Way off topic Volvo diesel

[charlesp]

I have a mid seventies Westerly Pageant with its original Volvo MD6A angine.

The engine starts well, pushes the boat along nicely, and generally appears to perform well.

However when under load at more than, say, 1200 RPM the temerature rises slowly at first, then sprints off into the red. It'll then stay there until I throttle back. If I stick with 1200 RPM then it can oscillate, from cold to very hot, and back again.

There's no smoke, no steam, and plentiful lukewarm water from the exhaust. The engine is hot to the touch, but not scalding. You can touch it, but you wouldn't want to lift it. That's the cylinder head/block. The thermostat housing remains cool.

There is no smell of hot engine, no blackened paint, no crackling and ticking of hot metal. Just this indication that the thing is overheating, then cooling, then overheating again.

'Faulty temperature sender' said the man at the local Volvo agents. So I put a new one in (Genuine Volvo part) and it's exactly the same as before. 'Thermostat' was the next suggestion. While the one I took out was undoubtedly due for renewal the new one (again genuine Volvo at massive expense) didn't make any difference at all.

'Snip the wire or don't look at the gauge' came next (not from the Volvo people, of course). But I really would like to get to the bottom of the matter.

Any thoughts? Is this typical MD6A behaviour?

Oh - before anyone asks, yes it has a Seagull as its auxiliary. It is an early EFNR long shaft, and it pushes the boat along at around 3 knots without a murmer

[philj]

Hi there,
sounds like you have a screw loose. Sorry, not you, the engine. Not sure how good you are with a DVM or an AVO but it sounds to me that you may have a loose connection or an air gap in your electric string. When the rev's increase so does the vibration, hence the problem appears. I suggest that when you have a problem with the temperature pull the wire from the sender and check resistance to ground through the sender. If this is constant and not changing randomly then its a problem with the wire to the gauge. At your leisure check the resistance of the wire to ground with everything turned off. This should be open cct, wiggle the wires and if you get a reading the wire is shorting to ground. Look for insulation damage around metal cable clips and or loose connectors, or slave in a new temporary wire and see if the problem goes away. If it does then run the new wire along the loom permanently.
I have chased these problems all around boats before when a new wire would have fixed it much quicker.
regards
Phil

[Vic]

Produced between 1971 and 1975 its going to have its cooling water system pretty clagged up by now!

First change pump impeller if not done recently , check all pipework for obstructions and general condition (the inner wall can collapse and restrict water flow, check the water injection elbow on the exhaust ... they can get choked up but you do seem happy with the total flow so perhaps no great trouble with any of those.

I'd look at the water passages in the exhaust manifold that take water from the inlet connection on its underside to the cylinder head. They can get choked up. and cause the overheating you have seen.

The fluctuating temperature is a symptom of choked water ways in the cylinder block. Really dont know what you can do about that on an MD6. (Some other engines have individual cylinder blocks that can be removed from the crankcase.)

The thermostat cover stays cool because all the bypass water flows through it!

Owners manual and workshop manual available at http://www.bluemoment.com/downloads.html if of interest.

Might be worth posting on the Reader to Reader forum at http://www.YBW.com/forums

Check availability and prices or gaskets and part before getting too deeply involved

[charlesp]

It did have a complete rebuild - by a reputable firm - a few years ago. The Thermostat housing and surrounding bits look clear of clag, but I'll take up the suggestion of checking all the other pipework that's readily available.

I will sit down with the AVO this weekend and I'll try slaving a wire too, it does seem plausible that the change of reading could be an electrical problem in view of the rapid fluctuations.

Thanks for the food for thought. These marine diesels have the unfortunate property of being rather too bulky to get home onto the workshop bench; on the other hand I'm pretty sure they come apart just like their smaller brethren.

Next time I chip the clag out of a Century water jacket I will be remember that it could be worse...

[Vic]

in view of the rapid fluctuations.

You did not say "rapid" before.
The symptoms I was describing would be slow oscillations, slowly rising the falling again, perhaps over the period of half a minute or more. Rapid fluctuations are more likely to be a problem associated with the electrics.

Check the voltage on your system just in case you have a problem there. Has a Dyna-starter rather than an alternator and stater motor?

[charlesp]

After lengthy and arduous sessions with contortions, bruised knockles and the like I can report that:

The Electrics are fine, slave wiring produced no change, voltages all OK, Dynastart appears fine.

