Logo
FULL SITE MENU

1996 7.4 Mercruiser 2 spark plug holes bridged over

D

[email protected]

Member
I was winterizing my 7.4, and 2 of the 8 plugs had a sandy/powdery yellowish dry substance on them, it was the back last 2 plugs on the port side. When I looked in the holes, said substance is completely bridged over the holes, (where the spark plug electrodes protrude into the combustion chamber. A few years back, I had a shady mechanic, who after doing a tune up, I mysteriously had water in the oil, it was foaming everywhere, he, at the time said I needed a new motor, but I refused, got the boat back, did a flush, changed the oil twice, replaced plugs, etc. The oil looks great. Could this be some sort of residue from way back? The past couple years the boat did have issues at higher rpms, but we mostly prefer to cruise at slower speeds anyhow, and this is apparently the smoking gun. I also put in a marine thermostat, the mechanic had installed a high performance one. My boat uses raw fresh water for cooling. Would it be safe for me to use a tiny attachment on my shop vac and suck this stuff out, then fog the engine real good, maybe squirt some mystery oil in those 2 cylinders, replace all the plugs and cross my fingers? Any advice would be greatly appreciated!! Happy Holidays​



​
 
I took a wire brush to them all, with the intention to do a final winter ization then replace them with new plugs, darn it!! Thanks for the reply. Someone else suggested possibly a head gasket leak at adjacent cylinders?
 
Remove all spark plugs and do a compression test. record results. If you have high hours on engine, expect to see ~140 psi per cylinder.

All should be withing ~10% of each other.

Without actually seeing the spark plugs with the residue it is hard to form an accurate opinion.

Typically if you have water getting into the combustion chamber the spark plug tips would be exceptionally clean as the water would act as a carbon cleaner.

Your observations may not be accurate so it is difficult to make a good observation and possible issue.

Did find this in manual


aaa.jpg
 
Thank you. I will try to take a picture of the spark plug hole, you can see the buildup, Its so bad, I don't believe those 2 plugs could even fire like that. All the other 6 plugs looked normal
 
I know this has been over 6 months, but I unfortunately have been too busy to do anything with the boat, but was about to try and get her ready to use, but here is a picture I managed to take. I was attempting to spray fogging oil into each cylinder, and the last 2 on the port side in the back the oil just dripped back out, and upon further investigation, there is a substance in each cylinder, and it is like semi solid, and in my photos, you can even see where the electrode of the spark plugs were setting. The plugs were not burned or anything, but I was not able to get a photo of the spark plugs These are the spark plug holes. The boat did have issues running beyond like a 10 mph idle the last couple seasons used, so this was obviously the issue, do you think I need to tear down to the cylinder? can this stuff be vacuumed out?

kghost said:
Remove all spark plugs and do a compression test. record results. If you have high hours on engine, expect to see ~140 psi per cylinder.

All should be withing ~10% of each other.

Without actually seeing the spark plugs with the residue it is hard to form an accurate opinion.

Typically if you have water getting into the combustion chamber the spark plug tips would be exceptionally clean as the water would act as a carbon cleaner.

Your observations may not be accurate so it is difficult to make a good observation and possible issue.

Did find this in manual


View attachment 28159
Click to expand...
 

Attachments

  • thumbnail (12).jpeg
    thumbnail (12).jpeg
    111 KB · Views: 46
That is a roughly $15,000-20,000 photo. Perhaps you can sell off this boat as a "mechanic's special" and get some money out of it. Or you can continue to run it as you have with the knowledge that it's maybe running on 6 cylinders and that it is either going to seize up or otherwise conk out with no warning. If you're just toddling around a lake I guess that's not too dangerous. You can always call a tow and that'll be the end of that boat.
 
Last edited:
Some times people use a short plug in place of the long reach one . Most Vortec motors use the extended reach plug such as mr43LTS or ngk 6bEFS . You cant bridge the hole because the tip protrudes into the cylinder.
 
The motor has been well maintained as far as oil changes, etc. I bought it about 6 years ago, and replaced all the plugs, cap, rotor, wires. Even now, the oil looks like new, it has never used any oil at all. It just seemed for several years to only want to run in safe mode, so I just took it easy on her since we enjoy being out on the water, then I discovered this when I was winterizing, was trying to figure out what someone thinks caused this. I understand the initial though is poor maintenance, but I was tinking possibly the had gasket has an issue just between those 2 cylinders maybe? It's a once through cooling system, so, there is no radiator, so no smoking gun of contaminated antifreeze. The oil is clean, so I would not suspect a cracked block
 
The 2 cylinders have something in there. If you look at the picture, that is a close up of where the plugs screw into the cylinder. You can even see the indent of the electrode from the plug. It was very hard to see this, I only discovered it during winterization, I was fogging each cylinder and saw those 2 it was just dripping back out, so I used a mirror to investigate further. This large engine in my boat allows very limited work room, it's tight
 
Another thing that comes to mind, I replaced the Thermostat at one point, and autozone recommended a high performance one, I believe it was like a 180 degree one, and I ran that one for a couple years then a boat shop informed me that I needed one like 120 degrees for that boat, so could that have caused issues?
 
