Mercury BigFoot 50 Overheating (long)

[Dimitri-2000X-Tampa]

I haven't been able to get too much good boating in during the last 6 months or so because of engine trouble...and it continues. I was able to get a little sailing in when the engine was only overheating over about 3500 RPM but then a couple weeks ago, I did 4-5 doses of salt away and it appeared to make the problem even worse. Now it is overheating even at very slow speeds. I'm going to try one more thing before giving up to the $500+ repair bill at the Mercury shop.

This saga started about 6 months ago. One weekend I was out running the motor pretty hard tubing with the kids and everything was fine. About 2-3 weeks later, I went out with my daughter and it started overheating, it has been trouble ever since. Coincidentally, my depth sounder stoppped working about the same time (meaning I have probably been in too shallow water kicking up dirt)...and...there was a big mud dauber party in my boat about the same time when they got in and mudded everything up. I've come to the conclusion that I have multiple obstructions in my motor, maybe from the bees, maybe from the Bay silt and sand, maybe from both. To make things even more complicated, these obstructions may be moving around.

The first thing I did was replace my 5 year old impellor. The one I took out looked almost as good as the one I put in. The motor did not overheat on the ear muffs after 10 mins or so, so I put it back in the water and it started overheating again. After the salt away treatments, I put the boat back on the trailer and took the lower unit off again (I have had the lower off 4 times now trying to fix this). Now it was so bad I got the overheat warning just idling with the flusher muffs on. When I shoot water up the copper tube, only a trickle was coming out the exhaust pipe but good pressure at the tell tale. I pulled the thermostat and shot water up the hose again, only a trickle out of the thermostat hole. I flushed back and forth for about an hour using house pressure. A big sucking sound happened and then my flow got much better. I figured I got a sand clog out. After this, I noticed that I was getting more water around the exhaust pipe...but still hardly any through the exhaust pipe itself. The motor was not overheating any more on the flush attachment. Even revving it up a bit. So, I figured I must have fixed it!......NOT

I put the boat back in the water and as soon as I get a bit of load on it in slow speed....beeeeeeep...damned whining overtemp noise. Shut it down again and put it back on the trailer today....again... Tomorrow I will pull the water jacket cover off. Seems to be my only hope that those passage ways are all bunged up. I was holding off doing this since Mercury wants like 30+ bucks just for the stupid gasket...and a 3-4 day wait to boot. This is my last shot at DIY, a couple more hours and I give up. If its not an obstruction in the water jacket, then probably the whole powerhead has to come off and everything will have to be cleaned out....at least that is what the Merc shop is saying as I have been consulting with them on this issue.

Motor is still running well and when the alarm goes off, I shut it down a bit and let it cool off...so, hopefully I have not done any serious damage to the powerhead yet although on second thought, maybe a good excuse to upgrade to a larger motor

Any mechanics out there have any tips about how to rod the passageways without removing the whole powerhead? I figure that is about all that is left now is the water jackets so it will be interesting to see what it looks like in there.

Another difficulty in all of this is that the shop manuals (both Clymer and Mercury) do not seem to show the whole water flow diagram so it is very difficult to figure out what passageways go where. Tonight I went to a bookstore and they did not have a single book on outboard motors.

[Divecoz]

Increase the water pressure ? What can it hurt ? Pull the thermo and attach your electric power washer / jury rig it to flush straight thru .I would think that whatever it is in there is coming out sooner or later. Its nothing big due to the size of the water pickup holes, but it may well be a mud silt sand blockage that dried to a brick of sorts and will just take time to wash away? Can you hook up something to keep water in the engine over night or to re-circulate it all night long how about adding some laundry detergent to break the surface tension.

[Robert]

Have you verified that the motor is actually overheating? Is it possibly a bad temperature sensor giving false overheat signals? Just something more to check out before you tear it down.
..
I would not put excessive pressure into the water inlets, that could make things worse by getting water into places where it is not desired.

