Volvo Md7a - Not getting to temperature

[JacoLTD]

The Md7a is notorius on the forum for overheating due to its clogged raw water passages. My dilemma is that I'm not seeing the gauge warm up to normal operating temps. Since I have owned the boat, I haven't run the engine over 2,000 RPMs.

- Starts reliably and runs without issue. Some whitish-blue smoke, but not enough to be concerned. Realize it's a tired old engine. Seems like little or no steam. A tad of oily sheen in the exhaust, likely unburned diesel from running cold(?)

- Raw water temp is in the low 40s (Rhode Island). Exhaust water is sufficient and feels properly warmed (haven't tried measuring with a thermometer.) I have not opened the thermostat housing.

- Temperature gauge works - moves maybe 25% after 20-30 minutes of operating above 1,500 RPMs.

[Lepke]

Oil in the water probably from worn rings - blue = oil. White = steam or unburnt fuel.

Most likely cause for low temp is a thermostat stuck open. With a properly functioning thermostat, the engine should reach operating temp no matter what the ocean water temp.

[JacoLTD]

Thanks. If the thermostat is stuck open, it's highly unlikely for the engine to get to temp, is that right?

[Lepke]

Yes. Even big diesels won't reach operating temps with an open thermostat. Running below temp uses more fuel. About 10%, partially because of the thicker oil the crank is dragging, and more effort to pump.

[barnakiel]

Pull the thermostat and boil it. Check when it opens and closes.


Cheers,
b.

[Wotname]

Originally Posted by JacoLTD

The Md7a is notorius on the forum for overheating due to its clogged raw water passages. My dilemma is that I'm not seeing the gauge warm up to normal operating temps. Since I have owned the boat, I haven't run the engine over 2,000 RPMs.

- Starts reliably and runs without issue. Some whitish-blue smoke, but not enough to be concerned. Realize it's a tired old engine. Seems like little or no steam. A tad of oily sheen in the exhaust, likely unburned diesel from running cold(?)

- Raw water temp is in the low 40s (Rhode Island). Exhaust water is sufficient and feels properly warmed (haven't tried measuring with a thermometer.) I have not opened the thermostat housing.

- Temperature gauge works - moves maybe 25% after 20-30 minutes of operating above 1,500 RPMs.

What is the temperature range of your temperature gauge?

Almost all temperature gauges start at 140F.

Your raw water cooled engine (and it's thermostat) should be operating between 120 and 140F.

If your gauge is like most and it's lower range begins at 140F and you are seeing it move, you may actually be running too hot. If so, you will be precipitating calcium carbonate (white scale) inside the raw water passages which further increases temperature causing more scale etc.

Unlike most salts, calcium carbonate (which occurs naturally in sea water) precipitates faster with increasing temperature.

[roverhi]

As Wotname says. If it's salt water cooled it needs to run at less than 160 degrees, actually quite a bit less. Above around 150 degrees salt deposits clog the coolant passages resulting in overheating.

Forget using cooling water">engine cooling water to heat water. A properly running raw water cooled engine should never get hot enough to take even a slightly warm water, btdt.

[JacoLTD]

Thanks for the inputs. I understand the temp vs calcium precipitate. Going to try pulling the t-stat and test. Just hope I don't ruin any old bolts or gaskets.

The gauge on the Sabre 28-2 starts at 100 and goes up to 240, so the middle of the gauge would theoretically be 170. Here's a picture. Note how most of the range between 120 and 220 is colored green.

Pushing the boat at 2,000 RPMs for 30 mins, I still haven't seen the temp needle move more than the equivalent of 110 or maybe 120, so just passed the red.

[hpeer]

The have seen this kind of behavior with my MD-7. I can’t explain it.

Last year I left on a longish voyage, water near freezing, it was running cold. I had to lay up for a few days then restated, and the damn thing would run hot and cold and in between. At sea I pulled the t-stat and impeller and cleaned up what I could, then it just ran hot! I aborted the trip, limped back.

Back at the dock I pulled the exhaust manifold off, it was pretty jammed up. After lots of mechanical cleaning, jabbing and poking I gave it an acid bath, a few actually, until it stopped gassing. I reassembled and repeated the acid bath circulating through the block until that stopped off gassing.

