AQAD41A Overheating problem

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brp

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I have twin 1989 aqad41a's on 290dp's which have been running very well.Recently the port engine just started overheating under load but as soon as i throttle back to around 1600rpm the temp comes down and sits at normal reading.Tried all the usual suspects, seawater pump,heat exchanger, flow restriction, exhaust elbow and even flushed acid through the system to try and clear any build up of deposits.Mechanic is now pointing towards cylinder head gasket although i have no loss of power, no white smoke or steam, no signs of water contamination in the rocker cover.The freshwater level has started to drop recently and the mechanic did find the freshwater pump slightly nleaking through the seals which he thinks may be getting blown out due to exhaust gases getting down into sump and causing this hence his pointing to cylinder head.I had a compression test done a few months ago and all was fine.
Would appreciate any help/advice on this matter as i am still convinced it is a flow problem
 
Think you have just about covered the usual suspects,presume the heat exchanger matrix was was removed and pressure checked and not just visually inspected ?
 
The matrix has not been pressure tested it was just removed and inspected by mechanic.A new one was installed about 18 months ago on that engine
 
FWIW,
Did this just start to happen or following work / service ??
Saw an almost identical to this which was resolved by re-fitting the heat exchanger end cover at the correct position. It had been re-fitted at the wrong position and was only 50% working so would run ok at low revs but overheat when pressed.
Beyond that I'd be looking for an obstruction somewhere. Debris in leg intake or a blockage restriction there perhaps - bit of plastic caused similar briefly for me in July last. Assume you've checked intake side before pump ??
Have you intercooler matrix also ??
 
The problem just started as i had been using it daily in July for around 2 weeks and then next day as i left the marina and put it under load the temp came up.At first i thought a plastic bag or similar had caught around the leg intakes but checked it out and all clear.We took the hose off before the pump and checked for flow and it was giving full pressure so no obstruction there.Seawter pump then checked for impellor damage but all ok.After that eliminated usual possibilities but still no change
 
Its clearly only happening when under load / high revs so agree the split leg input pipe would cause it - collapsing under high suction. Same would be for internal engine hoses. Have heard of the rubber pipes breaking down internally. Are your engine hoses original ???
 
If exhaust gasses were in the sump, it still would not put pressure on the water pump seals as its a closed circuit, so be carefull in what hes saying there., the 41/42 series engine can overheat due to any small thing at high load/rpm figures, as its a sterndrive once on the plane only the bottom of the leg is in the water as the inlet hose is then in fresh air, air will be sucked by the pump and not water, also the elbow on the h frame may be rotten away, or if its the engine that has the power steering pump fitted, it may be blocked.

If the head gasket had gone, it would not start very well as the 41a has a low compression and needs all 6 cylinders to start well when cold, the later 41b series ran a higher compression, but made the transom soot up, where as the a series always seemed fine, ive owned boats with both engines in.

If the liner seals had gone there would be water in the sump, sump level would rise etc.

I would go for the oil cooler as its first in the line, then intercooler then heat exchanger, if the boats uk based pm me if you need help/ me to take a look.
 
Not sure if there is a rubber pipe on the inside of the leg but i would say that alot of the hoses are probably original.I have replaced perished fuel lines but not any of the cooling lines.
I am also not convinced about the cylinder head because i do not lose any power getting up on the plane/top end and there is no water contamination in the oil or no visual signs of white smoke. .The freshwater tank has only recently started losing water which may be from the leaking seals in the pump but prior to that when it first overheated and i checked the level in the tank it had not dropped.My boat is based in Spain and mechanic has asked me to send down replacement freshwater pump.I am still convinced this IS a restriction/flow problem
Really appreciate all the help/advice that you are giving me
 
I have the same engines and legs and also see an overheat on one engine - seemingly proportional to engine revs.

Yes, I am a newbie so apologies if this comment is inane - but it is the case on mine! - have you checked the guages? Mine reports it's overheating but the engineers infra red probe disagrees - in my case it is simply the guage is wrong. I've got the port engine to give sensible readings after cleaning lots of connections. Sadly hasn't helped on stbd.
 
Unfortunately it is not as simple as that, although i wish that was the problem.I did experience exactly the same as you when i first got the boat as 1 engine runs hotter than the other and we checked the temp with infra red gun also but this is a different problem which hopefully will not prove too costly
 
Check out all the suggestions above, I've suffered from all of them and had two new oil coolers a few weeks ago. Also check the silly pipe from the header tank, it's held on with one screw and just an O ring on each end. How it dosn't leak all the time beets me.
 
What temperature is the engine rising to? Does it illuminate the over-temp light? If gauge and light, it's probably real; if not, it might be the gauge sensor; easy to swap with the other engine.
 
Just read your post and as just mentioned by Viscount thought about what is telling you about the high temperature, is it the temp gauge or high temp alarm, the reason being is because they come from different sensors and I have known the high temp switch to play up and had to be replaced.
Also as already mentioned, the water intake elbow on the outdrive, if it is an alloy one they corrode and become blocked or eventually break off, whilst going slow it is in the water however as soon as you open up and on the plane it does not allow full flow of water through (sometimes not at all) hence the high temperature.
 
I had a similar problem on my kad43's a couple of years ago although it was shortly after replacing the impellers. Called seastart who squirted a bit of washing up liquid down the raw water strainer onto the offending engines impeller and the problem was solved. It turned out to be the impeller just hadn't made a proper seal and wasn't pumping fully.
 
Hi brp. I had the same setup on my previous boat and exactly the same symptoms you describe i.e. only overheating when RPM was 2000RPM+ as soon as you drop the RPM's below 2K the temps went back to normal.
After a lot of poking around and trying to find the fault, I eventually stripped the cooling system apart and descaled it all (and I dont really think I needed to as the scaling was not too bad) but in the course of doing this I found very small pieces of impellor that must have been stuck in one of the cooling pipes U bends. Now even though I couldn't find this when the boat was in the water, it was only when I took the pipes home and they had thoroughly dried out and got bashed around in the back of the car did these little piece of rubber eventually fall out......and the overheat was solved. Even though these engines have sea strainers, mine were before the impellor so they would stop weed but not the impellor pieces (**** setup).
Other things to suggest are sea strainer seals not there, crack in covers from overtightening so sucking in some air. After cleaning I always filled up with water again to prime water pump and smear some silicone grease on plastic thread.
Water pickups from outdrives blocked. Water pump not pumping properly. I heard of inspection plates getting worn, so I flattened down with wet & dry to ensure better seal Water pumps not expensive (I think its JAbsco @ £250 so good to have one as a spare to try
Lots more stuff to go through its just time consuming but you need to be methodical to work through the whole system to find the prob
 
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