white smoke and overheating on MD7A - SailNet Community

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Old 06-10-2010
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white smoke and overheating on MD7A

I have some problems on my old Volvo Penta MD7A. Engine works fine, starts fine, but always I have that white smoke on exhaust. The motor oil looks good, not polluted by water, so I exclude the possibility that water enters in cilinders. Water flow seems to be OK. Since the engine have only warning sensor (switch) for overtemperature, I put additional termic sensor, which is mounted on the case of exhaust manifold just very near the original termical switch. So that additional termic sensor measures the temperature of the case and not the temperature of cooling water as temp. switch does.
The temperature switch closes at 203 F (95 C), thermostat starts opening at 140 F, fully open at 165 F.
On the run after one hour on 2000 rev/min additional termic sensor shows 212 F (100 C), no other alarm, water on exhaust can be touched ( by my feeling it was around 122 F).
Now, the main question is: What should be the "normal" temperature of the engine case (temperature measured near temperature switch on the case)? I think 212 F is too high.
On some posts I read on this forum, it is not advisable to clean cooling channels with chemicals, because these tiny channels could become even more clogged. So I meant to dismantle this exhaust elbow (I do not know - this is the part which collects water and smoke together), and inspect is something clogged by water stone or something. Is it advisable to do this?

Any suggestions? Should I do something or better leave as is and keep mooving.

WBR dobra vila
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Old 06-10-2010
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You need to clarify if it is indeed smoke or steam...

White smoke is unburnt diesel fuel and if you hold your hand over the exhaust for a few seconds and then smell it, it will smell like raw diesel fuel...if its steam it wont.

As far as no water in the oil proving that none is getting into the cylinders you are mistaken there...it doesn't necessarily show up that way depending on how its getting into the cylinders.

In regard to your "CASE" temperature...Im not clear on exactly where you have the new sensor mounted...Can you post a picture?

My actual head on my Perkins 4-154 will read 217 to 221 degrees when working hard and the exhaust manifold flanges right at the head exhaust ports will read close to 250..all the while the temperature gauge thats reading the coolant temp right at the thermostat is reading 190 degrees ..all well within operating range.

Checking your mixing elbow is a good idea as a partical blockage could do funny things.
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Last edited by Stillraining; 06-10-2010 at 02:27 AM.
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Old 06-10-2010
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The original sensor (switch) holds the additional sensor with thread. The new one is put into one big cable terminal (terminal for ground for example) and then screwed on case with the original one. I made so, because there is no need for drilling a new hole for sensor.
So as I understand (and is logical too) - the original switch measures the temperature of water which must not exceed 200 F, otherwise the alarm is activated, while the housing is normally on higher temperature. This temperature (on housing) measures my additional sensor. So, the housing is always on higher temperature than water. Is this my thinking OK? It seems that temperature of engine around 215 F is quite normal, and I should drill the hole for sensor to measure temp. of water instead of housing. The picture is useless, I made one but important details is to small. For now!!
WBR dobra vila
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Old 06-10-2010
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MD7a - know it well. You can use a non contact thermometer for troubleshooting. It works great for showing the hot spots. I had cooling problems with mine and got a good understanding of the cooling system, problems and solutions. The water passages are most likely rusted shut, but while time consuming it is not difficult to fix. If you can get a hold of a manual, it will help but I will try to describe what to do. This was 10 years ago so it is the best I can remember.

The MD7a was not designed for salt water cooling. The internal passages are i parallel and are quite small so it doesn't take much for them to plug. The good news is you can clean them out with a stiff piece of wire. Your bilge pump is going to get a workout. I would start by removing the exhaust manifold. You can then clean the manifold. You will find lots of rust in it but you can scrape it out if you poke around with the wire and flush it out with water. There are some small passages that were so plugged on mine that I needed to drill them out. You can also poke around in the head and loosen as much rust as possible too. Remove the engine drain valve on the block below the injector pump. Poke around in there to loosen rust. Run a hose into the water passages in the head to flush the rust out. (plug the exhaust ports so you don't get water in there) Replace the exhaust manifold and gasket. Leave the thermostat housing off.(I think) You can hook a hose to the copper water inlet that goes to the bottom of the thermostat housing to flush the engine.

