Drain Plugs for 1999 Mercruiser 4.3 Thunderbolt 6 cyl

[tdturnquist2323]

I am getting mixed messages and info on the # of drain plugs on my 1999 Mercruiser Thunderbolt I/O 6 cylinder. There are two on each side but I have spent over an hour looking for something on the front and it is not there. Is it possible that the 1999 doesn't have a drain plug for water in the front? There are no "blue" plugs and nothing brass either. The boat is stored in cold weather so I need to get an answer quickly. I just bought this boat used want to get it winterized this weekend. Thanks for the help. I attached two pics...one is the engine spec and the other is the engine from the front.

 

[Fun Times]

Is it possible that the 1999 (0L095465) doesn't have a drain plug for water in the front? There are no "blue" plugs and nothing brass either.

Hard to see in the photo and your engine design/year model, past service work situations may vary some here 'But the parts catalog diagram suggests there'd be a drain plug mounted to / on the Big water hose near the bottom part of the "U" between the thermostat housing and circulating water pump as noted here in the following parts link as item number 24 - 27, https://www.mercruiserparts.com/bam/subassembly/31480/2694/150

If there, that would be the 5th plug at the front of the engine and not on the engine block and i believe in 1999 they were using a black plastic plug not the noted blue plug normally used today.

Some models used a vent plug on the thermostat housing which yours looks to be a brass plug item number 2 in the link above that should be pulled when doing a quick water drain vs pulling water hoses like you're doing...You may want to pull the lower part of the big hose at the "water pump" for better safety measures.

Take Note of the "quantity used" for each plug listed.

Exhaust Manifold(One Piece)

Exhaust Manifold(One Piece)

www.mercruiserparts.com

Cylinder Block And Camshaft

Cylinder Block And Camshaft

www.mercruiserparts.com

Standard Cooling System

Standard Cooling System

www.mercruiserparts.com

4.3L (GEN +) (2 BBL.) GM 262 V-6 1998 - 0L010044 THRU 0M614999

4.3L (GEN +) (2 BBL.) GM 262 V-6 1998 - 0L010044 THRU 0M614999

www.mercruiserparts.com

Just wondering, you adding antifreeze after water draining? < It's optional...

Good luck.

 

[Lou C]

Maybe it’s the square plug on the front of the intake manifold. It’s hard to see but would be behind the serp belt near those disconnected hoses on the left side of that pic.

 

[tank1949]

Most Chevy V8s and V6s have block drain plugs on each side below Exhaust Manifolds. Get a mirror, light and look! They maybe 3/8 NPT or have bleed-valves.

 

[tdturnquist2323]

Hard to see in the photo and your engine design/year model, past service work situations may vary some here 'But the parts catalog diagram suggests there'd be a drain plug mounted to / on the Big water hose near the bottom part of the "U" between the thermostat housing and circulating water pump as noted here in the following parts link as item number 24 - 27, https://www.mercruiserparts.com/bam/subassembly/31480/2694/150

If there, that would be the 5th plug at the front of the engine and not on the engine block and i believe in 1999 they were using a black plastic plug not the noted blue plug normally used today.

Some models used a vent plug on the thermostat housing which yours looks to be a brass plug item number 2 in the link above that should be pulled when doing a quick water drain vs pulling water hoses like you're doing...You may want to pull the lower part of the big hose at the "water pump" for better safety measures.

Take Note of the "quantity used" for each plug listed.

Exhaust Manifold(One Piece)

Exhaust Manifold(One Piece)

www.mercruiserparts.com

Cylinder Block And Camshaft

Cylinder Block And Camshaft

www.mercruiserparts.com

Standard Cooling System

Standard Cooling System

www.mercruiserparts.com

4.3L (GEN +) (2 BBL.) GM 262 V-6 1998 - 0L010044 THRU 0M614999

4.3L (GEN +) (2 BBL.) GM 262 V-6 1998 - 0L010044 THRU 0M614999

www.mercruiserparts.com

Just wondering, you adding antifreeze after water draining? < It's optional...

Good luck.