You're asking for some magic bullet that will fix it. Engines don't foul like this under ordinary circumstances. SOMETHING is quite wrong. You don't believe the tech when he tells you, and you don't believe us when we tell you so where does that leave you? You say the block isn't cracked but you've offered no solid diagnostics to confirm this. There are tests that can and must be done to determine whether there is any point in proceeding with repairs or if it's time to replace.
Marine engines need oil changes at minimum twice per year...when you put it away for the winter and when you recommission in the spring. Winterizing you either drain it completely or thoroughly fill with strong mix of antifreeze. Cap rotor wires and plugs should be annual or at least biannual. Filters and belts naturally when you do the oil changes. Impeller on the water pump and lube of the drive if so fitted. Thorough check of electrical system and controls. If you let any of this go for a couple years it'll def bite you in the butt. The fact that you're shopping at autozone for marine parts is concerning.
#drainplugmafia
 
Last edited:
If you want my diagnosis, at some time in the past the engine was full of water. Someone tried to revive it by simply draining the water and running it with oil changes. Then it sat. Who knows how long, but long enough. The crud you see is a collection of jank that was in the bore. I guess those last plugs didn't get pulled out for one reason or another and whatever was in the stayed in there. Over the course of your ownership, the engine has been running on only 6, if that. It got warm enough to cook all that crap into a nice chocolate cake due to your way too hot thermostat.
Maybe you can restore the performance by pulling the cylinder heads off, cleaning up the mess and putting it back together with new gaskets and such. However, if the results of a pressure test, a leak-down test and a compression check show that the lump is too far gone, then you're back to shopping for another boat or engine.
 
Thank you very much! And I learned not to put car parts in it for sure! Your thoughts are in line with mine, I was thinking the same thing, remove the heads and replace the gaskets after clean up. If the block were cracked I would likely have water in the oil too, right? There was an issue it got water in the engine and we determined it was where I had ran a couple hours in reverse after my shift cable got ate up by the impeller and was stuck in reverse. And I did have someone do all the above, and you may well be right, they probably didn't take the last 2 plugs out. I don't think I know more than a mechanic, but I quit using the one I was using after I realized he subbed everything out besides routine oil changes and such. He always complained about doing anything to it because the motor was so big and hard to get to anything, (I agree). I am between work gigs right now, thus why I was reaching out to get advice, rather than just taking it to a reputable repair shop. I will go with and agree you know what you are seeing, only thing I was hoping, was someone would say they make a tiny sweeper fitting to clean something like that out, lol. Thank You sir!!
 
I'm glad that I noticed my fogging oil wasn't going anywhere, and investigated further. You can't just stick your head down in there and see what is in my picture. Hopefully if that is the issue, which there's been something wrong I knew, I didn't cause any damage putting around on 6 cylinders
 
You don't say what boat but some of them have a way of removing the entire cockpit deck as one piece (after you undo a bunch of fittings) which allows for removing the engine. Since you're already looking at being down for the remainder of this season, maybe get the motor out and on a stand where you can work on it at your leisure under cover or in (better yet) an air conditioned garage. Give it a thorough going over while you have the heads out to the shop for reconditioning. New circulating pump, hoses, top gaskets, crank seals, etc. Then you won't have to worry that something else will go wrong, at least for a while. You want all 350-odd horses working for you, not munching hay.
 
For your information,
DO NIT GO TO AUTOZONE FOR MARINE PARTS other than spark plugs and oil/filters.

If your boat 1996 454 is Raw water cooled the thermostat should be 143* per MERCRUISER MANUAL.

IF it is Closed cooling with heat exchanger,
143* with Thunderbolt IV ignition
160* with Thunderbolt V ignition.

There are very few if any marine engines that have ever used a 180* thermostat.

The cold temp set point is to keep the salt in salt water in suspension.

If the water temp gets to hot the salt seperates and sticks to the cast iron.
(Basic explanation)

You really should get a service manual.
 
