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Old 03-05-2015, 14:12   #1
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md7a losing power & cutting out

To cut a long story short...every time we motor into waves the engine slowly starts to lose power, (after roughly 20minutes) cuts out within a minute and then the overheat alarm comes on. After a few minutes it started up and ran again. Checked impellor, water pump">raw water pump, raw water outlet and for smoke...all appeared fine. We never had a problem motoring for hours on flat water.

Initially I thought it was an issue with back pressure because the exhaust was blocked but opened it up and it seemed fine.

Would anyone have any thoughts on this thanks.
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Old 04-05-2015, 15:46   #2
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Re: md7a losing power & cutting out

What RPM are you running at? Have you ever run at a higher RPM in flat water?

My guess is your cooling system is very marginal right now and in flat water (lower load) it is keeping the engine mostly cool. The increased load of driving into the waves might be making just enough extra heat that you are overheating the engine.

If you have an IR thermometer next time you are out check the temperature of the exhaust manifold and the head, check at both cylinders. I suspect you will find that you have hotter spots near #2.

The MD7A has a tendency to clog up the water cooling passages in the exhaust manifold and esp. between the exhaust manifold and the head. Pulling the exhaust manifold to clean it out is likely in your future. If you have access the job isn't terribly hard, just time consuming.

I'm in the process of putting my MD7A back together for the same reason. The exhaust manifold was completely clogged shut so no cooling water would flow through it and into the block. Ended up boiling the manifold and also soaking it in citric acid to clean its passages out. The head was only clogged right where it mates to the exhaust manifold.

While I'm at it I also built a new exhaust riser, will be converting to fresh water cooling, getting rid of that horrible metal pipe between the transmission and water pump and altered the cooling flow/path to take advantage of fresh water cooling.

Shawn
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Old 04-05-2015, 16:31   #3
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Re: md7a losing power & cutting out

Gut feeling, the overheat may be a red herring.

What size waves? 1 foot, 3 foot?

I would try running into the waves until you notice the engine slowing significantly, then turning and running with the waves. Does the engine continue to loose power or does it recover?

Also, out of curiosity what kind of boat?

I've not experienced this particular problem.
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Old 04-05-2015, 18:22   #4
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Re: md7a losing power & cutting out

I had a similar problem and it turned out to be algae getting sucked into the diesel pick-up tube. I had the fuel polished (in Tampa Bay I think) and that solved the problem. Wave action can stir up all sorts of junk in the fuel tank. I once removed a short section of insulated wire that blocked the fuel flow! I have no idea how it got there.
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Old 04-05-2015, 18:50   #5
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Re: md7a losing power & cutting out

I'll bet Cowboy has it about right.

Try loosening the fuel filter before running the engine, then again after a few minutes, then again when it starts to loose power.

If the intake is getting blocked you will build up pressure making the lid of the fuel filter difficult to remove.

I've put day tanks in both boats to resolve any fuel contamination problems.
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Old 05-05-2015, 06:17   #6
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Re: md7a losing power & cutting out

Hi Shawn 67
Thanks for your reply.I don't have an rpm gauge so can't answer your question.I just try to run at same rpm at all times judged by sound.Your idea about blockages in cooling passages seems quite possible.I don't have an IR thermoter to test different temps aboard.Access to exhaust manifold is not good but there may be no option.I ran engine on flat water for 20hrs+ with no issues and at same rpm into small waves for 20minswhen it stalled.I'm sailing in Carribean now so access to parts is limited.Only use engine when totally becalmed or entering or leaving port.I'll try running at lower rpm at those times and see what happens.Thanks


Hi jpeer
Thanks for reply.Its possible overheat alarm is red herring.The 3rd and latest time it started to lose rpm I shut motor down before it stopped and alarm was silent.The waves where only 2ft wind swell at each time.I'lltry your idea about running into then away from waves and see what happens.The wind is 20kn+ offshore for next week so I won't be able to try that for a while.Because sailing back into port won't be possible.Thanks
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Old 05-05-2015, 06:20   #7
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Re: md7a losing power & cutting out

OK, but listen to Cowboy....fuel starvation!!
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Old 05-05-2015, 06:27   #8
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Re: md7a losing power & cutting out

