Mercury Overheating

Hi Guys,
I am a first time boat owner and I have resently bought a 16" Boat with a 1986 Mercury 135HP V6 Oil Injected motor. The problem I am having is that at idle the engine runs fine and my temperature guage sits on about half way. However, if I put the boat into gear and the engine runs between 1000 and 3500RPM, after about 5 minutes, the temperature gauge goes to HOT. If the engine is running at over 3500RPM, the engine then begins to cool and after about 5 minutes, the temperature gauge drops to on or below the half way point. I should mention that there is steady water coming out of the small "wee nozzle" however it very hot. It is almost impossible to keep your hand on this water.
Would you guys have any idea what the problem may be? I have not yet had a chance to change the impeller but will do so however if it were the impeller, would it not run hot especially at higher RPMs?

Also, when the temperature does get to HOT, the alarm does sound.

Thanks in advance,
Callala.

Re: Mercury Overheating

To be on the safe side,replace the water pump and thermostats.also check the water tube going up to the power head,sometimes they will crack or get holes in the tube Good luck,Ted

Re: Mercury Overheating

If the water pump impeller is bad, it can start to overheat at low or high RPM, or both. It is best to change it. Remove and check your thermostats as well. Hang them by a wire in a pot of water on the stove and make sure they open at 140*F, or just replace them. check the poppit valve for proper operation. It is located on the lower starboard side of the block.

Re: Mercury Overheating

From yer symptoms, it sounds like the t-stats are sticking, (at least on the side that has the temp sender on it) or somehow the drain for them is blocked where it goes into the mid. It could also be a weak water pump. Check especially the face plate under the impeller.

The cooling at high rpms is yer poppet working.

Before I took anything apart, if there's opportunity, I'd run the engine till it shows hot on the gauge, then physically feel the heads. I'd bet only one is hot, and that's the t-stat that's bad. If they're both hot, then it's a general water flow problem, ie water pump or delivery tube and seals. It could also be either both stats or the t-stat drain system.

Just had another thought. If it's overpropped, it could heat up in midrange. What is your engine rpm at WOT?

hope it helps
John

Re: Mercury Overheating

1986. Go ahead and spring for a new oem water pump kit, t-stats and poppet kit. It probably deserves that much, ya got all tha bases covered and you will be ahead on maintance. Especially since your a NEW ta you owner ya don't know how good or old that stuff is.........D

Re: Mercury Overheating

Totally support the responses. If you are a curious problem solving kind of guy that likes to have the root cause of a problem so that he KNOWS he has the problem solved, I think the brown wire(s) goe(s) to over temp sensors in each bank of cylinders (I think I remember both sides are protected; would make sense that they would be with 2 separate stats) so if either or both overtemp you get the alarm.

There is a thermostat at the top of each bank. Gonna bet you that if only one side of the engine has a stuck closed stat, you will be able to feel the water jacket on that side and notice it is much hotter than the side where it is working properly. I have one of these $30 Harbor Freight laser thermometers and they are perfect for this kind of diagnosis.

However if both sides feel to be the same temp, then it would point to 2 stuck stats (remote possibility I'd say) or positively identifies a bad impeller in your water pump.....as long as you don't have corrosion or silt clogging passages.

The stats are 143F opening temp and there is a tolerance on that so give or take 5F like Chris said. 140F is the Normal temp setting on the dial of a domestic hot water heater to give you an idea of what that is.

As usual, my 2c; worth exactly what you paid for it.

Mark

Re: Mercury Overheating

Texasmark said:

Totally support the responses. If you are a curious problem solving kind of guy that likes to have the root cause of a problem so that he KNOWS he has the problem solved, I think the brown wire(s) goe(s) to over temp sensors in each bank of cylinders (I think I remember both sides are protected; would make sense that they would be with 2 separate stats) so if either or both overtemp you get the alarm.Mark

Click to expand...

If it has one on tha port head its for an optional temp guage. Tha one in tha starboard head is tha warning sensor........Guess ya could use tha same type sensor thats in tha starboard on tha port side, tie 'em together and get tha buzzer if either side heats up. Might work. Donno? Never thought about it. Never tried it...........

One other? Do you know if even has t-stats in it?

Re: Mercury Overheating

Dukedog said:

If it has one on tha port head its for an optional temp guage. Tha one in tha starboard head is tha warning sensor........Guess ya could use tha same type sensor thats in tha starboard on tha port side, tie 'em together and get tha buzzer if either side heats up. Might work. Donno? Never thought about it. Never tried it...........

One other? Do you know if even has t-stats in it?

Click to expand...