The Exhaust manifold was indeed blocked - at the root of the three way bypass. That was cleared, so also the small water way between the exhaust ports.

The Thermostat is new, so is the sender.

The impeller is fine.

The hoses and such at the gearbox are clear.

The oscillation - which was a few seconds either way - has ceased, but under load she still overheats. The exhaust water is now hotter than it was. I intend repeating all those clearing operations on the manifold, before taking the cylinder head off tomorrow afternoon to inestigate further.

It's not like a Seagull - you can't chuck it in the back of the car and put it on the bench.

[PeterW]

What condition is the pump housing in..? there is a tolerance IIRC for the impeller to housing in the repair manual as the impeller can be fine but if the housing is badly pitted then it may not be pumping properly.

Is the inlet strainer in decent nick - mine seemed ok, but was collecting rubbish very quickly. A mate redesigned his, I'll see if I can find the photos.

I find soaking anything that is furred up in Hagesan Blue, which is made by HG. I descaled a Seagull head with it (poured it in through the outlet) and I also use it on Landrover heads. It fizzes for a while, and certainly shifts the crud !

Cheers

Peter

[Collector Inspector]

I rebuilt a 5 litre GMH engine about 25 years ago and this problem is exactly a blow by blow symptom essay!

It was a head gasket, believe it or not.

Tiny leak thru a steel/glue shim gasket that was used , way back then, that pressurised the cooling system with air bubbles. Said air bubbles formed and gravitated to, of all places, the location of the temp sensor. High temps on the gauge but when touched, the block and heads were kinda normal.

Usually happened when cold ambient, not in summer. (Thermostat was WOT most of the time +30 Deg C ambient). Mind you this is an automotive application but had me guessing for ages. The gasket was replaced and the old thing did another 300,000 miles. Then the body rust ended it all.................

Just a thought Mate.

Regards

B

[Andrew Craig-Bennett]

I concur with Vic's diagnosis; my even older MD2 (1968!) displays the same symptoms.

Unfortunately, circulating descaler through the engine does not really do enough to make a difference, as by definition the most clagged up passageways are the ones that the descaler is least likely to get to.

Vic will know best but I believe that the only thing to do is to spring for a gasket set (at Volvo prices!) and then dismantle the engine and clean out the waterways manually, scraping the limescale off and then using descaler.

Encouraged by Vic's advice on PBO, I took the heads and manifold off and mucked them out a few years ago (gasket set £130 even then..) but did not have the courage to take the cylinders off, and the problem has recurred.

It is probably a good idea to replace the core plugs at the same time...

I am not any sort of an engineer and I chicken out of the thought of taking the cylinders off, replacing core plugs, etc.

Every professional marine engineer I know says "don't bother - fit a new engine" and I am currently contemplating this.

At the moment I have an engine which will run for ten minutes before melting the Vetus waterbox...

However, I reallhy can t afforrd a new engine, so am torn between sending good money after bad trying to get the old brute to go or saving up for a new one.

[charlesp]

Well it turned out to be a mixture of things, as ever.

The thermostat was a relic, as suspected, but a neww one made no difference. The bypass was clogged with fine clag, which was easily removed. That shifted the problem. The exhaust manifold had three holes blocked, all ,of them feeding water into the block. My mistake was to leave the third one blocked up.

Blockage now removed, new exhaust manifold gasket fitted, Runs fine, no overheating, good for a few years.

Thanks for all the tips.

[Andrew Craig-Bennett]

Bravo!

Most encouraging. I'll have another go at the manifold.

[rosbullterier]

I've just spent the past two days hammering, drilling and chipping rust and crap from the waterways around the cylinders in a block of a V6 GM.

Various shaped chisels were made, 10" long bits were used, sundry bent old bike spokes employed and two pairs overalls gone through. I wanted to avoid core plug removal because all new bits come from America; although parts are cheap, postage is horrendous.

Having spent many months on the boat before starting the mechanics, the idea is restore to good working order. There are several forums specialising in the specific mechanics (OMC) and sterndrives and funnily enough the most questions involve overheating.

I don't want to appear a smartass - but - with the flow space around the cylinders completely clagged up, it really takes complete dismantling and tedious chiselling out. This engine is 1981.

Now there's the other to start on.

[timberman2004]

Ummmm....ever thought of investing in a BRITISH seagull ???

[rosbullterier]

Do they produce 170hp (each)?

[charlesp]

Do they produce 170hp (each)?

They almost do if you believe some of the vendors on eBay !