Mercruiser Overheat

enterprise

enterprise

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Hi all,
Looking for the usual good advice.

Have standard 4.3lx mercruiser, 205hp.

The last time we ran her she overheated.

The problem is that we are having trouble finding the cause.

It overheats when under load, ie when planning at around 3500 rpm, below this at 1500 rpm the temp stays constant at 140-160'.

We have changed the stat, checked coolant flow from the impellar & removed the water pump for inspection, they are all working fine.
There is no water in the oil & no noticable loss of power

The exhaust note has changed noticably, now more throaty & all efforts to trace the the problem have upto now failed.

Before we have her lifted are there any more checks that can be done?

Lifting to remove drive & exhausts is going to be expensive if we find nothing.

Hope you can help.
Thanks
 
Change in exhaust note may indicate the exhaust is running partially dry (ask me how I know), so if everything else seems OK I´d assume the exhaust elbows (specifically the water injection nozzles) may be partially blocked, hence not enough water flow, hence overheat and exhaust note change.

Now this could be due to either foreign body (impellor failure recently??), or salting up (how old are the elbows/risers?).

If the water outflow is restricted then be VERY careful as the "something has to give" will probably be a hose leading to water in bilge and no flow down the exhaust (again, ask me how I know /forums/images/graemlins/mad.gif)
 
agree but would start with the heat exchangers then the elbow / risers. wouldn't the latter would tend to give overheating at even the lower running speed even if a little localised?

why do you need to lift out for this?
 
Low water flow would also be my first reaction to a change in exhaust note. Risers/elbows are pretty simple to check for blockage if you've checked everything else. Get a maintenance manual from Mercruiser, if you haven't already.
 
Things I forgot to mention in my opening post.

The oil pressure risses beyond the norm as the temperature starts to rise, the manifolds are 1 piece with no risers.

They get very very HOT! .

The boat is 1996, probably original manifolds but our mechanic seems unconcerned with there age!

What puzzles me is why there are no problems at 1500 rpm.

Another thing, when we are on the plane at 3500rpm the temperature rockets to overheat, if we throttle back to displacement 1500rpm the temp drops to normal opperating almost immediately.

We really don't need this problem all summer & beyond throwing silly money at it we are scratching our heads.

Do the symptoms sound familiar & what was the eventual cure?
We are waiting results of a compression test & have been told head gasket failure could produce simillar symptoms
 
enterprise, you should be able to take the manifolds and risers off with out lifting her out, worth doing before you need to left her to check further down in the cowhorns and drive exhaust passage - although the "pipe" is so large from there that I´d be surprised if it could be blocked.....
 
Thinking tops of "Y" cowhorn might be too close to waterline?

Also oppertunity to drop leg & check impeller, even though new one fitted durring pre season service.

Rise in oil presure ?
Any thoughts on shutters failure & exhaust restriction from them droping into Y pipe? could this restrict free flow of exhaust & cause the pressure rise?

All sudgestions are greatfully recieved.
 
If the exhaust gas exit was restricted it would kill the performance.

1996 is possibly old for the risers, mine ais a Volvo variant on the same block, seperate manifolds and risers but I had to change both.

Everything points to blocked riser water jets I´m afraid! Top of cowhorns shou.ld be well above the waterline I would have thought.

No overheat at 1500rpm as the engine is probably producing 1/10th the power that it produces at 3500rpm, so even the restricted water flow is enough to cool the engine.

Consider changing your engineer...... killer symptom is the change in exhaust note, this firmly indicates lack of water down the exhaust, and in that case it either can´t get in (impellor or blockage) or can´t get out (riser jets).
 
If the head gasket had gone you would have bigger running problems, but have you over looked the electrical side of things i.e lose connection or a dodgey temp sender switch???
or maybe when the engine is craked up to 3,500 rpm the impeller is slipping on the drive shaft make sure its the right impeller.
 
[ QUOTE ]
Rise in oil presure ?

[/ QUOTE ]

I don't know what could cause a temp-rise related RISE in oil pressure, this could be indicative of a dubious voltage regulator on the instruments - in which case they would probably all over-read.

How did you check the raw water output with the impellor in the leg? Just because it was OK/new before the season doesn't mean it is OK now, you may have picked up a bag temporarily over the inlet and lost some of the impellor lugs (and the bag then innocently floats away...) . It could be difficult to tell the difference between some water being pumped up the pipe when tested on the starter and proper output @3500rpm.
 
AAAAAHHH!!
If temp is OK at 1500revs but rockets immediatly on full revs, just a thought.
How does the water pickup enter the hull?
I once had exactly this on a volvo leg.
The water pipe on the leg had sheared off with the effect that at displacement speed the intake hose was under the water surface and able to suck water, result all OK, but once lifted onto the plane the hose was above water surface with the result of an instant overheat.
I dont know how the water gets into the hull on a merc setup but it might be worth having a feel around to check before going to the expense of lifting. Exhaust note more throaty would also indicate lack of water.
 
Had a similar problem with a volvo many years ago. I eventualy traced it to the cooling water hose between the outdrive and the mounting collar which had become detached.At low revs and low speed the hose was below the water and picking up direct and not through the outdrive. at higher revs and planing speed the hose was clear of the water and sucking air. I am not familiar with the mercruiser set up but it could be a similar problem that your experiencing.
 
Been there - done that

I have two of these motors. The usual problem is blocked risers. The risers have a defined life of 5 years and no more. Because its raw water cooled, the inner channels block up with large flakes of rust. To remove the risers is easy. Unbolt the 4 bolts that hold each of them on and undo the two SS worm clips that feed the risers into the rubber exhaust pipes. You should find the water passages into the risers blocked. Don't waste time trying to unblock them, chuck 'em and buy new ones. You may also see that the corresponding water channels in the exhaust manifolds are blocked. Remove the manifolds and use a masonery drill and copious amounts of water to remove the scaling right into the passages. Rod out either ends of the rubber stoppers (after removing them!). If the lands in between the water passages look thin and corroded, take the fannimolds to a precision grinders and have 1-2mm ground off to create a new gasket face. Don't worry - it works fine. I've done it.

By the way. Take a looka t what I'm talking about in the diagram below

RiserMerc.JPG


And you can read more about it HERE

You can do all of this without having to lift your boat out.

BTW - have you had any bust hoses yet? Or hoses popped off?

Good luck and let us know how you get on.

Dave
 
The general rule with raw water cooled Mercruiser and Volvo petrol engines is that the risers are good for 10 yrs max, over that is a bonus, the thing is they will corrode whether the boat is used or not.
So it sounds as if yours are due for replacement, have experienced this many times as a Mercruiser mechanic, at low speeds there is just enough flow to cool the engine, but as rpm therefore heat produced increases, the water flow doesn't increase to match. So the small amount you have circulating has to work harder, also if the risers are blocked the water stays in the engine a lot longer than normal so it gets even hotter. Hence the rapid overheating you see on the instrument.
When you say you checked the stat, did you check at what temp it opened, my mate had this problem with his 330 hp Merc and said he had checked the stat, what he didn't realise was that the stat was opening, but at over 100C! He went through all the things you did and was frustrated to find it was the actually the first thing he checked but assumed was OK.
 
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