Outboard cooling: can someone explain?

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Inselaffe

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Thanks for the replies to the last post, I will be changing the impeller, it's time for it.

But I don't understand why or how a worn or damaged impeller can give such an intermittent problem.

Sometimes when I start the engine (Yam 9.9 4-stroke 1990) there is no tell-tale & I have to blow & suck through the tube, blip the throttle & generally @rse around a bit until it comes through. Then it is a very strong tell-tale which remains very cool & never stops again even if run for hours.

Other times right from the start the tell-tale is strong & I have no problems at all.

What I can't understand is how a worn or damaged pump impeller can give such 'on & off' behaviour. If my problems were solely due to the impeller surely the flow should always be reduced or not there?

From my sometime car mechanic & engineering background it just don't make sense to me. But obviously I am missing something and would be really grateful if someone could explain. /forums/images/graemlins/confused.gif
 
It is not at all unknown for the inlet pipe, before the water pump, to close itself off. This can be from flattening under suction or, more frequenly delamination of the inner layers of the pipe.

Why is this symptom variable? because if this is the right diagnosis, the flattening will react to the differing speeds of throttle adjustment and, possibly the temperature of the engine bay.
It is difficult to find because, when there is no suction the pipe appears to be perfectly normal. It's probably worth the very small effort of fitting a new suction pipe between seacock and water pump.
 
The pump rate of the worn impeller will remain constant. What is quite likely is that at some previous event the old impeller broke and left some of itself in the cooling galleries or some other foreign body has been sucked up. Have you had the motor since new? I had a second hand outboard once and I spent ages blasting my way through the system with pressurised water and then air. Still a sporadic tell tale. In the end it was just a process of using the engine a lot which brought all the bits someone else had sucked up to the tell tale outlet. Keep a piece of wire with you and keep prodding it gently up the tell tale. It is amazing how long a pice of weed, or even the bark from a stick someone has used to clear the tell tale can remain somewhere in the engine.
Maybe this is useful. I hope so.
Nicki
 
Thanks Maxi,

But the outboard water inlet is directly through the little grills down on the leg (which have been replaced & cleaned behind a couple of weeks ago & still are clean). The only rubber tube is that from the power head to the little plastic tell-tale exit (also cleaned) which is under pressure not suction.

The motor is also in a well & can't lift out.
 
That would make sense to me, I will have to get a portable compressor & try a blow through. Would some sort of nozzle on the pontoon hose-pipe give enough pressure to use water too?

I have also removed the thermostat (new one ordered) to see if that was sticking shut sometimes (although it opens in hot water to more than the 3mm stated in the Clymer manual, but as I have no thermometer I havent checked if it is around 75 C).
 
The pressure in the hose could well be sufficient. Just get a nozzle small enough to fit into the tell tale and blast that way with the impeller removed. The thermostat sounds about right. Just use the engine hard, but keep a sharp eye (hand) on the temperature. The tell tale is there to show that the whole system is circulating, it does not specifically mean that its not working will result in overheating.
Nicki
 
One thing I forgot to say is that 60 degree celcius is about the absolute max temperature the skin of the hand can tolerate. Gives you an idea of how hot 75C should feel.
Nicki
 
Don't know this engine but the fact that you don't get a tell tale straight away may be due to thermostatic control, engine gets hot enough and the tell tale appears. Your mucking around may be just the time it needs to warm up.

Between typing and posting the thermostat post appeared... apologies
 
Last year I bought a boat with a Yamaha 6hp twin (1994). When running it in my wheelie bin there was only an occasional drip from the telltale under the power head.
I rang the local yamaha agent. He said he had replaced the impeller on this engine 2 years ago and it would not be that. He said it would be corrosion in the waterways and to flush it through, and he was right.
It was not easy clearing the blockage. I used a large paint syringe over the piddle pipe (with the drain plug at the bottom of the leg removed).

After 20 minutes of pressure/sucking all the debris broke free!

The engine now piddles like a hosepipe.
 
Had exactly the same problem with my '91 Yam 9.9hp. Over an hour on a helpline trying to cure the 'problem', eventually traced to the various tubes leading to the tell tale. Took ages to clear the 3 different rubber tubes as the previous owner had never bothered. In the end I got a long reel of monel wire and ran it through all the pipes - the CRUD that was in there, salt, sand tiny bits of shell, mud etc etc. Faced with the same problem again I would just order new tubes and fit them.
 
Thanks for replies,

Good to hear of similar problems, I will try the hosepipe & poky things first & then compressed air if no luck.

I think I will remove the water jacket cover too & have a butchers in there.

Since I have removed the thermostat, I have started it about 10 times without any problems, but of course before the problem only showed itself in situations such as when the wind dropped & I was right down at the river mouth with a strong current taking me out to sea! /forums/images/graemlins/smile.gif
 
I can't see how blowing up the outlet can help - you're not going to be able to blow stuff right through the engine and out through the pump. On my Merc I keep a bit of thickish wire for poking up the jet when it stops. It always seems to shift the blockage and it works OK after. I think it's creatures or weedy stuff growing in the innards that cause the problem. It only happens after the engine has not run for a day or two.
 
My experiencee is with old Johnson 6, however the thermostat when closed (cold) still directs water out throught the exhausr an d also the tell tale. Thhe thermostat either bypasses the cylinder area or directs the water around all the channels around the cylinders.

The water pump is in the bottom of the leg just above the gearbox. It does like to be primed ie filled with water to pump best. The amount of priming it gets ie natural entry of water depends on the depth the elg is inn the water. This usually is dependant on the trim of the boat. So stern heavy motor way down gives quickest priming and pumping. But motor up high with bow down can lead to a slow start if at all. (dependinng on how good a seal the impelller is.) Check the wear of the case as well. Johnson/Evinrude sell a water pump kit which includes a new case and end plate.
But yes of course corrosion products will block up the cooling channels in the engine itself. I have had a lot of trouble with blockage in the internal tube from the pump to the engine.
good luck olewill
 
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