Wired in parallel you bet your bippie both sides would be protected and I can't imagine Merc monitoring only one bank with two stats. Like I said the engine is not in front of me, but common sense should rule.......but as Bruce 58 states, common sense is not very common....lotta truth to that. But I do remember a sensor on the port bank, don't remember on the starboard bank.

And on stats, yes sir. without stats, the block would not fully fill, especially on the starboard side and the top cylinder would fry....I have watched the process troubleshooting temp problems on an '86 150.

Mark

Re: Mercury Overheating

Symptoms: No overheat at idle but overheat up to 3500. No overheat after 3500 and it cools back down when at idle.

Poppet valve is supposed to open at 1800 rpm. Stats are probably fine but replace them. Replace the water pump, it may not be producing enough pressure to open the poppet valve. Test it.. If its back to normal your good to go. If not, remove the poppet valve and check the nylon block insert for calcium deposits. You can try cleaning it but it's usually best to just replace it since your in there anyway. Replace the poppet diaphram also. The rest of it rarely wears out.

Re: Mercury Overheating

Some times I don't understand you guys. He's got a motor that he knows absolutly no history on 'cept what tha "other" guy says. Its tha "coolin' system. He has ta run it ta see if "THAT" part fixed it. Something that could fry tha motor in a second. Something you don't try and hope on one item at a time. Something that we are makin' an educated guess at. Change all of tha items mentioned ta NEW and be way ahead. A little more costly maybe but not very much compared ta what it could be. If it was a motor that you know well and how it is maintained then by all means track it down one item at a time. But a strange motor, don't take tha chance.................JMO

Re: Mercury Overheating

Coupla things I disagree with.
1. There's enough cooling so the motor is not in imminent danger of frying, provided it's not run for extended periods when it's overheating.

2. Unless someone re-rigged it, the sensor on one head is the overtemp alarm switch. The other one is a sender for the temperature gauge.

3. Shotgunning a whole bunch of parts in at once will usually fix it, unless you make a mistake, in which case you'll put a second fault into the mix and make it real interesting to troubleshoot. It's also the most expensive "repair".

If it were mine, I'd check out the pump and delivery tube and seals first. Put in an impeller at least, and pump housing or base plate as needed.

my 02
John

Re: Mercury Overheating

Its the thermostats , the pumps fine thats why it cools back down after 3500 rpm which is when the poppet valve is opening , if the pump was no good it would keep on getting hotter . i would change the all the above sugested items anyhow as the cost very little.

regards
bob

Re: Mercury Overheating

Dukedog said:

If it has one on tha port head its for an optional temp guage. Tha one in tha starboard head is tha warning sensor........Guess ya could use tha same type sensor thats in tha starboard on tha port side, tie 'em together and get tha buzzer if either side heats up. Might work. Donno? Never thought about it. Never tried it...........

One other? Do you know if even has t-stats in it?

Click to expand...

The OT switch is a spst snap action switch and grounds the attached wire when energized. Putting them in parallel allows any switch on the line (OT Fail line) to turn on the horn once it's kick in temp is exceeded. Will work just fine and if I had a V6 and it only had one, I'd put in the second one on the other side. Again, I'm surprised OEM didn't consider frozen stats that big of a problem to not put one on each bank with it's own stat.

Mark

Re: Mercury Overheating

I can see where that would probably work wired parallel with a carburated motor. EFI motors take up that spot (port side). Don't think it would make any difference though. Just never thought 'bout it............

No stats or some type of restriction in a lake motor will make temps do strange things. Lotta guys take 'em out thinkin' it helps an overheat problem. Thats why I ask. Do you know if they are even in there?..........

Re: Mercury Overheating

Lotta guys take 'em out thinkin' it helps an overheat problem. [/QUOTE]

Son had a 150 Merc and we were working temp problems which turned out to be a bad stat. Obviously V's have one bank higher than the other. The stats on this engine are at the very top of the water jackets. In going through the troubleshooting process, with the stats out (and had the stat covers off to inspect flow on a test stand so to speak) it was obvious that at the lower speeds, keeping the lower bank full of water was hard enough and until you got up around 2000 or so, you couldn't get full flow out of the upper stat and you could clearly see that the top of the starboard cylinder was high and dry. I really doubt the poppet valve kicked in as with the stats out and covers off, there was essentially no back pressure to unseat it. Besides, best I can tell, the exit hose from the very top of the block is where the poppet gets it's feed and there "twerent" any water up there.

So after seeing that, I will not run an engine without one and if one has overheat problems, I'd be finding out the cause.

But that's my opinion and worth every penny you paid. Ha!

Mark