Engine overcooling - and undercooling

[gunnarsilins]

I had this question mailed once before, with no good response, so I´ll give it another try.
My sailing yacht is powered with an Perkins 4.236 diesel, 24 years old with nearly 3000 hours on the clock. The engine is fresh water cooled from the factory. Starts and works perfectly apart from some cooling problems.
When running at cruising speed, appr. 1700 rpm, the temperature gauge shows a little above 90 degrees – perfectly normal.
When going faster or slower temperature changes in proportion to the rpm´s. 2200 rpm = 105 degrees and when idling around 70 degrees.

I have dismantled the entire cooling system, both heat exchangers, exhaust manifold, all tubing, everything. Throughfully cleaned tubestacks and everything else. Checked the thermostate, and put a new one in, just to be sure. The raw water side is dismantled and cleaned as well and the flow is normal.
The only non-original item in the cooling system is a calorifier plugged in to the system.

Anyone got an idea? How can an engine overcool – and undercool at the same time if it´s not the thermostate???
All suggestions appreciated.

 

[Guest]

Hi

Must say it sounds like the stat/valve, dunnit? Is this perkins the type with a single wingnut screw down on the primary raw water strainer? I think that they should have two o-rings, one in the perspex(?) cover the other in the metal groove belwo, but one that we had had only one. That too over and undercooled, resulting in (eventually) an overheat in the channel, temporarily fixed with makeshift seal in one of the grooves to mate with the o-ring in the cover.

 

[ccscott49]

See my reply in the PBO forum.

 

[gunnarsilins]

I´m not sure I´m with you....
What do you mean with "stat/valve"?
But regarding the raw water strainer, the yacht had when I bought her, a combined seacock/strainer in brass which was quite awkward to open and clean from weed etc.
So I rebuilt the intake with a new strainer of Vetus make, this has one size bigger in/outlets compared with original. The through-the-hull/seacock and associated tubing was rebuilt to this bigger size as well.
So I can hardly believe the problem originates from the flow of raw water. Besides, if that were true, my common sense says this should manifest itself by overheating and not be the source for the temperature to fall at low revs.

 

[ChrisP]

Both my engines run a little hotter when pushed to the line but normally the cruise at around 80C. Spoken to Mermaids and they recon this is about right for most diesels. Time to worry is when the engine produces more steam than water from the exhaust. Best not to run at too higher temp as it will affect the viscosity of the oil but should be ok for a short time. You could look at the oil system check for sticky bypass valve on the engine, as I believe on some engines leak by will bypass the oil cooler and pass the oil straight back into the sump. Elevated temp in the oil will cause a rise in engine temp. Try measuring the oil temp in the sump either with long thermocouple in the dipstick tube or surface type thermometer on the sump. You could also check the lube pump pressure and more important the flow rate off the pump. 3000 hours on a diesel is nothing really but the oil system does pick up a lot of gunge and combustion products so a good flushing with a good quality flushing oil could help. Refill with a good oil not the ASDA special offer type. I know this sounds daft but in a diesel the oil system makes up a good part of the cooling system and good oils certainly make a big difference.

Thats my lot so

Good luck

Chris

 

[boatone]

gunnar - do look at ccscotts reply - he may well have what is as so often in these cases the simple remedy before you go looking for more complex solutions.

TonyR
boatone@boatsontheweb.com

 

[Piers]

The starting point has to be - "is the temp gauge reporting accurately?"

A simple way to verify this is to borrow (or hire) an (infra-red?) thermometer from your local marine engine repair business.

With the engine running at different power settings, simply point and click at the same point on the engine and you will get an instant readout. Literally point and click - works up to a metre away.

Did it with my engines and it worked 'perfick'.

Piers du Pre
MBM Cruising Club enthusiast
www.dupre.co.uk

 

[gunnarsilins]

ChrisP!

