Zoso7

I have an 06' SL65 that kicked a check engine light upon startup after sitting for a month. I keep batteries charged, so no issues there. Anyway, the idle was smooth and it ran fine so continued on intending to pull the code later. A couple of engine starts later today and it was running rough. This is in Florida and the engine does get hot, even when parked in the shade like it was. So drove it home, put the scanner on it and I pulled P0300 with specific codes on cylinders 3,5,7,9 &11. So going by the numbering, this seems like it was something affecting both banks. I've owned the car 4 years (has 40k miles) and just before I bought it the owner replaced the right side coil pack and all spark plugs. I cleared the codes and it ran smooth again, but haven't tried driving it yet again under heavy throttle.

I'm thinking about starting with the "cheapest" part since it is a known wear item and that's the transformer since it is original. Based on those codes, does this seem like a reasonable approach? Since the transformer can cause pack issues, I thought I would start by knowing that component is good. I'm ready to replace at least the left coil pack, but thought I would give the transformer a go first.

tonylinc

Clark Rupp is the world's only and best rebuilder of coilpacks for the V12TT and when he rebuilds yours it's got better components than a factory new one. His website tells everything there is to know about these coilpacks, He will sell you a transformer to install first to see if that's the part with the problem, and if that's not the issue he will take it back minus I think about $50 "rental fee" for time and trouble. This way you can troubleshoot whether it's the transformer first, without laying out a big chunk of change. Clark is very highly regarded and reputable. He rebuilt my driver side coilpack a few years back i think the cost was around $699. Check for update on price.
PelicanParts sells them I think about $1100 each new. Suburban auto parts dot com has the left side coilpack (2751500780) for $933.
After replacing my left coilpack with rebuilt one the car's been perfect ever since. I replaced all 24 spark plugs while the one coilpack was away being rebuilt. Look on his website cause it has a ton of important things to know about your V12TT
If I recall it is V12 icpack dot com
Everyone who owns one of these V12's should know about Clark.
Find out if those cylinders are on the side of the engine that doesn't have the 'replaced" coilpack.
When my coilpack on one side of the engine quit you could feel the exhaust on the rear of the car and one side's warm, other side, Not.
This engine has 24 spark plugs and ONLY use the exact NGK that's specified, can buy direct from NGK on their own website!
I think those cylinder numbers you mentioned are all on one side of the engine, giving indication that your coilpack may be getting ready to quit, but in my experience it gave no prior notice, just quit one day.
If you do remove a coilpack to have replaced or rebuilt, you can hold the intercooler up outa your way on each side of the engine, using bungee cords from the hood. When putting a coilpack back on the engine use silicone grease on the rubber insulators so they'll slide on easier, and go slow take your time. Be gentle so you don't damage the coilpack! I did it so it can be done with patience. Be gentle on the coilpack removing so you don't break it and gentle going back on as well. Expensive parts!! LoL
Don't be in a hurry!
Best of Luck!!

Zoso7

Thanks, yeah I'm familiar with Clark and his website. Very smart guy and clearly knows whereof he speaks. I came very close to ordering his transformer, but on the SL, there just isn't a ton of room to put it. I guess it could go on top of the fuse box in the rear drivers side corner. In the interest of keeping it stock for resale when that day comes, I just ordered an OE unit from FCP. You can get them cheaper elsewhere, but I like their lifetime warranty, which makes it worth it. My codes were from cylinders on both banks and it started missing after a heat soak. I could still have a coil pack problem, but at least I'll be ditching the 17 year old transformer.

tonylinc

I don't recall the cylinder numbering system. Both sides, well that could be transformer maybe. Especially if it's ok one day then acts up the next, then okay again. Hope the transformer fixes it!

Billro

If the car has been sitting for a long period, you can get a CEL and diagnosis will show misfires on multiple cylinders. The problem is possibly bad gas; drive gently to burn most of it off and replace with fresh fuel and the CEL goes away.

Zoso7

Being a Florida boater, I know all about the shelf life of gas, especially when it has ethanol in it. That's not the case here, I do drive the car monthly and long enough to keep fresh 93 petrol mixed in the tank. Nice suggestion though, thx.

Abebundance

It does sound like coil packs to me, have you tried scanning the car yet? If so, what does it come back with, what are the codes? also... Central Florida, same here! Celebration, regularly at the shows.

Zoso7

Yep, scanned, misfires on both banks were recorded. I cleared codes and the engine runs smooth upon startup without misfire or check engine light. I'm waiting to drive it after I replace the voltage transformer later this week. No point in "testing" it until after I change that 17 year old heat soaked electronic part. If it's a pack or plug or insulator, I'll deal with that, but the transformer is an easy enough place to start especially considering the age.

(down in Winter Haven)

Zoso7

Update - I received a new MB voltage transformer from FCP Euro and after clearing codes and disconnecting the consumer battery, I swapped it out with my old one. I took great care with the engine harness connector, but otherwise this was a simple plug and play. The engine started and ran smoothly, I drove for a warmup, shut it down and let it heat soak in the sun. Whereas before after a heat soak it would misfire, now it runs smooth. I ran it up to 100 mph in several foot in it runs under load and it was a smooth rocket.

So it appears that for me, the voltage transformer was my issue. It was old and with the ill advised mounting location, the original transformer finally became toast. I like the idea of Clark's "Boost Box" and it's remote mounting location, but replacing the original is simple enough and FCP has me covered on any replacements from now on.