1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

[Longfisher]

Have a Merc 200 HP Offshore (1998) which sat idle last year (sunk all my money into a trailer rebuild). Stored it as usual. Started it every week or so.

Started fine this season but pee stream seemed weak while on hose recirculator and at idle or near idle.

Took it to the lake for shake down before going offshore and am glad I did. Motor ran fine at marina while in no wake zone and tell-tale had water coming out of it but, again, less than I'd seen in years past.

Put boat on plane after traveling only about 300 meters and noticed no water from tell-tale. Throttled down and then heard overheating alarm. Shut it off for 30 minutes while the kids swam around the boat to let it cool off.

Restarted it and after warming up for a couple of minutes the water came out of the tell-tale, again a little less strongly than I've seen in the past. Idled fine and even took a load at idle or just above idle without problem. Motored back towards the marina.

About halfway back towards marina pee stream became strong again so turned around and increased rpms to put it on plane. Pee stream stopped and engine overheat alarm came on pretty quickly after that. Neutral but then heard detonations so shut it off completely and let it sit for 20 minutes to cool off.

Started it again and pee stream was again weak. Motored back to marina and retrieved boat to trailer and came home. Wasted trip but glad to find problems before running offhsore.

OK, so what's the problem with this outboard given my description? Spun impeller? T-stats (not likely as pee-stream not apparent as motor warming up and then apparent after motor comes up to temperature)? Poppet valve? Obstruction?

Any help is much appreciated.

LongFisher

 

[gss036]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

First thinks first, when was the last time the water pump was changed? When they sit dry, impeller tends to get in a fixed position so it does not touch the edge of the housing. It will move a little water at slow speed/idle. If it has not been changed, that would be the first step. Second, if it were my boat would be to install a water pressure guage. Third would be to inspect the the thermostats and fourth, inspect the poppit valve. The poppit is designed to open at around 2500 rpm, so I would not suspect it so much as the cause of your overheating.

 

[Longfisher]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

Impeller was changed 3 - 4 years ago. But the boat has been little used during that period due to the cost of fuel and the need to rebuild the trailer last year (to the tune of more than $2,000). So, I thought the impeller would be OK to use this season.

Guess I was wrong.

I like the idea of installing not only a pressure guage, but also, a temperature guage. Only problem doing that is that there's little room on the console for more guages and switches. Suggestions?

One more thing. What's the difference between the impeller and the water pump. Is the impeller just a component part of the water pump? I see referrence to replacing the whole water pump on several forums, including this one, vs. just replacing the impeller. What would be your recommendation?

Oh, thanks for the response.

LongFisher

 

[Kev144]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

I would look into re-doing the water pump and a new impeller. Impellers are supposed to be done usually ever 2 years. I do it every year as its cheap insurance. I would defiantly say its something to do with the pump or impeller from sitting with out much use for a period of time. As said earlier, a water pressure gauge would be the important one first. If your not getting water pressure up, you better believe working that motor is going to over heat, its a given.

 

[Longfisher]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

OK, got the new water pump on and the lower unit again bolted to the mid-section. Did have some trouble with slipping the drive shaft into the receiver splines on the crankshaft. But figured out a trick.

I removed the fly-wheel cover and had my 15 year old rotate it a bit as I tried to "stab" the drive shaft into it. Worked like a charm. Oh, I first removed the spark plugs.

Also, had a bit of trouble getting the shift re-splined. But with some fidgeting it also synched up OK.

I saw the lower unit vent and fill screws had become scarred due to someone (undoubtedly the service folks where I take the boat) using a screwdriver with too small a head to remove them. So, replaced both of them when refilling the gear housing in the lower unit. Lower unit oil was pretty black, actually. Surprised me as it's been pretty much sitting for the last two years and the oil was changed about 2 years ago.

Lastly, replaced both thermostats. Both looked pretty bad with lots of scale on them and on the housing in which they sit. I spent a good deal of time scrubbing and otherwise scraping the scale from the mating surface for the thermostat seal.

