Hello, I am getting a check engine light and P0300 Random Misfire only. The boost box, both coil packs, and spark plugs are new from Clark Rupp V-12 back in June 2023. Running ethanol free 92 octane. When I got the check engine light and I assume the error, the car seemed to drive strong and smooth. I was driving it moderately at the time.

The check engine light would go away after a few drive cycles.

It seems like it’s serious?

Any suggestions of how to isolate further?

Johnny

but during the start of the issues, did you not do the coil pack before sorting the power supply that breaks them - so likely a damaged your first new coil pack - which is the very problem 9 out of10 owners experience

I'd sent it back for testing

Pull the codes. P0300 is just a general misfire code and you need to know whether it is the left side or right side. I would also clear the code and see if the Check Engine Light returns especially if you can't feel the misfire. I have used Clark's products for years and his quality control is good. He also warrants the coil packs for a year so you should still be good. Keep us posted. The S600 is a little finicky about ignition components.

Is that with a generic OBD scanner?
Xentry would probably show you more than this so that's my advice: have it scanned with Xentry.

Xentry or another Mercedes specific scanner (I use a Foxwell 530) would give you the number of the plug that is misfiring. That in turn will tell you whether it is a left or a right coilpack. On my Foxwell, when I get a P0300 it also says right or left.

The $10 Chinese scanners probably won't give you the details. You might try Advance Auto; they read the codes for free. Then once you know whether it is left or right, contact Clark Rupp, Jr. He can help you.

I din’t want to muddy up the troubleshooting story, so I’ll break clean:

The all spark plugs, both coil packs, and voltage transformer (aka boost box) were replaced with Clark’s stuff last June-2023.

I tie wrapped the VT connector to the top of the engine (aka barbecue)

The car gave up random misfire codes and identified just about every cylinder with Foxwell NT 510 Elite. Would go into safe mode, Run like poop. Would clear with new driving cycle.

Errors would return in minutes with new driving cycle.

Per Clark’s video, I put eyes on it and found the plug from the VT box had disintegrated and the pins were sticking out -5mm.

Tie wrapped cable back together, mounted up high off of the barbecue. Disconnect battery and cleared everything.

Car would run perfect for a few days. If I got roddled it in S mode, I would get random misfire and cylinder 1,6 consistently.

so, I got Clark on the horn, suggestion was to pull right side coil pack and inspect for black arcing.

The rubber insulator stiction was so strong, I snapped off cylinder 6 coil tube on the end.

Sent that assembly back to Clark, he quickly repaired, tested, and returned it.

It installed smoothly.

Car ran perfectly with no codes for a could have days when driven like Grandpa Jones in C mode.

Then, I put it in S mode, drive mildly spirited, and got the check engine light and only P0300. No cylinders errors.

Car didn’t go into safe mode and appeared to run perfect.

Disconnected battery to clear the check engine light because I was impatient for it to go away after many drive cycles of no more of same errors.

82k miles.

The coil packs, VT, and spark plugs appear to be solid now. Thank you Clark for excellent service after the sale.

Looked up on the in internet and suggestions are to check for:P0300- Random Misfire

• Vacuum leak (Spray starter mist around running motor and listen for idle change)
• Worn spark plug wires or coils (good)

• Defective fuel injectors
• Faulty or clogged EGR valve
• Leaking head gasket
• Faulty MAF sensor
• Faulty oxygen sensor. (Replaced wit Bosch)
• Engine mechanical problems (e.g. a failed head gasket or loose timing chain)
• Wiring problems (e.g. a broken wire or lose connector)
• Computer problems (e.g. software in need of an update or faulty hardware)

Got my list to go through.

Has anyone heard of components like EGR or MAF which tend to fail at this mileage causing this P0300 err?

Johnny

you are confusing me now making it sound sensible

last year you had a misfire - if you find the post I clearly said do the VT at the same time or you will regret it - you did only a coil pack (I presume listening to people who now owe you $1200) - and drove the car.... thus destroying the new coil pack - then fitted a new VT and the other coil pack ?

now you have the std issue of still having a damaged coil pack - we only identified the VT gets damaged as a result of the coil pack doing its designed to die trick, a month before you bought your car - the consequence of fitting a new coil pack with a damaged VT is yet another dead coil pack - you are not alone the dealers still haven't identified that sequence of failure modes....

any v12 owner needs to do the VT first (and or at the same time) before ever fitting a coil pack

