Hi all,

Any idea what could have caused a head gasket failure in a post-headbolts update m156?

I received the car with 17k miles and it’s never been overheated by me.

it appears to be the passenger side, out of fear I am wanting to do both sides because I have no idea why the gasket failed in the first place. What are y’all’s thoughts on that?

as always, thanks for any input

New bolts weren't torqued properly.

safe to assume the other bank might not be torqued properly too?

Yes, fortunately you shouldn’t have to worry about the bolts snapping on you, so you be able to get it with the engine still in the car.

Maybe, who knows what the previous "mechanic" did. It will be cheaper to have both heads removed and gaskets replaced at the same time rather than doing one side now and the other later on.

this is a factory assembled motor. I meant it was a post headbolt update vin. Sorry for the confusion.

So what exactly happened?

Aware of the head gasket failure but there is a little bit of detail missing like who owned it before you, when/how you drive the car, when did you notice something was wrong?

This helps when trying to figure out a problem where you dont have the car in front of you.

Sorry, I have a few threads but here's the summary leading up to Aug 26th:
- I have had this car for around 2 years. I actually got it with 27k (has 40k now), the previous owner got it with 17k and he had it for 5years before me. Well maintained, great service records, and looked after, and responsibly driven, but driven like you would want to drive an AMG. According to the VIN it is after the headbolt cutoff.
- For whatever reason when I got a new tune from 86 motorsports (another shout out to Barry who is/has been ridiculously helpful during this saga) I would get a very random cold start misfire. It would misfire for about 5 seconds, I would shut the car down and immediately restart and everything was fine. Sometimes I would get a lot of misfire, sometimes it would be a few. It was always on bank 2 and was always random cylinders, never the same ones.
- Replaced/repair/inspected EVERYTHING you could think of (rebuilt the intake manifold/plugs/coils/all vaccum lines/o2 sensors/exhaust manifold gaskets/etc.) and the random cold start misfire would still happen
- During this time, never once got a low coolant light or had smoke during startup. Never once overheated or showed signs of an issue. I did a 2 day track day at road atlanta in June, in 90* heat and it rain flawlessly.
- Every data log just showed strange o2 readings and random misfires on random cylinders, and that one bank always called for more fuel than the others. Barry could not see any vacuum leak in the fueling corrections.

On Aug 25th I drove to Atlanta for a concert. It was a hot day but the car never once got above 195* coolant temp. After the concert I crank up the car and there is a lot of white smoke out the exhaust...it smokes for 30min while I wait in the parking garage to be able to get out. It eventually clears.

Crank up on the 26th, no smoke but I do have a low coolant light. I add some coolant and drive back 4hrs at 80mph in 95* temp and oil/coolant are rock solid the whole time.

Since then, it runs incredibly rough, smokes on start and idle, and I had a oil/water film in my catch can that has become more watery as of late. And my misfires are all now on bank 1.

I just tested the coolant bottle with one of these style: https://www.testequipmentdepot.com/m...gaAm1CEALw_wcB and it shows engine gases in the coolant.

So I clearly need a head gasket somewhere, or I have a cracked head/block....which seems even less likely but given my frustration with this car, may be possible. I plan on scoping the cylinders and doing a compression test to see if I can determine which side.

A leak down test might be more valuable to you, (and more revealing), in this situation. I hope you nail it down.

For sure. That is on the agenda as well. Is it acceptable to use the cam alignment tool to lock down the engine for leak down testing?

Not knowing what caused it makes me want to do both sides.

That's a good question. I don't remember us ever having to lock it in place, but now I want to check. I don't think the rotating assembly moves because you do not need to use a lot of pressure. You are only after the percentage of pressure lost, (and from where). I will ask one of my guys..

You sure it's updated bolts since it's a 2011? What's your VIN?

I doubled checked today. It’s engine number 61,580