diesel engine heat alarm at low revs

[dylanwinter]

harmony has a Volvo 2002 - two cylinder 18 hp - about 15 years old

she is a slow starter - takes about ten seconds of turnover from cold

runs very smoothly

at tickover tiny amounts of water coming out the exhaust pipe

she overheats if on tickover for about five minutes

maybe three minutes on tickover in gear

but a few seconds of high revs gets the water flowing in fine form and the alarm goes off

weed catching thingy is clear

I swopped out the impeller for an older one - seems better but same symptoms

should I worry?

yours

deliberating Dyl of Littlehampton

 

[RAI]

Yea, do worry. It shouldn't happen. Maybe the heat exchanger needs cleaning out.

 

[Nigel-in-Oxford]

First try a new impeller.

 

[Tranona]

Put a NEW impeller in as first action. Check also the cover is not worn and that there is not a small air leak in the pipework. If waterflow is good at running revs then heat exchanger is probably OK (if it is fresh water cooled!). Poor flow at low speeds is usually wear in the pump or impeller or sucking air.

Are you following the proper cold start regime?

 

[rob2]

Some water pumps are prone to wear of the cam which compresses the vanes and thus builds water pressure. It may be worth seeing if the workshop manual gives a checking dimension for this. Again, some pumps have replaceable cams and some don't and even then, some can be fitted with different sized cams for varying applications. I recall that one engine manufacturer fell foul of this and fitted a smaller cam than required for a while with brand new engines overheating. The other common(ish) pump fault is for the bonding between the vanes and their hub to break down so that the vane doesn't always turn!

If the pump is working correctly, then an obstruction could limit the flow, particularly at low revs when the flow is already minimal. The most common, beside kinked pipes, would be build up of deposits in the exhaust injection elbow. It is fairly easy to remove the elbow to check, but often removing the deposits reveals terminal corrosion - the elbow has to be considered a consumable item.

Another, and more fiddly to access, culprit would be the heat exchanger. Disassemble the tube stack, rod through the pipes - a brazing or welding rod may fit -and soak clean in vinegar, much as in descaling a kettle.

Rob.

 

[Skylark]

Overheating after 5 minutes idle at no load or 3 minutes with a bit of load is indicative of a problem albeit resolves at higher engine speeds. A thorough and systematic check / clean of the cooling system would be my approach.

Perhaps tearing the engine out and replacing with an outboard may be preferable to you :encouragement:

 

[AngusMcDoon]

Perhaps tearing the engine out and replacing with an outboard may be preferable to you :encouragement:

Do you think a Tohatsu 10hp in a well would suffice?

 

[vyv_cox]

The cooling water path in these engines is something of a mystery that has had some discussion on the forum. This pic shows the layout.

Unlike other engines there is no diverter valve to push water through the thermostat after cooling the engine, in preference to passing straight through the tube to the manifold where it does no useful cooling. It seems that cooling depends upon a small restriction at the end of the brass tube but this cannot be excessive or the back-pressure before the thermostat opens would destroy the impeller.

Known problems are that the perforations in the brass tube become blocked by salts. It sounds as if that is what is happening in your engine. At tickover more of the water is passing the restriction at the end of the tube rather than going through the perforations, so the engine is less well cooled. Once the pump output increases the flow through the perforations increases and all is well.

I would not worry a lot at present but if the situation deteriorates it is fairly easy to solve by filling the coolant circuit with a chemical that removes the salts, such as Rydlyme http://www.rydlymemarine.co.uk/

 

[dylanwinter]

indubitably

Do you think a Tohatsu 10hp in a well would suffice?

I am sure it would be a great alternative

D

 

[AngusMcDoon]

I am sure it would be a great alternative

D

Should we start a discussion?

 

[dylanwinter]

Should we start a discussion?

love to

feel free to start one

I myself am a bit busy fettling an old diesel - while the back-up Tohatsu waits quietly for the time it is needed

 

[Blueboatman]

Can you leave it to run for half hour flat out, get the water and gubbins up to working temp, might self clear?

