Hello all!

So as the title suggests I have P0300 code (accompanied by some additional P030X codes)

I see a lot of posts regarding misfires, but none really address the issue directly or seem to have a good solution, so I figured I'd start a new thread and beg for everyone's input.

I don't just want to start throwing new parts/money at the problem, so I'm hoping someone reading this has had a similar issue, or understands the vehicles systems better than I do...

It first started a month ago, but I'd only get a misfire when I reached the top of the rev band (ie. near the redline), however now it happens all the time at any RPM and even at light throttle. If I cycle the ignition (even while still driving) the misfire goes away for a little bit, but always comes back after a short time, and it's happening more and more often.

I'm reluctant to think it's plugs or coil packs, I have a hard time believing multiple coil packs would fail all at once (car has 70,000 miles)... I also doubt it's the plugs since there are 2 per cylinder... but that's just my semi-educated guess. Maybe if the car detects a bad plug it shuts down the entire coil pack and/or injector???...But still, it would be weird to have no issues at all, and then have multiple random misfires all at the exact same time... You'd think with the relatively low miles you just get a single misfire if a coil started to go bad.... Which is why my gut tells me it's not plug or coil related.

My hunch is it's fuel pressure related?? Does anyone know if the Merc engine shuts off the injectors to random cylinders if it detects low fuel pressure?

I know it could also be a vacuum leak, blocked fuel filter, dieing fuel pump, or even a control module issue... But does anyone have a good idea about what's most likely the cause?

Thank you in advance for any help.

 

What are the other codes ? P0300 is a generic code but P030x (x=1:6) tell which cyl is an issue.


Most usual cause is a high voltage (coil, wires, plug) breakdown and they usually start small then get worse.


That last non-zero digit says which cyl it is (and a P030x just means the computer detected a rpm slowdown when that cyl was supposed to fire so could also be mechanical) but need to know what those other codes were.

 

I had oodles of P030x's and some O2 codes around 95,000 miles in the Graphite. Threw 12 plugs and wires at it, since it was time anyway. All codes GONE.

I'd measure the fuel pressure, I wish there was a way to measure it under load, but I think if the pressure is failing at all, you will see it on the gauge even when revving the engine when in Neutral. I'd then look for vacuum leaks, maybe clean the throttle body, perhaps even do that "Seafoam" thing with the gas.

If none of the above do it, I'd throw 12 plugs and wires at it. If the car had 100,000 miles on it, I'd do the plugs and wires first.

 

Google that code and you will find the possible causes, P0300 is as you say a random misfire and not caused by any particular cylinder.
Along with the mystery codes you should get a good idea of the cause, continued use when showing this code can cause damage to the engine.
Come back with the mystery codes and you might get better help.
Check the plugs and cables if you want something to do.

 

BTW don't forget that this is really a Mercedes SLK320 under the skin and for every code a generic OBD-II tester gives you, a real Mercedes scanner will find several each with about a page of information. These do not need to be expensive, there are a number between U$100 and U$150 that will do the job, more for 2004-2006 than later.

 

The X code changes after each code clear... It's always a random mix of 2 or 3 cyl's... Which is why I was starting to think it's not coils.

 

P0300 is a random misfire code, it just means more than one cylinder has been deactivated- The other codes change, sometimes they are p0301 and P0305, other times its P0304 and P0302, but those codes just tell me the car has shut off fuel and/or spark to a specific cylinder... So getting a P0304 code only tells me that cylinder 4 was shut down.... but it doesn't tell me why. The fact I'm not getting a consistent and repeated P030X code is what makes me think it's not plugs, coils or wires.

I will start By checking fuel pressure tonight and keep you all updated.

By the way, it's really great to have such passionate and helpful Crossfire enthusiasts at my fingertips... You guys are great!!!

