Volvo MD7B donkey not

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SteveB_Sigma33

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A friend of mine also has a Sigma 33 but her engine seems to be losing power. Basically if we have our boats motoring along together she pushes along at about 4Kn and mine at 6Kn (Unfortunately she doesn't have a rev counter but it sounds like it's working at between 2 2.5k)!

The problem seems to occur after running for approx 20 mins.

She has had a friendly diver give the prop, shaft and P bracket a good looking over and all seems sound.

A mechanic friend of hers has disconnected the temperature sensor (advised she should replace this ASAP) but I'm wondering if any distortion in the head could have taken place due to overheating!

Has anyone seen or heard of any engine ailments with similar symptoms before we strip the bloody thing down?!?!

All comments thoughts etc welcomed.
 
As th engine is a Volvo MD7A it will be 12HP but will also be nearly thirty years old so I very much doubt if it is pushing out anywhere near that power. Sounds as though the engine is tired. Rebuild costs on the MD7A is horrendous, check the cost of spares then take out a mortgage ! The old Volvos MD7A included usually run at about 2000 rpm max if all is well.
 
Not enough information in this post for rational analysis. have I missed something?

Why has the mechanic disconnected the temp sender? Surely it won't make the engine run better or worse, simply remove one level of safety?

An MD7 seems a bit small for a Sigma 33 which must weigh all of four tons. MD7 should give 13.4hp at 2,600 rpm. Don't Sigmas normally have at least 18hp?

The MD7 seems pretty tolerant of overheating in my experience (but no mention in your post of why overheating is the suspect in the first place). Although Volvo parts aren't cheap, it's unwise to dismiss the idea of an overhaul out of hand. New bearings, shells, rings, valves, et,c would of course be prohibitive; whipping the head off for a clean-up and checkover would not be.

But I'd check everything else first, including clean filters, which is the number one suspect in my view if you are losing revs. Tank full of bugs would give just the symptom you seem to describe.

best of luck!

Regards, Mudhook
 
Let me add some detail

removal of temp sensor - apparently because it beeped straight away!! My 2002 did this and I simply replaced the sensor for 45 quid. So I'm concerned then engine has run for the past season without a way of indicating if there is a cooling issue!

This is a 1979 Sigma33 as you stated yes they did tend to come with Penta 2002 engines pushing out 18 Hp but it seems they must have used the MD7Bs at the begining of the run.

All fuel filters appear good.

There has been some vibration hence why she had the P brackets etc checked.

I'm going to checkout the gearbox/shaft coupling this weekend but wondering what else I can check whilst I'm there.

Regards the head build I've just completed that on my own, was hoping not to have to do another one! Two in one season!!!!! Fun /forums/images/graemlins/frown.gif
 
MD7B is 17hp. glazing? rings? I had the engine out, top rebuilt by a local company in the Chichester area about £600. a couple of years ago. /forums/images/graemlins/crazy.gif
 
Is the Exhaust Injection Elbow choked up with carbon etc?? This will cause loss of power but usually accompanied with black smoke. It would pay to remove and inspect on the bench - clean up or replace if totally knackered.
 
Viking is right: the MD7B is a more powerful engine. They are quite rare compared to the MD7A.

Key symptom you describe is the "after 20 minutes" thing. Have you checked the daft things like air vent to tank blocked? Or throttle control vibrating from full open? It shouldn't take 20 minutes to warm up under full throttle.

Exhaust elbow is another suspect (except 20 minutes above). I've just taken mine off and the difference between the crudded-up one and a new one is remarkable. However black smoke would be expected with this in practice (which I've got!).

I think I'd have tested the exhaust sender as soon as I took it off. The water spaces are very cramped and restricted flow is easy to acquire. The engine won't always go bang but it's supposed to operate at around 60 - 70 degrees. I swapped my temp alarm for a gauge and put an alarm on the exhaust for safety.

Let us know what you find; us MD7-heads need all the help we can get!

Regards, Mudhook.
 
An outside chance this but easy to check. I have seen several cases where the rubber exhaust hose delaminates and closes down inside restricting the exhaust. You cant see it from the outside as the re-inforcement running through the middle supports the outer...... Not often seen but something I always check if an engine has unexplained loss of power and/or is difficult to start....
 
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