Thank you for your quick response and details. I am attaching another pic "close-up" of the square brass plug I believe you are referring to. Is this the 5th plug? If so, it's gonna be a ***** to get off. I really don't think it has ever been unscrewed so maybe it's not mandatory but I will attempt to get it off with a 6pt socket and pray I don't strip it. Also, yes, after I drain the water and put the plugs back in and fill all of the manifolds with the pink RV antifreeze until they are topped off.

Maybe it’s the square plug on the front of the intake manifold. It’s hard to see but would be behind the serp belt near those disconnected hoses on the left side of that pic.

is this the one?

 

[Lou C]

Thank you for your quick response and details. I am attaching another pic "close-up" of the square brass plug I believe you are referring to. Is this the 5th plug? If so, it's gonna be a ***** to get off. I really don't think it has ever been unscrewed so maybe it's not mandatory but I will attempt to get it off with a 6pt socket and pray I don't strip it. Also, yes, after I drain the water and put the plugs back in and fill all of the manifolds with the pink RV antifreeze until they are topped off.

is this the one?

Yes that’s it. There have been reports of the water passage under the thermostat housing cracking and that will put water right into the oil when you start up in the spring. I’m not sure why but they must have changed the casting when the change was made from the pre Vortec (up to ‘95) to the Vortec (96 and on). On my old pre Vortec all I have to do is pull the engine drains and disconnect the bottom end of the big hose from the thermostat housing to the front circulating pump. Never had a freeze up. So in your case I’d definitely try to pull that plug but I’d first pull the block drains & poke the holes, then disconnect the bottom end of that big hose as I said. Then try pulling that plug, if nothing comes out that’s good to know but if trapped water comes out that’s even better to know. BTW a cracked intake isn’t cheap to replace assuming you can to the labor yourself, approx $300 for a new one plus about 2/3 hrs labor if you are doing it for the first time.
On a different topic get rid of those one piece exhaust manifolds there were casting problems with them and they can fail unexpectedly and put water in cylinders. Barr Marine sells a conversion kit to replace them with the 2 piece units. Merc discontinued these because they couldn’t get consistent casting wall thickness.

 

[tdturnquist2323]

Yes that’s it. There have been reports of the water passage under the thermostat housing cracking and that will put water right into the oil when you start up in the spring. I’m not sure why but they must have changed the casting when the change was made from the pre Vortec (up to ‘95) to the Vortec (96 and on). On my old pre Vortec all I have to do is pull the engine drains and disconnect the bottom end of the big hose from the thermostat housing to the front circulating pump. Never had a freeze up. So in your case I’d definitely try to pull that plug but I’d first pull the block drains & poke the holes, then disconnect the bottom end of that big hose as I said. Then try pulling that plug, if nothing comes out that’s good to know but if trapped water comes out that’s even better to know. BTW a cracked intake isn’t cheap to replace assuming you can to the labor yourself, approx $300 for a new one plus about 2/3 hrs labor if you are doing it for the first time.
On a different topic get rid of those one piece exhaust manifolds there were casting problems with them and they can fail unexpectedly and put water in cylinders. Barr Marine sells a conversion kit to replace them with the 2 piece units. Merc discontinued these because they couldn’t get consistent casting wall thickness.

Thank you! Yes I have disconnected the big "U" hose and there is no water in anything that I have drained (I poked up into the 4 holes). But, I am pretty sure I won't be able to get that square plug loose and I know it's never been done. My mechanic told me to tilt the boat nose down while storing it so that any water in that area is less likely to freeze up. I will attempt to get it off anyways but I need to go buy a socket that will work (can you help me with that too?) Also, I'm not as technical as you...can you explain what you mean by replacing "the one piece exhaust manifolds" that can cause casting problems? I don't know what that is a reference to because I'm a rookie. And, I checked underneath the hoses and in the lower part of the engine there are not any plugs to be found. Unless I am blind its all metal down there with nothing that sticks out that is threaded.

Thank you so much for your help. I am including pics of my setup the way it looks right now.