 

Your problem lies in #2 and #4, they did not shut down, they were just not firing to spec. Switch the coils to other cylinders and see if the problem goes to the new cylinder position. switch the cables later if the problem stays in the #2 and #4 spots. Check the plugs as well.
It could be the injectors plugged or failing, but try the coils first. Looking at the plugs could give a hint about the injectors condition.

 

Really need to download the service manual, makes things much easier. P030x is not deactivation. It is an unplanned rpm loss. 9-343 in '05 SM.


(P0300) MULTIPLE CYLINDER MISFIRE (CONTINUED)
Set Condition: When more than a 1.0% misfire rate is measured during two trips.
POSSIBLE CAUSES
ENGINE CONTROL RELAY OUTPUT CIRCUIT
FUEL INJECTOR CONTROL CIRCUIT
FUSED IGNITION OUTPUT CIRCUIT
ENGINE MECHANICAL PROBLEM
COIL ON PLUG
COIL ON PLUG DRIVER CIRCUIT
SPARK PLUG
FUEL STRAINER PLUGGED
RESTRICTED FUEL SUPPLY LINE
FUEL PUMP
FUEL INJECTOR
POWERTRAIN CONTROL MODULE
For a complete Powertrain Control Module Circuit Diagram, (Refer to 9 - ENGINE - SCHEMATICS AND DIAGRAMS).

 

Sorry, what I am trying to say is all 6 cylinders fail, but it's rarely the same ones twice in a row if I clear the codes... It will be any random combination of 2 or more of the 6 cylinders that flag a fault code... Could it be that all 6 coil pack failed at the same time!?!?

 

Not likely at all. I thought you just had a few cylinders failing, not all. In which case I would look toward fuel problems.
Google lists multiple problems, see if any match your symptoms.

 

OK! this is really good feedback! It supports my theory that its a fuel pressure issue not a coil or plug issue.... I think I have a bad fuel pump and the car is shutting down random injectors to protect them when they are struggling to get fuel, and that would explain why I get a random misfire code and an ever-changing individual coil misfire code ... I will investigate this theory and report back!

 

I agree with you... I will start with fuel delivery. I will change the fuel pump and filter as its cheap and easy.... Wish me luck.

Thanks for the help! I really do appreciate it!

 

I had a 98 Dodge avenger (Mitsubishi motor), which kept giving me random misfire codes. turns out it was the crankshaft position sensor. May not be the case, but it's a cheap item, and relatively easy to change.

 

Thanks, I actually have a spare one so I will change it tomorrow.

 

which model crossfire do you have? are you losing coolant from the overflow?

 

I'd go with this first as well - they are known to bugger about, as does the cam sensor.

 

Well did anyone ever figure this out?

 

Hello all, I'm posting in this (older) thread because I have exactly the same problem with with my SSB SRT6 roadster. I replaced the Crankshaft Position Sensor with a new BOSCH CPS today. No difference. Car runs good until I "get on it" then in the upper RPM range it throws the P0300 code and runs terrible. That is the ONLY code. I can turn off the ignition / restart the car and it runs OK enough to limp home. When I clear the code, it runs good again until I "get on it" again then same code P03000 comes up and it runs like S#*^ again. Recent set of NGK Laser Iridium Plugs and Magnecor KV85 8.5mm wires so I know that's NOT the problem. Are we thinking this is a fuel issue? I have an SL55 fuel pump and the the adjustable fuel pressure regulator. I ran out of time today but will look at the fuel pump area tomorrow and see what the fuel pressure reading shows. Any other ideas are welcome . . . THANKS,

 

What scanner are you using please ? The reason I ask is that P0300 is a generic code that means there is a misfire issue but not precisely what. If a DTC only scanner like Torque Pro, it does not do in-depth scanning required for a Mercedes (Crossfire is really a Merc SLK320 or SLK32 (SRT) that has been rebodied.

To read the underlying code you either need a Dealer scanner (DAS/STAR) or a handheld that has a MB module (like an ) or something equivalent.