Mercury BigFoot 50 Overheating (long)

[Dimitri-2000X-Tampa]

The latest status is that I dropped my foot again, totally rebuilt the water pump..again...with all new parts. Did a lot more flushing of various passageways, reamed with weed wacker line, etc. I also pulled the water jacket cover and found it pretty clean. This taught me a lot about how the water goes through the manifold and the head though.

One interesting thing when I was flushing today. Out one of the little exhaust passage ways, what looked to be a few small pieces of magnetic tape (like what you would pull out of a cassette tape) was stuck in the hole so I pulled them out...the tape was rather brittle. To come out the exhaust hole means the foreign matter would have had to go all the way through the motor's cooling system. Anyway, does not seem to be running any different than the last time I did this (after I got the water flow back up) so I have a lot of good spare water pump parts now.

I have a boat mechanic who lives in the neighborhood. I got his attention the other day and he seems to think that my tell tale hole is letting out too much water pressure and that it should be plugged when the motor is running. That's funny...I've used the motor almost 3 years with it pee-ing happily out that hole. Anyone else have a 2000 (or older) Merc BigFoot 50 which has a covered up p hole (where the hose threads are)?

Anyway, he sold me a one way valve that closes off the flow. I had actually argued this point with the guy at the Merc shop last week. The fact that if the P hole starts getting too large (from being reamed and broken off, etc.) whether it might start starving the top cylinder for cooling water. The Merc guy shrugged it off as nonsense. Funny thing is that today, after I got everything back together, I ran with the muffs for about a half an hour and experimented with a meat thermometer. With the p-hole water blocked off, the area near cylinder 1 (top) on the head was a good 15 degrees cooler, same with the area around the overheat sensor on the water jacket. It was pretty consistent. Open the p hole and you can see the pressure slowly drop in the stream. When the pressure drops, the top cylinder starts getting hotter. Plug the p hole up and the temp starts dropping immediately. I did it back and forth about 3 times to make sure that it was a correct trend. The exhaust water stays about the same temp...around 97 degrees. But if you cover the p hole, it goes up to about 101 for several seconds and then cools back down to 97..as if it had just carried the extra heat away from the top cylinder which was getting water starved. The overheat sensor is near the top cylinder of course.

I asked the neighbor mechanic..why have I been able to run fine for almost 3 years? He seems to think I was just on the verge of overheating but staying just under the warning horn. This explanation makes me a bit skeptical though.

Anyway, I'll try to put it back in the water this week to test under load. Obviously, running with the muffs is no load and even revving it up a bit, I could not get the overheat alarm to go off. So whats the deal, anyone think I could do any damage by blocking the p hole off? About the only thing I can think of is if there might be too much pressure build up at higher rpms. I'll take my new knowledge (and my one way valve) out in the bay and see what happens. Whether I can make it overheat anymore with the regular p stream (which is about as much as the 1/4 npt size water dispensor stream from my fridge's exterior ice and water dispensor) and if so, whether stopping off the p-stream makes the overheat warning go off.

This is strange stuff but the plumbing is pretty straight forward really. Another interesting thing is that there is an exhaust bypass hole right under the p hole. With the p hole open, no water ever comes out of that. But with the p hole blocked off, water spits out at times. Like before the thermostat has opened, it seemed to spit out quite a bit but after the engine heated up more, no more came out of there (presumably because the thermostat is now open). I'll have to find someone else with the same motor and see what it looks like. If my p-stream has gotten a bit too wide, then it could be like a pressure cooker without the weight on top. The weight is just enough to keep the pressure right. Pressure definitely drops when I unplug the hole, you can see the pee distance decreasing slowly as the water must leave the top cylinder but still have enough pressure to cool the other 2-3 lower cylinders. Anyone ever make similar observations?

[craiglaforce]

SInce you have gone to all this effort, you might want to also post your question on iboats.com

They have a very impressive motor forum.

[elcid79]

After reading some books on the subject, obstructions are actually supposed to cause the pressure to rise.

I very well could be mistaken on this, being that I am not a boat mechanic. But, an obstruction in a system like this would subject the water to what is known as a venture effect. Much like in a carb. The Velocity of the fluid would increase, but the pressure would decrease. JEM

[aya16]

Ok Ill try take a stab it this... If you pull your thermostats out and run it what happens? does it run cooler? (too cold)

I take it your pee hole tube is opened more than stock for some reason.
Have you decided to replace the pee hole parts?