Hi Cowboy sailor
Thanks for your input.I had fuel tank out and opened up last year.Cleaned it out fully then and replaced all fuel lines while I was at it.But I'll check it out again for blockages.I thought it was a fuel problem at first but when alarm went off and motor started straight after I thought cooling issue.The tank was close to full each time it happened.Thanks
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Old 05-05-2015, 07:06   #9
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Re: md7a losing power & cutting out

Fuel starvation on a Diesel won't cause it to overheat, it's possible there is more than one problem, although I have seen an overheated engine lose power, seen a couple seize too, overheat isn't something to take lightly.
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Old 05-05-2015, 08:13   #10
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Re: md7a losing power & cutting out

Fuel problems would not result in overheating. Do you have an oil pressure gauge? If so, what does it show when motoring into waves versus motoring on flat water? If there is a problem with the engine's lubrication system (clogged pickup, bad oil pump, blocked gallery, etc., etc.) it is possible that the boat's motion could result in gradually reduced oil pressure and eventual overheating. Are you sure the alarm you're hearing is an overheat alarm and not a low oil pressure alarm?
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Old 05-05-2015, 08:43   #11
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Re: md7a losing power & cutting out

It the engine starts right away after getting the alarm then it is doubtful it is overheating. However without a temp. reading it is impossible to say so have you checked all cooling water passages from thru hull to exhaust exit?

Could you have an unsupported fuel line that somehow gets blocked, pinched, twisted when jostling into waves? Is the fuel line solid or hose up to the filter? Is it crushed or clamped to tightly anywhere? Hopefully it is hose from filter to lift pump so is there any chance that the hose could collapse and block fuel flow when bashing into waves?
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Old 05-05-2015, 11:27   #12
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Re: md7a losing power & cutting out

Hi all,

Jkishel - we don't have an oil pressure gauge. As for the overheat alarm I fitted it myself between the engine and the water lock. And it's definitely this alarm I'm hearing.

Deepfriz - I opened up the raw water pump, then opened the through hull valve, water poured through the pump. I opened up the exhaust down line of the water lock, there was just a light coating of carbon on the pipe. There appears to be the same volume of water coming out of the exhaust exit as usual but it's hard to be certain. The fuel line is solid all the way to the primary filter. I'll have to check about hose being crushed, clamped or collapsed anywhere.

We've owned the boat for four years and 'bashed' into plenty of waves at home in colder water without a problem. Maybe the fact we are in warmer water now is why the overheat alarm is going off. Might it be necessary to install another raw water pump to boost the cooling water through engine?

Thanks everyone for your input.
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Old 05-05-2015, 12:46   #13
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Re: md7a losing power & cutting out

Before you start replacing or adding components, I'd suggest you determine which system is malfunctioning. A coolant temp gauge or $25 IR thermometer will help determine whether the cooling system is OK, though there's really no reason that it would be fine on flat water but inadequate in some small waves unless the cooling water through hull is way too high (near the water line) and/or there is an air leak upstream of the raw water pump that becomes problematic under reduced inlet submergence. If the cooling system is OK, move on to oil. Add an oil pressure gauge, you should have one anyway. It will tell you if you have adequate lubrication. Wave motion can affect a defective lubrication system as described earlier. If cooling and lubrication are OK, check electrical. Is the kill switch electronic? You say you installed the exhaust temp alarm yourself. Did the problems begin shortly thereafter? Disconnect the kill switch and see if the problems recur. Is there a short somewhere that occurs only when the boat is rolling? If all that checks out, you have a fuel problem. This can be as simple as crap in the fuel tank that is stirred up when the boat rolls or as difficult as a tiny air leak that opens up only when the boat rolls. Remember to investigate one system and one component at a time in a logical fashion.
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Old 05-05-2015, 13:18   #14
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Re: md7a losing power & cutting out

The installation of the temp. sensor and alarm leads me in the same direction as jkishel. Sure would be nice if your kill switch was hooked up to it (grin).

Bye the way...nice boat.
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Old 05-05-2015, 13:36   #15
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Re: md7a losing power & cutting out

Hi Jkishel
The thru hull is adequately low in the water so I can't see this as an issue.
I'll try and source an oil pressure gauge as you recommended but I'm in Curacao now on route to Panama so I may not be able to find one before I leave.
The kill switch is not electronic and the exhaust temperature alarm didn't sound until the first time the engine failed.The electrical system is so basic I don't think a short is a problem.
If dirt gets stirred up I don't understand why the engine will start again. As for air would the fuel system not need to be bled to restart the engine?
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