If I run the engine flat out (2400 rpm), which I only tried a couple of times very briefly, I got a slightly "whitish" exhaust. Probably steam. But there is no significiant difference in colour within the tickover to 2000 rpm range.

I will definetely look into your suggestions regarding the oil system. I have no idea what so ever which temperatures I might have in the oil.

But this won´t explain the decrease in temperature when idling!?

 

[gunnarsilins]

Piers and Boatone

Please take a look on my reply to csscott in the PBO-forum regarding suspect senders/gauges

 

[ChrisP]

Re: ChrisP!

The coolers and pump will have been sized to remove the heat from the engine when running at its maximum load. As the manufacture of coolers an pumps for each engine size would be cost prohibitive they use stock sizes. Usually using the next size up from the calculated heat transfere requirement. This means that at idle the exchanger is actually oversized. The effect wouldn't be enough to cause the thermostat to close but would be detected on a temp. gauge. One of the biggest problems on the river is that big engines are not running hard enough to overcome the efficiency of the coolers and come up to the full running temperature. This causes the cylinders to run cold producing inefficient combustion and smoke. The most common thing you hear in the marina is "as soon as you open her up the smoke clears". There is no special cure its only the engines coming up to temperature. Idealy you would have one set of coolers for the river and one for the sea. The difference is only a few degrees but the effect is remarkable. when manufacturers rate their engines they rate them for all day use or hour use. The difference is the size of the coolers to keep the engine running at the correct temp. Smaller coolers means less weight and a better power to weight ratio which looks great on the adverts but means you can only run flat out for a restricted period.

ChrisP

 

[gunnarsilins]

ChrisP, again!

So your verdict would be, the low temp when idling due to an oversized cooling system and the high readings when running hard due to high oil temperatures.

The thing with oil can be investigated and sorted out, but I´ll not get an other heatexchanger for river use!

What is you opinion here, sometimes I am forced to go for several hours on very low speed, in rivers and canals for instance. Will this hurt the engine when considering the low temperature? Or can I just forget this part of the problem?

 

[ChrisP]

Re: ChrisP, again!

Rather than install another cooler you could install a temp controlled by-pass valve between the pump and the existing cooler. This would open when the engine is running cool and reduce the amount of cooling water through the cooler thus allowing the engine temp to rise. Setting it up could be a bit fiddly but it would be effective.
In all honesty by letting the engine run cool the only problem you are likely to have is a bit of smoke and more combustion deposits which shouldn't cause a problem if you change the oil and filter regularly. Cool engine is fairly good, hot engine is very bad, warm engine is best of all.

ChrisP

 

[ccscott49]

Re: ChrisP, again!

Isnt that what the thermostat is meant to do? But of course the thermostat can only control the temperature over a range, maybe the temp range of the thermostat could be tightened up, I have never seen an adjustable thermo stat, but tighter ranges I have seen. Rolls Royce used to have them in military engines.

 

[gunnarsilins]

To everybody assisting me...

Please go to the PBO forum and read my final posting in this issue under the headline "To everybody assisting me"

Thanks a lot for your time and help!

Gunnar

 

[Guest]

Check your skin fitting , some skin fitting have a small lip ( ie a bit that is raised) which should be aft ,so that water is forced up the sea suction as the vessel goes ahead. if this is not fitted correctly you will have overheating as speed of vessel increases. (Venture effect ). You could fit a small scope to help push water up the sea suction, as I have done this to my friends boat who had a over heating problem and it solved it.

 

[ChrisP]

Re: ChrisP, again!

Thats what it there for but unfortunatly the hysteresis of a wax stat is too wide to operate over the small range of temperature that we're looking at here. It probably starts to open at about 60C and at low revs will over cool the block. You can buy externally sensing temp valves at most good plumbing merchants.

 

[ccscott49]

Re: ChrisP, again!

Didn't know about the externally sensing valve affair, interesting, I'll stack that away in the old memory banks, thankyou.