Also it doesnt seem like the pee hole would cause an over heat problem no matter what the flow. ( more flow more cold water pumped through the engine)

maybe it will make a little diff. but If the thermostat is open at what ever deg. it suposed to then the head should read around the same deg.

Other things could cause an overheating problem

Timing of the engine
running lean at the carbs
turning a larger prop
If the water pump impeller was only changed the body for it could be so worn its not pumping water as much. Not making a good enough contact with the walls of the pump. You can put greese in the body then install the impellor this will increase the pump flow for a short time. to check this.
bad head gasket (gaskets)
also if you have a transducer installed near the engine it could be causeing air bubbles to be sucked into the engine as well as water. (you should be able to hear this when the engine is running and pushing the boat at med speed.)
obstruction in the lower unit pick up, by useing the muffs you force water into this but in the water its not being forced its sucked in.
are your impellor fins bent in the right direction? when installed in the body.



I would at this point take the thermostats out and run it the engine should
run a little hotter then the water you put in. You want to put them back in though after your done testing.
The mag. tape you talked about doesnt sounds like it would be all through the water system more so down in the lower unit where it sucks water. But it doesnt seem like it would be in the head unless it broke up into little chunks. Seems like it would be wrapped around the pump before it went into the head. You can back flush with the water pump out through the thermostats holes. you can then put the muffs on the lower unit with out the pump in and flush up

Now there a good chance that when you put the water pump in and put the lower unit back on that the seal for the tube going up into the engine is missing or not making a good seal.

if all this fails to fix then its pull the head time I think. and check for a crack in the water jackets.

Fixing these things ourselves will have us changing and fixing all kinds of things that dont need to be. Because of our lack of experience. On the other hand taking it to the dealer or some one else doesnt mean they know any more then you. I have a suzuki car that was overheating and
not charging the battery very well. I put a new water pump changed the alt. and still was bad. I took it to a shop and they said I needed a radiator
and a battery. That done it still had the same problems. One day I noticed that when I turned the lights off at night the temp. gauge went down to normal. Turn the lights on and it went to hot.. Turned out to be a bad ground from the engine block to the battery.Many hundred dollars latter.

Check the hot and ground wires to the engine..............That will cause
misreadings......good luck

[elcid79]

you said @ one point that it starting hissing if i remember correctly? That sounds like a blown gasket. When you fire it up, is it still making that hissy sound. I would be VERY cautious of running it hot. That can cause detonation, and / or preignition. Both of which can have catastrophic effects on the motor. ie. holes in the cylinder walls, busted valves, and cracked cranks. Have you checked the mixture? Is your oxygen sensor (If boats have them.) working properly. If its carborated is the carb flowing enough fuel (I think you said it was an EFI but I dont remember.) If its running to lean it will cause high EGT / CHT readings. Which could detonate or pre-ignite. And would make the temp rise allot. Is the timing adjusted properly. This can cause heat problems as well. JEM

[Dimitri-2000X-Tampa]

I ducked out of work today for a while to try out the motor under load. Pretty much up to the same shinanigans, however with my new p-hole shut off valve, the problem is much more manageable. With the p-hole uncovered, it will still overheat under slow speed conditions. With it stopped off, it will only overheat when I run it hard. I am able to run it at WOT for a few minutes before it overheats, slow it back down to 2000 rpm, and the overheat horn stops sounding after several seconds.

Its pretty obvious I have a water volume problem, the flusher testing I did over the weekend just seems to be masking a problem. More pressure and volume with the flusher on, guess it is forcing the water into the pump at a higher pressure.

The Velocity of the fluid would increase, but the pressure would decrease.

At this point, there does not seem to be enough pressure or velocity or water volume.

If you pull your thermostats out and run it what happens? does it run cooler?

The p hole is at the input to the cylinder head. If there is not enough flow at this point in the system, I don't think the thermostat is the problem. Besides, I've tested them in hot water so it is opening.

Have you decided to replace the pee hole parts?

At some point I may since the end is somewhat broken off, but since I had the p-hole completely blocked and I can still make it overheat, this is not the problem as I was thinking it could be a couple days ago.

Other things could cause an overheating problem

Timing of the engine
running lean at the carbs
...

I considered this, but as the engine is running perfectly when it is not overheating, seems unlikely. I had my X up to 19 mph today which just would not be too likely if something like this was wrong.

The mag. tape you talked about doesnt sounds like it would be all through the water system more so down in the lower unit where it sucks water. But it doesnt seem like it would be in the head unless it broke up into little chunks.

