Overheated this weekend - 0 compression in 5 and 7

The vitals:
2006 Launch 21V
Indmar Assault 325 - non-Cat (I believe) with half system
1890 hrs
0 Compression is cylinders 5 and 7.
All 8 spark plugs have some sort of dry black "dust" on them. Wipes off pretty easily, but comes back after running for just a couple of minutes.

What happened:
A couple weeks ago I replaced my alternator, I checked the belt to make sure it looked fine, no fraying or anything. Everything checked out and I installed the belt. I couldn't seem to get the belt tight so my friends and I got the bright idea to use a ratchet strap to tension the alternator. This past weekend I took the boat out, we were cruising a long just fine, I brought the boat down to an idle for a few minutes because I wasn't familiar with the water around me. I look down at my temp after I've been idling and I see the needle pegged. I shut the motor down and start checking everything. Eventually I see the alt belt in the back just laying their with teeth missing and split. I had purchased an extra belt, just in case (I had a gut feeling). I put the new belt on at the dock (after getting a tow back to the dock, and letting the motor cool down for a couple hours). I cranked the boat up and everything seemed good.

I took the boat out the next morning and after about an hour I started to notice white smoke coming from the exhaust. Got back to the dock and checked the oil, it looked clean, but had definitely burned some (it was full when I started the weekend, it was at half way on the dipstick). I checked for leaking around the manifolds, nothing there. Coolant levels looked good, temperature looked fine, oil pressure was starting to dip, but I assumed that was because of the decreasing oil level. So I decided it was time for a compression test. I pulled all of the plugs out and they were all black on the end (I'll take a picture in a couple hours). While I had them pulled, I had the motor cranked to make sure it didn't spit out any water or coolant, nothing came out.

I changed the plugs and cranked the motor up and waited for it to warm up. I gave it a little gas once it warmed up (around 2500 rpm for 10 sec), brought it back to idle, then shut it down and looked at the plugs. Black again. Back to the compression test. Port side (2, 4, 6, 8 ) were around 160, with 8 being 155. Starboard side was 1: 180, 3: 120, 5: 0, 7: 0. So, sounds like I've either warped my head or wrecked my gasket on the starboard side. I have a few questions though.

Questions:
1) has anyone replaced the head gasket on one of these Indmar motors? If so, any tips or things I should no before I dive in head first?
2) I don't understand why all 8 of my plugs were black on the end even after replacing them! Any ideas?
3) Should I replace both gaskets while I'm doing this work, or should I leave well enough alone on the port side?
4) Is there anything else that I have missed when checking? Anything I should check when I pull the heads off?

At 1890 hours once I had the heads off I would have the valves done and the machine shop will check to make sure the heads haven't warped. I would get or you can probably down load a manual on the engine. Have you removed heads before? Familiar with internal engine work? Make sure there is no damage to the pistons ect. There will be a sequence for torquing the head bolts. Unless I am missing something generally most spark plugs have some black carbon on them from the ignition process. Don't recall ever seeing a perfectly clean spark plug upon removal.

I've never done any internal engine work before, but I'm not afraid to get my hands dirty. I was thinking about getting the valves done as well, and definitely going to have someone check the head for warping and cracks. I found an engine manual that seems to show what to do (https://www.bakesonline.com/images/M...epairSpecs.pdf). I do have a friend and his dad that have done this kind of work before (his dad was a mechanic in his younger years).

Will I or should I pull the engine for this kind of work? I'll snag that photo of the plug while I'm at work, I'm used to seeing a golden brown, this was like a pitch black almost powder covering the entire end that was in the cylinder.

The blCk powder is likely just carbon dust from unburned fuel now that you've lost 2 cylinders.
First off, do or have someone do a leak-down check on all 8. That's gonna t ol you way more than a compression check. You're problems are way beyond a bad head gasket unfortunately. If you can crank and run the engine with no water leaks, the head gasket is fine.
If you got the motor hot enough twice to burn up some of your oil, you likely burned either a couple valves or most likely trashed some piston rings which are now in little tiny pieces down in the oil pan since you said you saw smoke (I'm guessing blue smoke - oil) in the exhaust.
The leak down test will hopefully tell you where the issue is. When you shove the air in the cylinder if you hear it coming of the crank case vent (PCV) your rings are toast. If you hear it coming out the throttlebody/carb, your valves in that cylinder are trashed.
Good luck with it all, but hey, motor work is fun! Put ya some dome top Pistons in if you have to tear it down and get a couple more ponies outta the ol girl. These engines are nothing more than old school Chevy 350's. There's all sorts of parts out ther to help her out!
If you need any help or have questions feel free to pm me.

I'll look into the leak down procedure. The smoke was definitely white. I've seen the blue smoke from a car before, but this was definitely white.

The attached picture is the plug from number 7. It's the only one that had crud around the electrode, the rest of them had the same black powder though. (the little rust spots were not originally there, those have developed since it has been out of the boat).
Attachment 16550

Interestingly, the motor does sound alright when it's running. No excessive noises. A bit of RPM shifting around the 3200RPM mark if you leave the throttle alone (shifts up and down about 50RPM as per Perfect Pass). I do notice there was a loss of power at take off, it seems to take forever to get up to speed and plain (or plane, I can't remember the correct grammar right now) off.

You will have to pull the heads to see how bad it is. Common overheat issues include:

Stuck valves, cracked head, melted Pistons, stuck rings, cracked block.

Any or all of these definitely occurred to get zero compression in 2 holes. No matter what, the intake and heads have to come off to see what you are facing. A leak down check will give you an idea where the compression is going...but there is nothing that sends compression to zero that can be fixed with the heads on the motor. You might as well pull them and get a head start.

Also, do not get your hopes up. This is a serious failure that very well could leave the motor an anchor. I have never seen a motor with a serious overheat run right afterward without a full rebuild.

Sorry...

That's sucks, but I'll figure it out. I'm only able to work on it during the weekend, so progress will be slow, but I'll definitely keep the thread updated. I'm going to perform a leak down test on Sunday and begin disassembly.

I have a slight issue with storing the boat, I am unable to keep it in an enclosed area, it will be sitting in the driveway. My plan is to spray fogging oil on any exposed components and wrap the engine in a plastic sheet. Anyone see a problem with this or have other suggestions?

Also, any idea why the plugs in the "good side" are covered in the black powder?


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The plugs likely suffered from pre-ignition and oil burning (picture of it here) http://www.motoredbikes.com/showthre...539-Detonation which was likely caused from the overheating. Unfortunately CJD is pretty much spot on and this will most likely a full rebuild since the pistons likely (guaranteed) scored the cylinders, and your valves probably stuck, if not bent. Michigan Motorz has great prices (2,300 last time i looked) on long blocks which can be cheaper than a rebuild

you cant do a leak down with blown head gaskets , look there is alot you can do and alot you can spend .. how long was the engine hot ?

if we are talking about 3-5 min then put some gaskets on it and check the valves ,

more than 10 min , put gasket on do a leak down , rebuild and deck heads ARP studs

i blew my head gaskets out once under way , i never stopped it reached 220 degrees , my heads were fine ..

.2cents.

Before tearing it all down just do a simple test. Put a couple of teaspoons in the cylinder that is at 0 compression then do a compression test. if the compression comes up then it's your rings, piston or block. if it doesn't then your in luck and it is probably your head or gasket. Then just to make sure it is just your heads have them sent off for a test. Hopefully you will get luck like i did and it is just a vortec head. 400.00 from napa and very simple to install. It's just torqueing in the right order. Start in the center and spiral outwards from there.
Best of luck