them the details and they got it cleared out by introducing a fuel system cleaner and driving it *spiritedly* for about 45 mins. I took it home fine (~1 hr) parked it in the garage, and had it backfired in the garage about 45mins after getting home when I came to unlock it and run some errands. Thought it was a one-off occurrence and drove it normally. Didn’t have any other occurrences until I hit “E” on the fuel gauge on the way to fill it up and it started misfiring again while running.
I took it to the shop and they replaced the spark plugs and injector #3. This was during covid when supply was low. They said that all new ones were out of stock and the refurbished ones were even more expensive than new. They mentioned that they had ‘known good parts’ around the shop so they replaced it with one that was in another vehicle.
It seems to smell like it’s running rich, but I’m not sure what to look into. Not sure if it’s the injector since it happened before one was replaced so I’m looking for advice on what I should investigate – fuel pump, fuel filter, other injectors, tune, flex-fuel sensor etc.
I’ve taken it back to the mechanic a couple of times and they can’t replicate it for some reason. They’re about 1hr away (merc specialists), but I may have to go up there and replicate it for them if I can.
Appreciate the help!
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**Car**
2014 Mercedes C300 (facelift W204 w/ M276 engine) – flex fuel vehicle
---
**Problem**
Car backfires while parked after driving. Or at least it seems like a backfire. I hear a loud-ish bang/pop and smoke/exhaust from the hood. I pop the hood and the smoke seems to come from everywhere – no specific spot I can identify – possible the driver’s side.
Used to happen when I came back to the car and hit unlock on the fob, then eventually started happening on its own around an hour after parked and locked. It’s not always consistent and thought I had solved the problem, but after changing the battery it’s back happening on its own.
After this happens the car doesn’t start as smooth, probably because it’s been knocked out of timing or something. Car works perfectly fine when running and no codes shown. Had been using E85 ethanol previously, but running the standard 91 premium in it since issues arose.
---
**Circumstances**
Running errands around town – get the car warm by driving around 20-40 mins. Park and lock the car. Might make a couple of stops before leaving it for ~1hr when I will hear the backfire.
I've heard the car do a "settling" sound after parked. It sounds similar to hitting the unlock on the fob, when the systems prime, but this is after it's parked and sitting for a bit.
---
**Background**
It’s a flex fuel vehicle, so during the warmer months I used E85 ethanol, since it seemed to idle slightly better and give a touch of increased performance. I had an unscheduled trip and left E85 in the tank at the airport, which got cold over the 5-7 days I was away. When I got back it drove home fine (~1.5 hrs) until the turn into our neighborhood and it started to misfire. I forgot I had E85 in the tank which gets slushy when below freezing.
I took it to a mechanic after telling them the details and they got it cleared out by introducing a fuel system cleaner and driving it *spiritedly* for about 45 mins. I took it home fine (~1 hr) parked it in the garage, and had it backfired in the garage about 45mins after getting home when I came to unlock it and run some errands. Thought it was a one-off occurrence and drove it normally. Didn’t have any other occurrences until I hit “E” on the fuel gauge on the way to fill it up and it started misfiring again while running.
I took it to the shop and they replaced the spark plugs and injector #3. This was during covid when supply was low. They said that all new ones were out of stock and the refurbished ones were even more expensive than new. They mentioned that they had ‘known good parts’ around the shop so they replaced it with one that was in another vehicle.
It seems to smell like it’s running rich, but I’m not sure what to look into. Not sure if it’s the injector since it happened before one was replaced so I’m looking for advice on what I should investigate – fuel pump, fuel filter, other injectors, tune, flex-fuel sensor etc.
I’ve taken it back to the mechanic a couple of times and they can’t replicate it for some reason. They’re about 1hr away (merc specialists), but I may have to go up there and replicate it for them if I can.
Appreciate the help!