It was in very little chunks and it came out of a water output hole (after the head). To be honest, it could have been small pieces of seaweed too, just happened to have the color and texture of tape. Probably not relevent unless there is a bigger clump of it somewhere deep in the motor.

Regarding the water pump and connecting tube, I have replaced all of it now with brand new parts including the pump housing.

When I had the foot off over the weekend, there are a couple clues that point to some sort of partial obstruction.

1. When I backflush it, most of the water comes out around the exhaust, only a small flow comes out of the half inch wide copper tube that feeds the motor. Doesn't seem like enough comes out of the fill tube and since water will follow the path of least resistance, points to some sort of partial blockage where the water comes into the motor from the pump.

2. When I put a hose onto the copper tube simulating what the water pump should be doing, there is a lot of backpressure. I really have to hold the hose on there hard to keep forcing water in. Eventually, I can force enough water in there to fill the head and start coming out around the exhaust but it takes all my house pressure and all my strength holding the hose on to do so. House pressure is probably at least twice as much as the water pump can put out.

While I had the foot off, I did run weed wacker line up the copper tube while I was backflushing but maybe that is not rigid enough if it is partially obstructed with something hard. The Merc shop said that they sometimes see the grommet which holds the water tube to the powerhead slip sideways and partially obstruct the pipe. Only way to fix that it is to remove the powerhead from the midsection.

About the only thing left I can think of doing is reaming something harder in there, like a drain snake or thick wire for example. I hate to have my perfectly good powerhead taken off if there is an obstruction I could dislodge doing something like that. I guess I have become pretty good at taking the lower unit off and on so I could do that in 20 mins or so. Also, I should also start running it in a tub to see what the real water pressure is, flusher muffs are misleading.

Craig, thanks for the iboats suggestion, I'll give that a try too. I think most people would have given up by now and taken it to the shop. I just have so much time invested already, and a bit of stubborn foolishness.

[aya16]

I would then back flush the engine through the thermastats with out the pump installed What ever is in there doesnt seem to want to go through the head. It would be easy to see if the lower unit is clogged by taking the pump off and running water with the muffs throught the lower unit it should shoot right out of the water pump houseing.

But it sounds like your tube is clogged before the engine so back flush this and it might come out. Go through the thermasat houseing with out the thermastat. Even if you get some junk out by flushing keep flushing what ever is in there is probably in a lot of pieces hope fully you get a big burst of water coming out and what ever is clogging comes with it.

you can see the tube going into the engine for the water . run a coat hanger up there with a little hook on the end and turn it when it hits you might be able to break what ever it is free. I think the more you run and flush your driving the stuff up deeper. If it gets into the block your never going to get it with out tear down. Unless its degradable and over time it just dries up and breaks apart like sea weed. But a back flush just might do it. Maybe even use hot water from a service sink or something. It wont be too hot to hurt anything but dont burn your self.

[Theo]

Aya,

That is a fantastic suggestion. I will have to try it myself.

Theo

[aya16]

One reason its so hard to get water through is the thermostat isnt open and your trying to force water through there. you need to take them out and flush the other way. The thermostat opens the other way. so I dont know how your getting water to even go as far as you did.

If there is a clog all the things I mentioned else where has to be done
thermostat has to come out and the lower unit has to come off and the
water pump needs to be taken off then back flush the head and the lower unit. Water should flow through these like nothing is there. any clog at this point should break lose.

You mentioned a hissing sound when running? that would be a head gasket maybe. You can check a head gasket leak by
checking the compression
milky oil
water leak around the head


you also said the engine will run great till it over heats. A lean condition will make the engine run like it had 10 more hp but will overheat the engine.

If back flushing doesnt work and its running fine its time to buy a temp gauge and see if it really is over heating. Over heat condition will cause the pee hole to steam also it will discolor the head, I know its black but it should change to a dull black. You should also be able to put your hand on the head when its running and it wont be that hot you should be able to leave your hand there. you cant see steam from the exaust because its under water. If the engine isnt over heating but the sensor says it is the computer will make the engine run like crap.

Is the oil full? dumb question but low oil will make it heat up too.

But from what you found so far it sounds like a clog to me. and all the things your trying is only forceing the clog higher into the engine and it will never pass the thermostat because the water your useing is to cold to
open them when its not running. Even if you could get them open they only open a slit and the stuff will have a hard time passing that way.
look at it this way you never will have to pay for a water pump change again you took it apart so many times you could do it in your sleep.
Im like you, you started this and you want to finish. Go for it dont get
frustrated, and think it through. Even if you have to take the motor off its no big deal. But I would save that for last. Do all the simple stuff first. The gromet thing the dealer said sounds like they are grasping at straws. now you will have that on your mind. Look up into the body with the lower unit off and you can see where the water tube connects to the engine. use a flash light at night. If there is a problem with the gromet you should be able to see it. It doesnt sound likely to me, If water is getting into the head at all the seal is fine. The gromet they are talking about is only a rubber seal.
good luck let me know what happens.