I'd be a bit skeptical on being able to salvage the engine...if it is mineral buildup in the back cylinders, and its been there a while, its likely to have fallen into the bore and scarred it up...you might get lucky...you may not. until you get the head off, its all speculation...
 
Thanks for the feedback! And I never ran the 180 thermostat in salt water, and mine is a raw water system. I didn't have the high performance thermostat in there for very long. I would not even recommend saying it's safe to use plugs from auto zone, if they aren't marine plugs/stainless, they will sill sell them to someone unbeknownst and if the boat sits for long at some point they will rust into the head and break off. I've also read stories where people use automotive starters in a boat and they cause explosions
 
It's a rinker 285 fiesta vee cabin cruiser. I know the motor has been out since I owned it, I was just not part of the process, I just wrote a check, but I think it comes out of the hole. I know it's a tight space. If you drop anything at all, its gone to the black hole. You almost have to pull the engine to even get to the oil drain plug. And I sure hate the idea of being down until next year!! Thanks for your input
 
Yeh it's a bugger not to have the boat in working order. Express style cruisers tend to put the premium on plushness and not so much on the mundane aspects like being able to get to the back of the engine or check on a seacock. Who cares about that stuff when it's time to PAR-TAYYYY!!!
To paraphrase a saying among us Land Rover folks, Rinker Boats: Turning owners into mechanics since 1945. I guess your choices are getting narrowed down rapidly here. You can throw yourself on the mercy of some shop or you can roll up your sleeves and get about the business of doing this yourself. I think you should strongly consider the purchase of a new engine. That or look for a good used running take out. Thing is, you're standing at the edge of a very steep slope with no visible bottom with the current engine. You could put many tens of hours into removing and reconditioning a lump that is more or less one foot in the grave. A reality-based cost benefit analysis will likely put you in the same spend-o-rama as just replacing it. Do some diligent research, ask around at the local shops and marinas for a possible runner.
Last but not least go see Mr Mercruiser about a complete repower package that you can finance over the next ten million years. The new engines and drives run better, cleaner, have more power, less maintenance, more reliable, blah blah blah. You may find that the Mercruiser repower is the best deal over the course of your ownership. That way, you sign a piece of paper and the rest is on someone else. Then you just turn the key and go and if there's a problem someone comes out and fixes it.

PS Just checked. Mercruiser Repower financing is 10 percent down with 144 months, so it can be very attractive for that 8.2 and Bravo 3 package.
 
Last edited:
I would at least like to take the head off and see what's in there. is that a huge task? Would like to see if there is any scarring in the cylinder. I have all the tools and mechanical skills, but being I would have to follow a manual,since I have never done this before, how many hours would you think it would take for a novice to get down to the evidence? I can't find ANY videos on you tube of this task. And I have spent a lot of time putting around on this old lady and I think she is fine still, just my gut feeling, and I am a pessimist.
That old mechanic I used to use didn't take those 2 plugs out. I quit using him once he told me compression checks don't tell you anything lol
 
Well I’ve done it on a much smaller engine & you need good access to do this with the engine in the boat. I had to use a 1/2” impact gun to get the cyl head bolts out (raw water cooled in salt water). They all came out without breaking. The challenges in this job:
label wiring & keep parts organized
cleaning parts that will be re used, I cleaned the deck surface of the block several times & cleaned out the cyl head bolt holes with a thread chaser (raw water cooling they will be crudded up) also use carb cleaner a gun brush & compressed air.
I hope you have a lot of physical strength in your hands arms & back, or a helper. Those cast iron big block heads are HEAVY! Might be a job for 2 people and if you drop one end on your foot it will be an ER visit. The ones for my 4.3 are only like 40 lbs each for the BB Chevy they could be 2x that.
I put it back together with a pair of reman cyl heads, Fel Pro gaskets & ARP cyl head bolts (originals were very rusted). Still running well 5 years later.
 
A lot!
First you figure out how you will organize the parts and label all the wiring & take photos. You will have to remove first:
carb/EFI linkage
alternator
bolts for power steering pump
carb/EFI hoses
carb/EFI unit
intake manifold
exhaust manifolds
THEN
you get to remove those heavy cyl heads.
On mine the intake manifold gaskets were baked on & it would not come off. Took me a couple days to figure out how to get it off without damaging the sealing surfaces. The actual removal of the heads took like an hr. Cleaning parts, couple hrs. Figure 2 days if you have all the tools & a helper…
 
Before you do anything, you need to pressure test the block, have a leak-down test and a compression check. If it passes all that, then move forward with pulling heads.
 
Back
Top