Now this is really the key part. Pull the soft plug (freeze plug) on the front and rear of the head. This gives you good access to the head water passages and allows you to clean the most important ones which are directly below the plug openings and are undoubtedly plugged. There are 2 water passages that feed water from the head into the block. If you poke around while running water you should get a lot of rust flushing out once you get them open enough to flow.

The plugs can be replaced with the rubber double washer through bolt type sold in auto parts stores. You could try to replace them with the Volvo ones but it is not needed.

If you have a manual this will make more sense as you can see the holes in the head and block in the pictures.

The basic problem with this engine is if the water inlet is not shut off when the engine is not used for some time the water level will drop to sea level back through the cooling pump. This leaves a water air combination in the engine which allows rust to form.

After you clean the engine and get it working properly you can start shutting the water off when leaving the boat for a few days. Hang the engine key on the handle so you don't forget to turn it on.

I know this might sound daunting but if you have some mechanical abilities you will figure it out. Be aggressive as possible in the cleaning process as you won't be able to get at all of the passages anyway.

Good luck
Gene
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Old 06-11-2010
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Stillraining and Gene, thank you for your answers and suggestions. I decided to dismantle the exhaust manifold, try to clean the channels and - remove the cilinder head, grind it and put a new gasket. Somehow I forgot to tell you -
two years ago we went on holidays. On the first day, after few hours on the way the engine suddenly stopped. The buzzer was not working and I did not see that yellow warning light on the sun. It was lit. It seemed, the machine was overheated. After a while the engine was cooled down and started normally. We slowly reached the marina. At marina I dismantled the thermostat. It was all rusted, not working. So I could not get the new thermostat right at that moment, we went forward without thermostat. At that time engine worked normally, I did not put much attention on that white smoke on exhaust. After holidays I got a new thermostat and put it. Now I know that probably at that overheating, the head gasket maybe gone. Still I am asking myself, what technically happened in engine when it stopped. It has no security means to stop engine on overheating. Is it because of lack of compression or this engine has some security valve for this purpose, I do not know. Still be all good and thank you again.

dobra vila
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Old 06-22-2010
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Please allow me to cut into your thread with a similar (but not as severe) situation.
I also have a Volvo MD7A diesel. I have had what I considered some overheating issues but the boat is still relatively new to me so I'm still learning what is "normal".
My instrument panel has a temperature gauge that has a wide green band and a narrow red band. At rpms below 1,500, the engine seems to run about 20% into the green. At rpms around 1,800, it runs around 60% in the green. At 2,000 and above, it gets to about 95% into the green. At the higher rpms, the needle will slowly climb (sometimes going just into the red) and then suddenly, I’ll hear an abrupt change in the sound of the exhaust. It gets louder and very “throaty”. At the same time, the temperature gauge takes a nose dive and drops from the top of the green band to near the bottom. Keeping the rpm high will result in the same cycle to repeat about every 5 minutes.
My thought is that at the lower rpms, the engine isn’t producing enough heat to open the thermostat (the thermostat has a bypass line and also has a small hole in the face). At the higher rpms, the thermostat finally opens and when it does, the temperature drops. But I don’t think this should be normal operation for a thermostat. It should keep the temperature at one point, not have it cycling 30 or 40 degrees every 5 minutes.
So I’m wondering if I should replace the thermostat or take some other corrective action?
Any thoughts or suggestions are appreciated.
Harbin2
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Old 06-22-2010
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In regards to the white smoke issue try moving the timing ahead about 1 or 2 degrees on the injector pump. On the pump there should be an arrow showing the direction the pump turns, you would want to pivot the pump in the opposite direction. This will move the timing of the pump ahead slightly and may very likely reduce or eliminate your white smoke problem.