[craiglaforce]

My Tohatsu warning buzzer was always going off and kicking onto the SLO circuit. THey had defective overheat sensors, although it might have been getting a little warm.

I would replace the impeller which you already did, the thermostat and feel the engine with your hand briefly when the buzzer goes off. If you can leave your hand on it for a second, you should be fine. If you scream in agony then it is overheating.

But it sounds like you do think you picked up a load of mud in there.

I wonder if Tohatsu made the motor for your manufacturer?

I would also use premium motor oil liikemquicksilver, and also add a little extra to the gas tank now. Tohatsu recommends this for the first tank of the season. a little extra oil is a little extra insurance in my opinion.

Is it giving a good Pee stream?

[Divecoz]

You shouldn't be able to harm the cooling chamber. If you did it with the motor running it would NOT have time to cool off the motor . So what is the temp of the water leaving the motor ? Is it possible (don't know why it wouldn't be ) to test the high temp sensor as well?

[Dimitri-2000X-Tampa]

Thanks for the responses. Here are some comments/answers to the various questions:

It may be a good idea to keep water in it for a long time. I just havent done it cause I don't know of a good way to recycle the water and it makes a big mess in the grass.

Have you verified that the motor is actually overheating?

I haven't actually used the thermomelt sticks, but I'm virtually positive that it is really overheating. When I was still able to go low RPM's and I would kick it up on a plane, the engine started missing right after the alarm went off. I immediately shut it down. Also, I can feel the heat coming from the cylinder heads and when I shut it off, steam comes out of the P hole.

By the way, the Mercury dealership also warned me about trying to force water into the exhaust pipe...definitely a no-no. So far, I have only forced water up the water tube (where the impeller pushes the water into) and down the motor either through the P hole or through the thermostat hole.

I wonder if Tohatsu made the motor for your manufacturer?

At some point (2001 I think), the Merc Big Foot changed to fuel injected and I'm not sure where they got the motor from. I've always been under the impression that it is a Yamaha powerhead mounted to a Mercury lower unit, but I could be wrong.

Is it giving a good Pee stream?

See now this is where it is a bit tricky and I don't have a pressure gauge nor a benchmark to judge from. There is definitely a P stream, and there always has been. But when I first noticed the problem, I thought the stream may be a bit weaker than it used to be. The shop manual does give pressure readings which do go up a lot when you rev the motor. I've been wondering if the pressure goes up enough when the motor is revved. Doesn't seem like it.

When I put the new impellor in (and replaced the thermostat too), it still seemed a bit weak. But there is definitely water coming out. After reading some books on the subject, obstructions are actually supposed to cause the pressure to rise. The question is whether the stream would be slightly lower if there was a powerhead obstruction but water is still getting through alternate paths. This is why a diagram would sure be useful here. I don't know exactly how the branching of the paths work. But I'm pretty sure that there are multiple paths.

So what is the temp of the water leaving the motor ?

I haven't actually measured it but maybe this is a good idea before I pull the water jacket cover off. Just putting my hand on it with the motor idling with the flush attachment, I would estimate that the water coming out of the prop hub is about 100 degrees and the water coming out of the P hole is slightly cooler, maybe about 90. What will the water temp tell me? If anything, I would have thought that everything is fine since its carrying heat away from the motor...but obviously, something is still wrong which is why I think there are multiple water paths and one of them must still be blocked. The way it is now though, the alarm won't go off unless I put the motor under load.

Thanks again for all the suggestions.

[Divecoz]

Forcing water in / up the exhaust would . . .could . . push water right into the cylinders !

[Theo]

Dimitri,

Hang in there bro! I have been dealing with a similar situation for well over a year now.

Where I am at now is, my engine, a Suzuki DF50, purrs at lower rpms under 3500 and overheats within minutes at higher rpms above 4500. My Clymer manual is pretty much worth a handful of cat crap as far as detailing the cooling passages.