[aya16]

I think your going to have to use a test tank to run the engine. A trash can full of water will do but running the prop will empty it out real quick. There is a test wheel you can buy, borrow that will replace the prop and wont force the water out of the tank. If the boat is in a slip tie it to the dock real good and run the engine there in gear at 3000 rpm or so till it heats up then put your hand on the head it should be hot but not so hot you will get burned.

[Dimitri-2000X-Tampa]

Aya, thanks for all the good tips and support. I hadn't thought of looking up the input tube but I did look up the exhaust with a flashlight and didn't see anything interesting. I'll try one more time with a coat hanger or other piece of wire. I did actually try flushing for a long time without the thermostat last weekend. I also put water through the water jacket exit hole that the thermostat water would go through. I suppose it is possible that the backpressure I am feeling when trying to simulate the water pump could really be anywhere in the system but it seems most likely to be in the input area. I think this is the key, has to be a partial obstruction at this point. I'm also still not real sure about the flow path. My neighbor mechanic thinks it only comes in at the water jacket and does not split off. Graham's comments imply the water splits, but he has a 2003 bigfoot which I'm pretty sure is a different motor (changed in 2001 when they went to EFI). If it does split off, then the problem is likely on the other side (starboard) where the intake manifold is. Good chance I sucked up a load of seaweed glued together by dirt (and maybe mud dauber spit) if that is what came out of the exhaust hole. Hey, in some parts of the world, they probably make roofs out of it, so its likely some tough stuff that would take years to decompose.

I'm waiting for my broken goal post to be welded before I try to put the boat back on the trailer...its been windy lately and I think trying to (singlehandedly) load it with just one goal post could be a bad idea.

As for pulling the powerhead off myself, I guess the biggest thing I don't have is an engine crane. Seems like I would spend a lot of time and/or money trying to rig one or rent one. I had pretty much figured that would be the point i would turn it over to the Merc shop. Water jackets and lower units are fine, but when I get into the oil system, I get nervous that I don't have all the right tools/experience to do the job properly and a mistake there could be fatal for the motor. For example, a water pump base came with my kit, but I didn't put it in because you are supposed to pressure test the oil seals. You are right, I have gotten pretty good with water pumps and lower units though. Last time, I didn't even have to turn the shaft, it went right in on the first try!

I also did post this issue on iboats.com yesterday (link) but so far, no response. Anyway, I think this is becoming a comprehensive thread on outboard overheating problems. I don't seem to be getting a break with this problem, and maybe Theo's is similar. At least with a full obstruction, you would know that for sure.

[midget]

Dimitri--sorry to take so long, check your email, and turn it right side up(sorry)

[elcid79]

One thing we have hit upon, that you havn't responded to yet was the Mixture. Are you sure its not running to lean? Try upping the mixture allittle and see what happens. Worst case scenario you will foul your spark plugs. Which you can remove and clean without there being any damage. JEM

[Dimitri-2000X-Tampa]

Midget, I did not receive any email from you. Perhaps you got the wrong address. If you use the email button at the bottom of this post, it should get to me. It would still be useful to see a flow diagram, even if the motor is slightly different due to the model year.

Elcid, I'm not even sure if this engine has a mixture screw to be honest with you. I don't remember it having one when I had carb problems a couple years ago. But my main rationale for thinking it is a water problem is that the engine still runs fantastic when it is not overheating and also the fact that the water is way too dribbly compared to what it used to be. Also, the fact that there was a much better water flow with the flusher (and house pressure) than there is with the motor in the water. I'm still waiting on a trailer fix, but the next time I try to troubleshoot this, I will run the motor in a barrel instead of with the muffs. I honestly think that I have ruled out every other possibility and that a partial obstruction somewhere seems to be the only rational explanation any longer. The question is whether my motor will have to be completely torn apart or not. I've decided to give it one more try to ream the passage ways with a stiff wire instead of weed whacker line during a lot of backflushing.

[Night Sailor]

Maybe a small diameter piece of wire cable, say 1/8", frayed on the end to resemble a wire brush, then inserted into a drill and rotated at low speed while pushing through the passages would help?