Took it out for a half hour spin, making good water and coming up to temp. IF I ran it hard I could see T-stat working. By now weeks have gone by and the water at the dock is probably mid-50’s. So I called it good and hauled the boat to do some hull work as the season was shot any way.

I can’t explain why it ran cold at the start or why it would fluctuate as it did beyond saying miscellaneous piece is crud were blocking passages at random. The deep cleaning did it some good thats for sure.

[JacoLTD]

Little update after some additional use. So air and water temps have warmed up a bit and I actually see the temp needle coming up when the engine is under load above 1500 RPMs. I think the temperature/thermostat situation is fine. As soon as load/rpms drop, temp drops too.

Now, for the white smoke. There’s quite a bit. A few weeks ago it was leaving a notable sheen on the water, but i dumped some starbrite Star-tron diesel conditioner in the tank and that seems to align with the timing of when the sheen mostly went away. It’s still there but has greatly improved.

Still, fair amount of smoke from startup through use. Thinking it’s old injectors. (Originals). Can’t imagine it’s head gasket because the amount of smoke is the same from start up thru use. Still fair amount of diesel scent in the exhaust water. Haven’t done a compression test but engine fires up with a second or two of cranking and seems to deliver power as rpms increase. Read a bit about adjust the injector timing, but thought to throw this out there for fellow md7’ers.

[hpeer]

In 50-60 degree water I had to run mine pretty hard to get her to the point where I could see the tstat working.

The white from the exhaust; smoke or water vapor?

I think I could tolerate a bit of diesel on the water, especially just after start up. I have to believe that the engine works more efficiently when pulling a load at a decent RPM. It may be working just fine under those conditions.

I would take her out a few times and forget about it. If some issue comes up that makes you think then think about this.

[JacoLTD]

Smoke is definitely exhaust and not water vapor. Probably closer to gray. Not cotton ball white. Pretty sure it’s just the age of the injectors coupled with some junky fuel.

[Compass790]

Originally Posted by JacoLTD

Smoke is definitely exhaust and not water vapor. Probably closer to gray. Not cotton ball white. Pretty sure it’s just the age of the injectors coupled with some junky fuel.

You may well be right. If your injectors have never been serviced maybe it's worth a shot if you have the budget. Try cetane improver if you are in part of the world that has diesel below 50 cetane.
Does it use much oil?
If you pull injectors good idea to compression test then but if it starts easy good chance it's ok.
Have you checked tappets?
I don't know MD7a specifically, just general diesel idea but it sounds like you have a handle on it

[MicHughV]

I'm very familiar with the Volvo MD7 engine. Had it in my first boat, back in the 80's. After 10 years or so, I replaced it with a Volvo 2003 series engine.

But, as you point out, the MD7 did not appear to run at very high temps compared to the 2003 engine.

The MD7 is raw water cooled, and I think that is the difference. An engine with a heat exchanger, runs pretty much at the temperature of the coolant in the system, like a car, probably in the 200 degree F range. ie, at or near boiling. A raw water engine gets a continuous supply of seawater, which is substantially cooler.

My MD7 got water in the cylinders one time. Long story. After beating on the piston with a sledgehammer to get it moving again, I re-assembled everything and it started right up. Always smoked a little after that experience, but never skipped a beat. I had the injectors tested, but they came back ok, so the smoke had to be from some internal friction somewhere, but it didn't seem to harm the engine nor cause any oil related problems that I could tell. It always started right up and ran for hours at a time sometimes.

Even though the coolant passages would gum up with saltwater residue from time to time, it always ran at the same temp.

It's an old time diesel engine....whopping big flywheel and all....a relic really...no longer made...raw water cooled...really ?....but it ran !! I am surprised that many older boats are still running with this engine.

After I pulled my engine to replace it with the 2003 series engine, I was surprised to find a buyer for it. Seriously surprised !!

[Compass790]

JacoLTD here is a thread that will interest you:
https://www.cruisersforum.com/forums...-134515-7.html