Some things to try before you cough up the big bucks for a pro.

1) I did not read that you had checked the thermostat. Do that first. Mine was stuck. To check it put it in warm water in a pot on the stove to see if it opens up. Your manual should give you the temp specs and you can use a candy thermometer to measure the temperature.

2) Test the temperature sensors. You will need a multi meter and your manual should give you a schedule of resitance readings at various temperatures. Also examine the sensors for cracks or other problems. When I checked mine, they gave the proper readings but one of them had cracked and broken. I only noticed this when I went to reinstall it and it stripped. The sensors in my Suzy are notorious for being faulty.

3) Try and find some thermomelt sticks in the appropriate temperature range. I have had no luck finding them locally here in OC. You can also get an electronic temperature reading gun for about $50. That might be my next step. Verify that your engine is truly overheating.

4) Finally, my motor has a high pressure valve (probably spring loaded) that opens up to allow more water flow at higher rpms. It is a total bitch to get at and might involve pulling the power head just to get at it. Just by the symptoms, I suspect that is where I am headed. I am just dreading it. I have been trying to hang up my wrenches for years now but find that I mostly just get my intelligence insulted by the "pros".

I am at the stage where I am replacing my second temperature sensor to see if that helps($38 bucks). After that I will need to spring for or borrow a temperature reading gun ($50). Finally I will need to get at the high pressure valve or pay mega bucks to have someone else do it.

A thought to leave you all with. We are being horribly short changed on performance with our engines. I also do not understand why outboard manufacturers insist on using raw water cooling. It seems to be extremely inefficient and prone to failure. Not what you want on the water! Consider that Suzuki manufactures a 750cc engine (50cc less than my DF50) that warps out well over 100 ponies. More than double. Note that this engine has a pressurized radiator cooling system just like a car. They are considered ultra reliable. Engine failure at freeway speeds on a crotch rocket can be fatal. Weight is not a factor. As amatter of fact the engine would probably be lighter. The whole motorcyle that this engine is attached to weighs less than 400lb (GSX 750)!! Go figure. Rant over!

Theo

Ooops, reread your post and you did check the thermostat! Good job.

[Night Sailor]

Dimitri,
Before you started having this problem could you reach the max rpms recommended by Merc with the same prop and load you now have? It's concievable that if you haven't, that the prop has too much pitch and the engine is struggling to push the boat without overheating. The faster you go the harder the struggle until you get up on plane.

I have also had faulty spark plugs and faulty thermostats right out of the brand new box. Make sure your thermo is actually opening at the specified temp.

[Dimitri-2000X-Tampa]

Yea, I've been boiling thermostats on the kitchen stove In fact, I boiled the old one first to make sure it was working. Then I replaced it with a new one which I also recently boiled to make sure I hadn't gotten a defective one. Both thermo's work fine...unfortunately. I wish it was that easy of a problem.

As for the prop (and boat bottom which the Merc man asked me about), they are perfect. I had been using the same 14X10 prop for over 2 years and probably over 100 hours of run time with no problems. I had made 80 mile "motor only" trips at high speed (even with a slightly crusty bottom) with no problems. I also just redid the bottom paint late last year so it is nice and smooth now.

One thing I am still not positive about is how much water is supposed to actually exit through the exhaust versus around the pipe. The shop manual just says that the water exits through the drive shaft area...which is just too vague. The other vagueness is how it talks about the thermostat allowing additional water to exit the cylinder block...which implies that there is a primary area of water exit that is not the thermostat plumbing. I just can't understand why there are no flow diagrams available.

Theo, I agree with you about the weakness of the cooling system design, however, a split water system isn't all goodness either. I owned a traditional (inboard diesel) keel boat for 7 years before trading it for a Mac. It had a split system and I never did have a problem with the fresh water part of the system however, I did have problems with the raw water side including a bad impellor, failed connection between exhaust and water exit tube which had to be welded, and the biggest problem was a heat exchanger which had gotten clogged up with bits of the sacrifical zincs that had broken off. The heat exchanger had to be professionally cleaned out by a radiator shop. I may be into a similar problem with the outboard, but unfortunately, the heat exchanger is the whole darn engine block!

