I took off the flame arrestor. There was a film, but nothing hard or crusty. The flame arrestor seemed pretty dirty and there is a sensor in it (MAT Sensor) so I don't know if it is cleanable like the one on my 5.0GXI was.

Couple of thoughts---

buy "throttle body cleaner" instead of carb cleaner. Does the same thing but leaves less residue and evaporates MUCH faster.

--about the 'valve monster' (I like that), aka the IAC (idle air control) valve--if you're TB is so dirty you need cleaner, it's possible that the IAC itself is sticking. They can be removed and cleaned, but I'd suggest a repair manual...

And remember our thread on dirty MAF sensors? That could cause many of your symptoms...

I will have to look into removing and cleaning. I have the EFI manual already and will hopefully have the engine maunal by next week.

I don't see a MAF sensor on the schematic.

Those sensors and valves are expensive. Makes me a little scared I might mess them up. Even the o-ring to take the IAC off is almost $14.

There are no MAF sensors on I/O boats, only MAP sensors.

SB, the amount of coking that causes problems is very small, and what you cleaned sounds like the normal build-up. I am not familiar with the flame arrestor on the 8.1L, but if it has a MAT sensor, can't you move it? I clean my flame arrestor in mineral spirits (parts washer). I don't think mineral spirits will hurt the sensor, but you have to be careful not to physically damage it, obviously.

GM IAC valves very rarely cause problems. Ford, on the other hand, has had problems with theirs on cars and trucks since, oh, the beginning of time.

I have cleaned probably over 1000 throttle bodies since the 80's using regular old spray carb clean. It works just fine.

SB, how many hours are on Baby Blue, and what type of spark plugs are in it (Delco Double Platnium or the old Delco MRxxyy)?
Did FLB ever have them changed?

I'm actually going to check the part # for the plugs tonight to see what they are. I did some research about a month ago and I found only one number and it was not a Delco #. I'm hoping these have a good number to cross-reference off of.

I tugged on the MAT sensor which is a temp sensor I guess. It will spin, but it doesn't come out. I will try to get a mirror and see if there is a clip on the inside. The flame arrestor is a wierd design geometrically so there is not much access.

Baby Blue has about 330 hours I believe off the top of my head. I don't know if they are the original plugs. FLB might have told me, but I don't remember at this point.

Glad to hear the GM IAC valves cause few problems. I think I will not touch it for now, but I will order the gasket so I have it if I need it in the future.

I happened to have Throttle Body Cleaner already so I used that. I only used a rag to prevent getting any in there. I did not spray any in the intake at all. I just sprayed it on the blue cotton paper towels and wiped out as far as I could reach in with my fingers.

I don't have the energy to deal with mineral spirits, but I think I will use some water and simple green to see if I can get it cleaner using a toothe brush from the outside. If I mess it up I have a new one on order already (almost $100)

My biggest problem is our longest boat trip of the year is next week and I don't want to have issues on the lake, nor do I want to cause damage by not fixing something. I'm up against a wall and we don't have local dealers that stock parts. Everything has to be ordered in.

Keep the questions and progress reports coming. I'll do my best to help, but it's tough to diagnose over the internet.

Thanks for all your help. Hopefully I can take it out on Saturday and see how she runs after checking over everything. I think I will be ordering the data logging system tomorrow and it will be here by Friday. It would be nice if they put a check engine light in the boat. At least you would know if a code was present.

May have found the (a) problem.

Actually found 3 issues while digging around the motor tonight. I don't know if they are the cause of the issues.

1.) The spark plugs were installed in May of last year. They are RS12PYP which is the same plug that my 5.0GXI used and I thought the plugs were different plugs. So, perhaps the wrong plugs. Perhaps not.

2.) One of the plugs was not even finger tight. There was not much soot around the plug so it doesn't seem to have been leaking bad, but it definitely could have caused some issues.

3.) Whoever did the wiring on the motor was having a bad day. The harness is not in the proper position creating an issue with one of the plugs on the ECM. First, the plug is pulled to the side which could be a problem. Bigger than that is that because of this, the insulation was damaged on at least one wire.

4.) There was something else I found, but I cannot remember at the moment.

So, I need to determine the right plug. If these are wrong change them for the right ones and retorque all of them. Take apart some of the hoses on the engine and reposition the wiring harness. I also need to seal those wires with liquid electric tape to ensure there is no moisture or corrosion issues.

The good news is that through this inspection I found the serpentine belt to be just about worn out. So I have a new one on order so I can change that out.

B.O.A.T.?

More like 1/2 B.O.A.T right now. But it could go to full B.O.A.R. Hopefully not.

I didn't know EFI boats DIDN'T use MAF sensors, good to know!

It is good to know.

It will be a busy weekend as I try to get it fixed and get it back on the water by the end of the weekend.

DING DING DING! I think we have a winner!!!! Nice find!

If this is let go for too long, then the threads in the cylinder will be junk because the escaping combustion acts like a cutting torch.

It seemed to thread in fine. Hopefully no damage was done, though a repower could be fun.