Should I be concerned about scale in the rest of the cooling system? If so, is there a product or procedure that someone could recommend that would remove the scale from the system?

Lastly, the new thermostats seem to work a bit differently from the old.

With the old there was about a 2 - 3 minute delay after starting the engine before water began coming out the tell-tale. At first, it was pretty weak (only one open, I guess), then it became stronger (both open, I guess), then it'd shut off almost completely (cool water admitted to the water galleries which then probably close them), then both reopened almost in synchrony and the stream was constant thereafter.

With the new thermostats there appears to be much most "modulating" of the coolant. I see the opening and shutting of the thermostats much more often now and it takes about 10 minutes of running before the engine cooling system appears to come to steady state (as judged by the constancy of the stream).

Am off to the lake early tomorrow to see how she does at high RPMs as this was the problem in the first place.

Anyway, should I worry about the scale?).

LongFisher

 

[jerryjerry05]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

Gear oil should be changed every year at least!!! If you use it a lot then change it a lot.The scale is nothing to worry about as it will happen to all motors.The impeller should be changed every 2-3 years at the least. Again if you use it a lot then change it more often.If you let it set then it gets a memory and stays that way and looses function.Jerry

 

[Longfisher]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

Took boat to lake with skis and kneeboards and intent to test out repairs.

Firstly, got pretty good tell-tale at low and intermediate rpms. No alarms at all.

But have seen a pretty weak tell-tale at WOT. Probably the poppet valve? Intend to replace it tomorrow.

Also, I've seen a good number of instances where the tell-tale stream is changes in velocity during running. Sometimes it's really strong. Sometimes it's pretty weak. Can this be attributed to the thermostats being new and just doing their jobs or to something else?

I see other merc's on the water with an incredibly strong stream even at idle. But the orifice seems smaller on those smaller motors (don't see that many 200 HP on the lakes). So, maybe the smaller orifice accounts for what looks like increased stream velocity.

Finally, at one point I heard what I thought was detonation after pulling a skier and circling back to get him while powering off plane. It sounded like someone taking a flat stick and running it along a chain-link fence. Stopped in a few seconds. But it was unsettling.

I really don't want to ruin this motor as I barely have the money any more to run it at all much less replace it. So, please help me by either comforting my anxiety at these strange sights and sounds or telling me that I've got a real problem that must be dealt with.

Also, what does detonation sound like and am I hearing it given the description above.

LongFisher

 

[gss036]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

The sound you are describing almost sounds like straight cut gears in the lower unit are flutter, maybe from quick decrease in speed and water flow backwards against the prop. I doubt if you would hear engine noises like that. Again one never knows, keep your ears open and pay attention to what is causing it.
Do that poppet valve to be on the safe side.

 

[Longfisher]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

Took boat to lake for skiing.

No alarms at all after replacing water pump and thermostats. Like I said above, the thermostats seem a lot more "active" now as they seem to open and close more frequently while in the driveway on muffs and in the water. I don't always have a stream at idle out of gear. Sometimes it's a dribble. Also, at WOT I don't see stream, but rather what looks more like a dribble. But it's hard to see from the helm as there's a lot of spray too from the hull.

Did once see what looked like a lot of steam coming from the exhaust. Happened at the beginning of the day and after the first short period at WOT.

Later after WOT run with skier, came to low rpms to pick up a skier and heard a rattling noise. It was very disconcerting. I've never heard such a noise before.

Still no overheat alarms, though.

Decided to call it a day. Then heard like a faint whine from the engine when not in gear. Put it in gear and it went away. Very reproducible.

Want to take the boat fishing offshore but an afraid to do so. Don't want to ruin this engine and can't seem to figure it out. I don't have a lot of money so before I take it to the shop and let them charge me a ton of money can any one help with this one.

Longfisher

 

[j_martin]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

A bit of basic merc cooling theory.