[Night Sailor]

I don't recall if your bigfoot is a 2 or 4 stroke but in either case, if the timing has slipped or become maladjusted, the engine could overheat, but usually there is also some noticeable indication of performance fall off at idle and top end. Since you didn't mention that, I can only conclude that you are right... every water passage needs to be cleaned out, block and all.

[Theo]

Dimitri,

From what I can gather, the P hole is actually before the thermostat. This would make sense because you should get a healthy pee as soon as the engine starts, not just after she warms up and the thermo opens. The best I can gather is that it should give a good enough squirt to coat the urinal top to bottom, not just a dribble. Mine has a little nozzle on the end that can be turned in any direction. Turn it straight up and if it gives a good stream your impellor should be fine.

I have a Clymer manual as I suspect you do. The high pressure relief valve is buried somewhere in the section about the "power head adaptor" at the very bottom of the power unit. It also refers to vague water passages but has no good diagram. If necessary I would invest in a good factory shop manual before diving in there. They should give a comprehensive diagram of the water/coolant passages. The Clymer manual does specifically mention a high pressure relief valve for higher rpms. Again no good diagram. I thought it would be after the thermo but on further review I just assumed that. It could be before the thermo and just kick in at higher pressures. Either way, I would scour your manual one more time to see if it gives a good indication of where the coolant exhausts are located. You might be able to get at them without pulling the powerhead.

As far as a heat exchanger on an outboard. Not necessary. The motorcycle engines I refer to use radiators, not heat exchagers. Take a look at a rice rocket. The radiators are only about the size of a shoe box. Easily mounted around an outboard cowling.

Good Luck

Theo

[Dimitri-2000X-Tampa]

Here is an update: I took off the cooling jacket cover...and for the most part, it was clean. Only exception is at the very bottom part of the cover, it opens to the exhaust manifold. This part of the cover is very caked with carbon, strangely, there is a openable port at that point which makes no sense to me unless you want to monitor your exhaust gases. Ie, I don't think there is any water in this part. So, I doubt that is my problem.

I did actually browse through a Mercury factory shop manual at the Merc shop and it was only slightly better than my Clymer manual, still no water flow diagram.

Dropped my lower unit again and ran water up the tube with the jacket cover off. That gives me an idea of the flow. By deduction, it looks like not only is the P hole before the thermostat, but it is before the water even goes through the head (it has only gone over the exhaust manifold at this point). This actually concerns me because my P hole has been mangled up over the years, like part of it is cut off. I'm starting to wonder if a larger opening here is possibly reducing my water pressure, so I think I will replace that piece.

As I still haven't found any major blockages other than the one I pushed out a couple weeks ago with pressure through the thermostat hole, I'm beginning to go back to the theory of insufficient water pressure again. The way the temperature sensor is designed, it is up near the top cylinder. By virtue of gravity, if there is a pressure reduction, it is going to be the top cylinder which is starved for water..which of course, will trip the sensor even if the other cylinders are getting more water and cooling down more.

What concerns me is that when I put a water hose up the tube before I took the jacket cover off, I got a much more forceful P stream than with the impellor. So, I think my strategy now is to replace the impellor...again (with one from the Mercury shop versus the one I got from boating world), replace the rubber connector tube grommet, the P hole and possibly replace the water pump housing too. If this doesn't fix it by increasing the water pressure (volume), I'll have to take it to the shop before I miss all the best FL sailing season. you know how it goes though, once you have invested this much time into it (have pulled the lower unit like 5 times now), you just keep trying out of stubborness.