 

[Lou C]

For the square plug, just get an adjustable wrench, don't need a socket really. Spray penetrationg oil first. The 4.3 and all the V8s all have a plug on each side of the block low down, you gotta find them.
Here's a pic of my old 4.3 the plug is in front of the starter......see the 90* drain fitting, they used this when the old big starters were in the way, not needed with the new smaller starters, that's the driver's side (starboard), on the other side (second pic) you can't see it (too dark) but its between the motor mount and the oil filter. That pic is from when I did my top end overhaul with new cyl heads etc.
On your engine the other thing I'd do is point the raw water intake hose down in the bilge to drain it and then hold it up and fill with -100 marine AF to push water out of the hose, and water passages in the transom mount and drive till the AF comes out the outdrive water intakes. Put the drive down first. This will put AF into the impeller housing as well.
To fill the engine with AF:
1) fill manifolds via the feed hoses that are disconnected at the thermostat housing till AF runs out the exhaust (why you MUST use no tox marine AF), then reconnect those hoses
2) to fill engine, reconnect bottom end of the big hose, disconnect top end at 'stat housing. Then fill the engine via the hose till AF spurts out of the stat housing. Then re-connect the hose.
I don't like -50, or -60 in my climate, they get hard at a few degrees over zero, not the best in a cast iron engine. I prefer -100 it stays liquid like to -40--50*. Now AF is optional, in fresh water you probably don't need it, I use it in salt to cut down on corrosion and Merc does say they advise the same.

Remember, to prevent freeze ups you MUST FULLY DRAIN
adding AF is ONLY for corrosion protection
and
NEVER use the suck AF Up the drive kits, they can result in a cracked engine, if the stat does not open fully on a cool fall day.

 

[tdturnquist2323]

For the square plug, just get an adjustable wrench, don't need a socket really. Spray penetrationg oil first. The 4.3 and all the V8s all have a plug on each side of the block low down, you gotta find them.
Here's a pic of my old 4.3 the plug is in front of the starter......see the 90* drain fitting, they used this when the old big starters were in the way, not needed with the new smaller starters, that's the driver's side (starboard), on the other side (second pic) you can't see it (too dark) but its between the motor mount and the oil filter. That pic is from when I did my top end overhaul with new cyl heads etc.
On your engine the other thing I'd do is point the raw water intake hose down in the bilge to drain it and then hold it up and fill with -100 marine AF to push water out of the hose, and water passages in the transom mount and drive till the AF comes out the outdrive water intakes. Put the drive down first. This will put AF into the impeller housing as well.
To fill the engine with AF:
1) fill manifolds via the feed hoses that are disconnected at the thermostat housing till AF runs out the exhaust (why you MUST use no tox marine AF), then reconnect those hoses
2) to fill engine, reconnect bottom end of the big hose, disconnect top end at 'stat housing. Then fill the engine via the hose till AF spurts out of the stat housing. Then re-connect the hose.
I don't like -50, or -60 in my climate, they get hard at a few degrees over zero, not the best in a cast iron engine. I prefer -100 it stays liquid like to -40--50*. Now AF is optional, in fresh water you probably don't need it, I use it in salt to cut down on corrosion and Merc does say they advise the same.

Remember, to prevent freeze ups you MUST FULLY DRAIN
adding AF is ONLY for corrosion protection
and
NEVER use the suck AF Up the drive kits, they can result in a cracked engine, if the stat does not open fully on a cool fall day.

Thanks for all of the info and additional tips! Again, I have already removed two drain plugs from each side so that takes care of finding the ones on each side of the block down low (right?)and will attempt to get that square one loose without cracking something.

Also, this is a fresh water boat used in Minnesota. So, you are saying that AF really isn't necessary because it's not a salt water boat but a recommended precaution against corrosion??

 

[tank1949]

Thank you for your quick response and details. I am attaching another pic "close-up" of the square brass plug I believe you are referring to. Is this the 5th plug? If so, it's gonna be a ***** to get off. I really don't think it has ever been unscrewed so maybe it's not mandatory but I will attempt to get it off with a 6pt socket and pray I don't strip it. Also, yes, after I drain the water and put the plugs back in and fill all of the manifolds with the pink RV antifreeze until they are topped off.

is this the one?

Unscrew and find out! Go back with sealant!

