Outboard overheating, but not all the time.

[davez104]

Hi all,
Having a bit of an overheating drama with my 1990, 135hp V6 Merc. Running from idle up to about 1800rpm, it seems to be fine, the guage sits at about half. from about 2000 - 2800 rpm is where the problem is, the guage will climb, all the way to hot if you keep going. Anything over 3000rpm, the boat is planing cleanly and the temp again sits on around half. There appears to be heaps of water flowing from the telltale at all times, so I'm thinking that the water pump is allright. I think these motors have thermostats in them, is this right? If so do they just have one, or one per side? I was thinking if they had one per side then one side could get hot while still flowing plenty water through the other side. Maybe its a timing issue, is this possible? Any idea's on where to look would be great, if there are simple things to check, then I can do them myself and save on the end bill a bit.

Thanks.
Dave.

[ryank]

hey mate.

they have a thermo one each side if u look at the top of the head there weill be a little cover with about three bolts attatched to the head if u take those out and pull the guts out of the thermos and just put the ring back in it might fix the problem. so water will be flowing all the time. you will most probably need to put new gaskets on if you take them off.

good luck.

[Chimo]

May I suggest that before you pull it to pieces you check the temp of each head when the problem occurs. Then look further if one is hotter than the other.

Cheers

Chimo

[davez104]

Do you reckon I would be able to tell just by feeling them? Don't think I can do too much harm pulling the thermostat covrs off, I've had the boat in the local dam a few times lately chasing redclaw, so its been churning up mud and weed and stuff, I'm guessing that this is when the problem started. The flow from the telltale was a little slow, so I pulled the hoses off and flushed a bit of cr@p out, that got the flow back to normal, but did not fix the problem, I'm guessing/hoping that its just a bit more rubbish partly blocking one of the thermostats, but I guess it could really be caught up anywhere.

Thanks guys.
Dave.

[FNQCairns]

Yeah feel each head when it is at it's hottest to give you some idea.

If it were mine I would be replacing the waterpump - complete kit possibly (dont know how long since you last did the total job)

At low speed the engine makes low heat and the pump doesn't need to pump a great deal to cool, at medium revs but on the plane- medium heat but the pump is flat out cooling as water pressure has not taken over most of the pumps job yet, high speed water pressure does most of the cooling with the pump vanes sometimes laying over.

I don't know which type of pump the mercurys have but if it were some other engines the pump could easily be the culprit.

Then again you may have some engine buildup that needs the extra pressure high speed running to force flow which your pump would handle OK if there was no buildup.

Anyway I would go the pump, better to replace a bit early and know it is good as they are a basic maintence item.

The pee stream has no good diagnostic value so don't believe the pump still works OK because of the stream, a water pressure guage IMO is a must on all O/Bs.

Good luck,

cheers fnq

[INTIMIDATOR]

Hey mate
Not trying to be smart but how long ago was it serviced?
You realy need to take the leg of ALL motor's every 12 month even if you don't run the hrs up.Then atleast you can get the thing off when you need to,this also give's you a chance to clean & inspect.
Cheer's Payney!

[Roughasguts]

davez104, mate think you answered the problem your self, running around in muddy weedy water. Yup that will block things up.

So pull your thermostats, drop your leg stick a hose up the water in let pipe and flush your motor out, see how much sediment comes out the thermostat housings, You can use a air hose as well to float the debris up better.
Also check your thermostats by sticking them in water and heating it up see what temp they open at.

Hey while the leg is off change the water pump and housing, and stop running the motor through the mud.

[armoz]

Hey Daves
Had a similar type problem with the OB. Had the heads replaced and she is right as rain, cost about $300 to fix and sweet as......

[davez104]

Okay, just a bit of an update. I pulled the thermostats out, tested them in some hotish water, they open and close just fine. Put the covers back on without the thermostats so I could take her for a run just to make sure they were not the problem. Nothing changed, the problem is still there. I've ordered the water pump kit today, so that should be in in a week or so. I'll give it a good flush out while I'm at it, change the oil in the leg etc etc. I'm going to have a crack at it myself, never done it before but can handle most things mechanical. Anything in particular I should lookout for? I know about the possibility, (probability??) of coroded bolts, I'll cross that bridge if I come to it, but any thing else? Should I be able to handle the job on my own or should I line up a mate to help with lifting the leg off/on? Any special tools?

Think I'll be pretty right with it though, and fingers crossed it fixes the problem.

Thanks again for all the help.

Dave.

[Kleyny]

as others have said possible blockage in the line somewhere.
do as suggested earlier and you should see the junk come out.
nothing too hard removing the leg but I buy some kind of penetrating spray to help unlock any corroded bolts. (i normally spray them once or twice a day for three or four days)
once you get the problem sorted I would personally run the thermos again.

neil

[fishingjew]

Mate had a similar thing happen when I lent my boat to my brother one weekend after he went dreging with it had a 1988 135 merc after a few flushes and a couple of bukets of mud and three trips past the seaway with allarm still coming on managed to track down a nut? on the righthand side bottom of water jacket faceing the transom behind this nut? was a springthingy? that was full of mud once clean no problems ill try and find a diagram and post.

[Splash]

I luv overheating outboards .... ;-)

[fishingjew]

http://www.marinepartsman.com/Mercur...blies-299.html

EXHAUST MANIFOLD AND EXHAUST PLATE NO/ 14

7 to 16 pretty sure thats what I pulled out IT was full of mud .

[big moose]

Hey Dave I agree with the flushing comments but would most strongly recommend against leaving thermostats out. they provide flow control to allow proper heat transfer, faster water is not better

Good luck bm

[bastard]

I would cut the guts out of the thermostats,they are definately not needed here in australia,replace the water pump,if this doesnt fix the probleme the water jackets inside the head can block up with corrosion.