And this engine was professionally serviced/layed up eh? Any advisories?

Perhaps time for an honest discussion with the previous owner as to All Known Faults. No skin off their nose, is it..(?)

Edit: On a car I would say remove the thermostat next and let er run hard n hot too..

The slow to start thing might be first attempt after winter..slight air somewhere that self bleeds? Heat plugs? ( Cant remember, and have used a brand new shiny V2003 for a whole season too ahem)

 

[robmcg]

You mention it being a slow starter too. Are you doing the Volvo cold start procedure for the engine which makes a big difference to starting from cold. As for the overheating, I am in the camp that says check the pump housing and cover plate for scoring and wear. If the plate is worn, try fitting it inside out and see if the problem goes away.

 

[sailorman]

You mention it being a slow starter too. Are you doing the Volvo cold start procedure for the engine which makes a big difference to starting from cold. As for the overheating, I am in the camp that says check the pump housing and cover plate for scoring and wear. If the plate is worn, try fitting it inside out and see if the problem goes away.

also check the wear plate in the back of the pump, remove the impeller & the pump cam to get it out

 

[VicS]

I swopped out the impeller for an older one - seems better but same symptoms

should I worry?

yours

deliberating Dyl of Littlehampton

There is only one way to deal with old impellers.

chuck 'em the fecking skip

buy two new ones. Fit one, keep one as a spare.

while doing the impeller consider inspecting bearings, replacing seals, inspecting shaft for wear, cam plate, pump body and cover plate. Now is the time to do it all , not miles from anywhere off the Scottish coast.

also consider the merits of a "Speed seal " cover

 

[corinthmistral]

Had exactly the same problem on my volvo 2003 - same as yours with an extra cylinder. As suggested, check / change impeller first. If that doesn't fix it clean out the heat exchanger. Mine had some growth and barnacles in it which we're restricting 25% of the heat exchanger pipes. Cleaned it out and it has worked perfectly since. On my engine the symptoms were an alarm at very low revs or very high revs. "Normal" revs somewhere in between were ok and the alarm didn't sound. Hope that helps.

Edit: nb my previous boat had a volvo md7, same symptoms. I took out the thermostat which was gunked up and seized and it ran perfectly. In the end I binned the thermostat and never replaced it. Worked perfectly for 5 years till I sold her.

 

[BarryH]

Before you go spalshing out on spares and getting involed in engineering work measuring clearences and cam runout etc. Check that the strainer lid isn't leaking air in. Start with the simple stuff first is my motto..........

 

[sailorman]

Had exactly the same problem on my volvo 2003 - same as yours with an extra cylinder. As suggested, check / change impeller first. If that doesn't fix it clean out the heat exchanger. Mine had some growth and barnacles in it which we're restricting 25% of the heat exchanger pipes. Cleaned it out and it has worked perfectly since. On my engine the symptoms were an alarm at very low revs or very high revs. "Normal" revs somewhere in between were ok and the alarm didn't sound. Hope that helps.

Edit: nb my previous boat had a volvo md7, same symptoms. I took out the thermostat which was gunked up and seized and it ran perfectly. In the end I binned the thermostat and never replaced it. Worked perfectly for 5 years till I sold her.

no thermostat will allow the engine to take for ever to get to the designed working temp causing wear

 

[dylanwinter]

the heat alarm seems to get more infrequent

I do know that the boat hardly moved last year so my plan is to motor her a a fair bit on the way round to Chichester and see what the symptoms are wile she is under way under a proper load

but only after checking the strainer

I have had the lid off but I will have a look at the seal

the engine has a speed seal on the impeller housing so that helps a bit

before dismantling stuff I will consult the mighty Dave Kay

https://www.youtube.com/watch?v=1OHj60qNyqQ

D

 

[Colvic Watson]

Put some Vaseline on the lid thread, it helps getting the strainer lid on and off but it also helps seal it.