The only water that gets to the tattletale is coming from the thermostats. If you are in cold water, or the poppit opens early it would not be unusual to see little water from the telltale.

About the only use I have for the durned thing is when I launch, I wait till it pees before I blast off, and avoid a cold siezed engine.

hope it helps
John

 

[MexMangoMan]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

I have a similar problem with my 225, the alarm went off but does not engine does not seem to be overheating. I decided to change the impeller just to make sure, dropped the lower unit and found a rubber hose that appears to be plugged and I cannot find anywhere that it looks like it should go. Clymers shows it in their picture but says nothing about it. Anyone know what it is or if I need to do something with it? After I get it back together I will test and post if that cleared up my problem or if I need to check the thermostats or poppit valves. Thanks, Dan.

 

[Longfisher]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

The only rubber hose I'm aware of in the lower unit goes to the speedometer. There's a break in the hose (two hose lengths joined by a small plastic pipe with barbs on each end) which allows the speedometer hose to be disconnected when the lower unit is removed.

I understand that the speedometer relies on water pressure coming into a small hole and then into the tube in the lower unit. I guess there's a diaphram somewhere that senses the water pressure. The small hole is right on the front of the lowerunit and a bit below the cavitation plate. I've had to clean it out once to restore the workings of the speedometer. And, when I replaced the water pump a couple of days ago I noticed that the hose was worn and replaced it.

On reattachment of the lower unit to the midsection you just make sure the rubber hose protrudes upwards alongside of the gear shift rod and then connect it to the complimentary rubber hose that comes down from the midsection.

Has your speedometer not been working? If you don't know where to connect that hose you probably have not had a working speedometer.

LongFisher

 

[Longfisher]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

Took boat to the shop for a professional to look it over. The detonation without the alarm going off concerned me. I can't afford to replace this engine.

I'll let you know what they say.

LongFisher

 

[Longfisher]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

Poppet opening early?

Thanks for the primer on Merc cooling systems. But can you explain further about how the opening of the poppet, either early, normal or late, would effect the tell-tale?

The yards taking forever to look at the boat and I'm thinking of taking it back and looking into it again in the driveway.

Thanks,

LongFisher

 

[Longfisher]

Re: 1998 Merc 200 OB Overheating at High RPM, No Water from Tell-Tale

OK, $400 later...the yard didn't find anything particularly wrong with the motor. For no parts charge they replaced the already new impeller with what they said was a redesigned one and inspected and approved my repair. They also replaced the plugs and fixed a leaky recirculating line. They compression tested the head and said all were good and very uniform across the head.

They had no explanation for the slapping noise on a chain link fence that I'd observed, but then, a previous poster on this thread probably nailed that as being flutter caused by water streaming past the prop on deceleration. Of course, the yard couldn't test that as all they have is a tank in which to test the engine.

Took the boat to the bay a couple of days before the hurricane hit south texas and got in some fishing. But noticed something odd when we launched the boat, something I'd never seen before.

With the boat still on the trailer but the motor in the water on the ramp we always start the motor and let it run intil we see the tell-tale pee. Same thing this time.

But on warming the engine up at the ramp I also noticed that the small exhaust port (about 1.5" X 2") dirctly under the tell-tale spit a lot of water this time along with the exhaust. Water continued to, well, bubble from that exhaust port the whole time we ran the boat that day. It certainly did so when we were at low rpm, but I don't know about WOT as from the helm station I can't view that port and pilot at the same time.

When we got the boat home and cleaned out the saltwater using muffs we didn't see water coming from the exhaust port. So, it's at least possible that we only see the water coming from the exhaust port when the lower unit is submerged over the cavitation plate and perhaps above the point where the lower unit is joined to the rest of the motor. At WOT I think water only covers the lower unit up to about the cavitation plate.

Is this a problem? Did the mechanic do something wrong to introduce water into the exhause stream? Is maybe a gasket misassymbled?

LongFisher