Thanks Botus. I did replace all spark plugs and both coil packs out of the gate first week of ownership. The car ran well. Many weeks later I replaced the VT (aka Boost Box) and it still ran well. Only started getting P0300 and P030X codes 5k miles later after the VT plug fell apart on top of the barbecue. It looks like the ignition problem most likely is cleared up now and coincidentally I have something bad or sticking like an EGR valve, bad Mass Flow Sensor, or something global to the P0300 error code?

this has some ideas on how to play - https://www.obd-codes.com/p0300

you could try a 50 50 water meth mix atomised and sprayed in after the air cleaners gently on a hot engine - its will de gunk inlet manifolds throttle bodies and inlet valve - warm engine 500ml over 20 mins blipping open and closed the throttle to 3k
just keep of the wiprer blades it will make then fail quick

after that some CAT cleaner in the petrol tank and a decent run - I doubt will make any difference to the misfire but the clean up will help

If the coil packs and VT are known good and the P0300 code continues; I would suggest you also have a vacuum leak. On my S600, I had a hairline crack in the Y Connector air pipe; and under sharp acceleration, the crack would leak and throw a misfire code (even though I felt nothing). My indy said someone had tried to repair it by over-tightening the clamps; but his advice was simply to replace the Y Pipe (I think it was $78). I would re-check your engine for vacuum leaks.

I use a 20 oz plastic bottle of Techron every other month. Normally, I would tell you additives are trash; but my S600 loves Techron. Case quantity is available online.

Thanks for the suggestions and link.
I added 2 bottles of Techron and will get a spray can of diesel starter spray and see if idle jumps. The Y connection could be cracked due to man handling it numerous times.

Anyone have a diagram where the EGR valve is located?

does this have water cooling on a charge cooler ? under the engine cover is a second expansion bottle - its often empty and the car will hold back when trying to use its performance

A few notes....

Your 275 powered car does not use ‘air mass meters’ or any other manner of ‘Airflow meter.’ Engine management happens via ‘Speed density.’ AKA MAP (Manifold Absolute Pressure) and the other sensors. I’ve never seen an EGR valve on a 275. Be thankful.

Botus we got the same (Undersized) Water-Air charge cooling you Brits got. PITA to bleed the charge cooling circuit, but needed. OP the charge coolant pump is plenty old enough to be suspect. When starting up after a hot soak the charge coolers will be about 11,000 degrees (sarcasm) and it will take the charge cooling circuit a while to get the heat out. Boost is reduced with high intake air temps. MB did some upgrades to charge cooling circuit for the 279. Someone on the board did a nice write up for how to install the 279 charge cooling reservoir and hoses. Worth doing as is increasing size of heat exchanger in front of A/C condenser and adding another charge cooling pump.

Using starting fluid can and will make for instant short lived FIRE when (yes; WHEN) it finds ANY manner of electrical source of ignition. There are better ways to trim hair.
I prefer propane delivered via a wand with a tip that will self seal should it get lit. The wand will allow you to direct propane to exact spots VS spray from a can. Inexpensive tool and a bottle of propane will last a long time.
Any boost leak will make added noise when in boost. You know what your car sounds like when healthy/happy. It will hiss, whistle with a boost leak. Finding those leaks requires a smoke machine.

As for your misfire.... any codes for fuel pressure? XENTRY is your friend for collecting ALL of the codes and dissemination.

Spark plugs all looking happy?

Crank position sensor fresh? Always good preventative maintenance to have a fresh one.

O2 sensors (front) fresh? They tell lies as they age.

Plugged up exhaust kittahs will make random misfire codes before becoming not drivable.

new SI unit of temperature

Coolant level in intercooler tiny reservoir was 1/2 cup low. Topped it off.

Replaced upstream O2 sensors a couple of months ago.

In advance I checked the clamps on the y connection into intake manifold and found one on right side a little loose. Moved car outside away from the natural gas hot water heater, started engine, sprayed sparingly diesel engine starter fluid and got no hits.

Car is running perfect in Comfort mode. Ran it in Sport mode for only a few miles tonight, is running like a top, and got no errors.
Will run it harder in S mode this week and see if she holds.

Johnny

Today I drove everywhere in Sport mode and executed many good strong accelerations and she’s back running superb again.
Thanks for the great suggestions from everyone. It’s looking like the Y pipe clamp into the passenger side of the motor was borderline loose causing the P0300 Random Misfire, no cylinders identified?

Another case solved and closed.

When this car is behaving, it’s one amazing machine.