 

[Lou C]

That's what I think, but I know Mercruiser does now recommend it, it is really up to you. The main thing is get all the water OUT. There has been a debate about the benefit of antifreeze forever it seems, so its really up to the owner. BUT, I'd rather see you just drain, than use AF the wrong way, that can cost an engine (those suck the AF up the drive kits, no good on raw water cooling).
And remember you really SHOULD get rid of those one piece V6 manifolds. They are a ticking time bomb. If they crack, or rust through, you will have water in one or more cyls. Might think about getting it done over the winter season. Not cheap but engines cost a lot more!
I did the same conversion on my '88 OMC 4.3 that had OMC's version of the same design. I never had trouble with mine, just lucky I guess. But changed them every 5-7 years because you have to in salt water.

 

[nola mike]

For the square plug, just get an adjustable wrench, don't need a socket really. Spray penetrationg oil first.

I'm 99% sure this isn't necessary. The Merc manuals don't mention it, it doesn't look like the other drains, there's a *ton* of space in that cavity, nothing to break even if it freezes. And the water doesn't stay there when you pull the t stat hose. After pulling the hose that cavity has been completely empty the 3 or 4 times that I've tried it. Didn't pull that plug when I winterized last year

 

[Bt Doctur]

There are 2 plugs, one on each side of the motor just above the oil pan, if there are no drain plugs on the bottom of the manifolds simply remove the bottom hoses to drain.

 

[Bt Doctur]

around the 2:12 mark

 

[tdturnquist2323]

I'm 99% sure this isn't necessary. The Merc manuals don't mention it, it doesn't look like the other drains, there's a *ton* of space in that cavity, nothing to break even if it freezes. And the water doesn't stay there when you pull the t stat hose. After pulling the hose that cavity has been completely empty the 3 or 4 times that I've tried it. Didn't pull that plug when I winterized last year

Thanks for your input Nola Mike! This is what I wanted to hear! I can tell for a fact that square plug has never been unscrewed in this old boat's history. It is rusted on and if I crank on it I"m gonna break something and be back to square one with a whole new issue. At this point I've pulled all 4 drain plugs, released and drained all hoses and tomorrow I am going to fill those voids with marine antifreeze and book it for the winter.

 

[achris]

Man, I'm so glad I don't live in a freezy region! Middle of winter is some of the best fishing you can get. Coldest I've ever been out in is about 6 degrees. Brrrr! But the fish seem to love it!

Chris.......

 

[Lou C]

This is one of the reasons why I think all inboards should be closed cooled. The thousands of engines ruined in the great Texas freeze up this winter prove it. As far as breaking things I did a top engine overhaul on this engine after at least 15 years of salt water use and all the bolts came out not one broke and I had to use a 1/2” drive impact gun to get the head bolts out. But if others say you don’t need to remove that plug then they have experience to indicate that.

 

[achris]

This is one of the reasons why I think all inboards should be closed cooled.

Completely agree, but you get some cheap-a** manufacturer who'll take the dollar option (less dollars is better ) and that forces other manufacturers and suppliers to follow suit.

Case in point. Back in the 90s and 00s, Yamaha were offering outboards of the 'CV' flavour. One I was competing against all the time was the 90hp. It was a good $1,000 cheaper than my Mariners of the same horsepower. It wasn't until you found out what 'CV' meant. Yamaha called it 'customer value', I called it 'cheap version'. It was a standard 90hp with the oil injection, power trim and tilt, and one other thing that I have forgotten, removed. But as most customers shop on price only, we lost a lot of sales to them. I'm happy to say that Merc/Mariner never went down that particular path.

Chris..........

 

[Lou C]

The situation with ruined engines in places that don’t usually have winter (so owners never felt the need to drain) is so bad that one major engine supplier told me they don’t expect any new 4.3s maybe ever because GM doesn’t build the old cast iron version any longer apparently. So then all you can find are remans.

 

[achris]

The situation with ruined engines in places that don’t usually have winter ...

Cue the 'climate change deniers'.... (Idiots!)

We (in Perth, WA) are also experiencing something of a cold snap. Been as low as 5 degrees a few mornings. Very low for this time of the year... And we're expecting it to be more record highs this summer... We (humans) have really done a number on the planet's climate!