[Graham Carr]

Dimitri
I have 2003 50 bigfoot.
you are correct the water that exits the peep hole is before the t-stat but the water splits before that to run through the block. See flow info at the bottom.
Not sure what dealer manual you saw but mine is far superior to any after market manual that I have found. Maybe the dealer didnt want you to see all the info. But flow charts are in the book including oil flow, fuel flow, smart craft charts and color wire diagrams plus a ton of other info.

The pee flow only shows the pump is working or the water inlet is not blocked. You can have a great flow but still overheat. The water flow splits (see below in the flow info). Likewise you can have a low pee flow with out the engine overheating. The problem is if the flow is down then the fuel cooler and fuel pump would over heat. You can also have a t-stat that opens when tested but still restricts the flow enough to cause the engine to over heat. I had this very problem last year. It turns out that the t-stat look good but had just a slight bit of corrosion on the back side (block side) so it reduce the flow. I found out that the owners manual is wrong on temp for flushing. They state only idle the engine while flushing. That is what I did for two years. This is what I found out. I install the smartcraft system monitor (this is not the Guardian warning sounds). This is a gauge system that monitors engine functions. I found that when the garden hose is hooked to the engine and running at an idle, the temp NEVER reached the opening temp of the t-stat. I have to run the engine a little faster to reach 120. So it seams hat I only flushed part of the engine. Salt water must have stayed at the back of the t-stat. I did have my test results but I lost them. Ill run some more test this season and post the info this fall. A new t-stat solved my overheating problem. The old one looked almost as good as the new one. The Guardian system will not allow the engine to run at higher RPM only enough to limp back to port.

T-stat testing:
1. Place the t-stat in a pan of water
2. Place a thermometer in the water and heat slowly.
3. 118 - 123 F (48 - 51C) starts to open it
4. AT 145 F ( 63C) you should have a minimum lift (opening) of 0.12 in. (3mm)
If you dont, replace it.

Engine coolant test (ECT):

1. Place the ECT in a pan of water.
2. Place a thermometer in the water and heat slowly.
3. Measure the resistance at different temp. if the reading is off, you will need to replace the sensor


30 F = 32 (KΏ)
60 F = 15 (KΏ)
90 F = 7 (KΏ)
120 F = 4 (KΏ)
180 F = 1 (KΏ)
240 F = .5 (KΏ)

Water pressure test:

This should be done on a warm engine with the thermostat at 140 F (60 C). You need to remove the test access port plug from the base of the thermostat housing and screw an adapter in so you can attach a pressure gauge.
Water pressure measurement:

1000 RPM = 5.5 psi
2000 RPM = 8.5 psi
3000 RPM = 12 psi
4000 RPM = 14.5 psi
5000 RPM = 17.5 psi
6000 RPM = 20.5 psi


Water flow info:

This info is based on the 40/50/60 EFI (4-Stroke) Water flow diagram which can be found on the last page of the Mercury service manual.

The numbers reference the parts shown on the water flow diagram.

The cooling water flow is as follows:
1. Enters water inlet-
2. enters water pump then exits to-
3. Driveshaft housing into-
4. Water tube which channels the water into the-
5. Adapter plate and into the-
6. Cylinder block. At the base of the block the water splits. Part of the flow is directed into the base of the water jacket cover. The water exits the base of the jacket via tubing and is routed to the (10) Fuel cooler, then the (11) fuel pump and then exits the (12) Tell-tale (pee hole). The rest of the cooling water flows up through the cylinder block channel (water jacket). This channel routes the water into the-
7. Cylinder head, then it flows back into the cylinder block around the cylinder walls until it exits the block at the top via the
8. Thermostat and then back into the water jacket. This flows down to the-
13. Exhaust water discharged.

Hope this helps.
Graham Carr

[midget]

Dimitri--My Merc Bigfoot manual has a water flow (and fuel flow!) chart in it. Maybe I can scan it and fax you a copy if you'd like. PM me if so.