Johnny

My S600 is also very sensitive to small vacuum leaks. Frequently throws a P0300 but nothing else. Keep the connections tight, and you should be good to go.

motorbikes don't follow the same phasing as cars Re. emissions control cycles - for 2024 bikes now need to meet euro 5+ (which sounds like what cars had 15 years ago but likely tighter limits) - bikes until recently didn't need the post CAT O2 sensor, only the upstream one to tell the system how to behave - the post CAT sensor is to ensure the CAT is still operating efficiently (or throw teddies). So like cars, new bikes with Euro 5+ no longer just have the first sensor that should be making it run to std and just pretending its all good

the same article went on to say (with over fueling as per the Merc style - of I know I have a cyl down and I could easily switch it off, but I like to keep injecting fuel just to destroy things), a small misfire is enough to wreck the CAT as modern ones are super sensitive to CAT temperatures and a small amount of unburnt fuel nicely leads to severe overheating and a dead CAT



.

Killing a catalyst has always been easy via feeding it unburned air and fuel from a misfire. Seems even more so today with healthy cats delivering exhaust that is cleaner then the air ingested to make it run. “Improvements” since about the advent of OBD-II mostly cost a LOT more and make for painful failures. Direct injection comes to mind. Injectors that fail for full spray of fuel with engine on or off. I’m good with my old tech port injected junk that makes more than enough power.

I am having a similar issue and using this thread to plan my resolution. What is the "VT" that you're speaking of?

I just got a malfunction and codes show random misfire and misfires on 7 and 9 cylinders. Incidentally, this all happened right after I changed my starter battery because the previous battery failed. I want to make sure I change everything needed to resolve the issue without damaging new parts.

07 S65
32,000 miles, assuming original plugs and coils

The VT is a Voltage Transformer and it sits at the top of the engine (a rectangular box). It is responsible for raising the voltage from 12 volts. The two coil packs and the voltage transformer suffer from heat and eventually fail. They are considered maintenance parts and typically need to be changed after 50K miles. Symptoms of a bad VT are multiple misfires usually on the same side. Symptoms of a bad coil pack usually start with one cylinder and tend to creep into multiple misfires. You need to visit Clark Rupp's web site V12icpack.com to learn about this stuff.

If you haven't already done so, and you won't like what I am going to say, you need to replace both coil packs and the VT (Clark now has a replacement VT called a Boost Box which has a longer connector cord and allows you to move the VT Boost Box off of the top of the engine where temperatures are close to BBQ status). This is called a tune up on a V12 and you will need to do it every 50K miles. Expensive and not fun but after you will be really satisfied with the way it runs.

I now have 108K miles on my S600 and have replaced coil packs and VT twice and will need to consider it again later this year. Keep us posted.

Thank you for the detailed reply. I gave a quick look to Clark's website and will carve out some time to deep-dive into everything he has there.

Since the car has only 30k miles and we doubt it's the plugs and I'm learning the VT can hurt the coils, do you think it could be wise to simply replace the VT with the Boost Box FIRST and see if that resolves the issue, before I go deeper into the coils? So far I've only thrown a code once for misfires and the car has been parked, since.

If it was me, I would start with a new VT. The Boost Box is fuse protected so if it turns out you also have a bad or shorted coil, you will not have a fatal result. You said you had misfires on 7 and 9 which are both on the same side. I would also purchase a replacement coil for that side, because I suspect you have problems in both the VT and the coil pack. But you are closer to it than I am. And it is so easy for me to spend your money. Typically when you replace the coil and the VT, you are also supposed to replace the spark plugs also. All 24 of them (which might be a religious experience when you price an Iridium spark plug).

But I promise, once this is behind you, the car will run so nicely you wont even know the engine is on unless you look at the tachometer. That smooth!
Study Clark's stuff. You can also buy coil packs (OEM not Clark's improved refurbs) from FCP Euro which are also very expensive but supposedly have a lifetime warranty.

Keep us posted on what you do.

Also, just based on my own experience and since you said you had two misfires on the same side; sometimes the valve cover gasket on the misfire side leaks oil into the well of the spark plug causing the misfire. So if and when you pull off the coil pack on the misfire side, check for oil seepage.

Finally, re-read or search the posts on misfire especially the issues associated with the Y Pipe intake leak. This also happened to me. As the engine was put under load, a small crack between the Y Pipe and the adjustable clamp caused a vacuum leak popping a P0300 code. Tightening the clamp only worked for awhile since my Y Pipe actually had a hairline crack in it at the base (probably caused by some clown over tightening the clamp). Replaced the Y Pipe and code ceased.