Earlier this year I had my Injector Pump rebuilt and when it was reinstalled I had a lot of white smoke, not just a t start up but at anything below 2000 rpm, moving the pump ahead pretty much eliminated the white smoke, except for a few seconds at start up.
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Old 06-22-2010
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Quote:
Originally Posted by harbin2 View Post
Please allow me to cut into your thread with a similar (but not as severe) situation.
I also have a Volvo MD7A diesel. I have had what I considered some overheating issues but the boat is still relatively new to me so I'm still learning what is "normal".
My instrument panel has a temperature gauge that has a wide green band and a narrow red band. At rpms below 1,500, the engine seems to run about 20% into the green. At rpms around 1,800, it runs around 60% in the green. At 2,000 and above, it gets to about 95% into the green. At the higher rpms, the needle will slowly climb (sometimes going just into the red) and then suddenly, I’ll hear an abrupt change in the sound of the exhaust. It gets louder and very “throaty”. At the same time, the temperature gauge takes a nose dive and drops from the top of the green band to near the bottom. Keeping the rpm high will result in the same cycle to repeat about every 5 minutes.
My thought is that at the lower rpms, the engine isn’t producing enough heat to open the thermostat (the thermostat has a bypass line and also has a small hole in the face). At the higher rpms, the thermostat finally opens and when it does, the temperature drops. But I don’t think this should be normal operation for a thermostat. It should keep the temperature at one point, not have it cycling 30 or 40 degrees every 5 minutes.
So I’m wondering if I should replace the thermostat or take some other corrective action?
Any thoughts or suggestions are appreciated.
Harbin2
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The water passages between the head and block are plugged. Read my earlier post in this thread on the procedure. You could also pull the head to clean the passages. The problem is there is no water circulation through the front of the engine. At higher RPM the engine heats up more until the water in the block and head flash (boil). You will hear a different sound in the exhaust when this happens. Cold water then fills the engine through the already open thermostat and it cools rapidly. Note there is a bypass circuit past the thermostat which confuses the issue too. The hole in the center of the cast thermostat housing needs to be an exact size. It tends to rust and get too big thereby throwing the whole cooling system off. So that could be an additional problem. Putting a restriction in the feed pipe will fix that however.

Gene
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Last edited by Gene T; 06-22-2010 at 10:25 PM.
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Old 06-23-2010
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Last week I dismatled the exhaust manifold as I told I will do it. It seems, the previous owner of my boat has already dismantled it, because everything looked OK. There was quite new bolts, gasket OK, no excess debris on input and output holes. I put out the termostat, it was a little suspicious so I took it in winegar for few hours. It cleaned very well. Then I tested it in hot water with thermometer and it worked fine.
When the exhaust manifold is dismantled from the engine there are three holes on gasket surface regarding water. One is towards back side, little lower located, rounded aprox. 1/4" dia, which I persume is input. The second is towards front of engine on same height, rounded 1/4" diameter. The third hole is triangular, quite big ( 1" by 1") and is located above the second. When I forced winegar in the first hole it runs out easily from the second and third hole. So I persumed that water pasagges are OK and clean. Is this my thinking allright?
So I did not put out so called freeze plug (made from aluminum) on the front for two reasons. One is I do not know how to pull it out and I wonder how to seal it again. The second reason was that water pasagges I persumed are clean and there is no need to further tear apart the engine.
So, after I put all together and went to a test drive. Surprisingly, there was no white smoke anymore (??). But the housing of the engine still heats to approx 212 F, water on exhaust has app. 130 F. If I would not have that additional thermometer fixed on the housing I would say OK, engine works fine it is all OK. But now I am still in doubt is it normal (temperature of engine itself, not the temperature of cooling water) or not.

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Old 06-23-2010
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Perhaps you should consider,... that un burnt diesel in your exhaust will give you BLACK SMOKE, and water/coolant getting INTO the combustion chamber will give you WHITE! Since you already have WHITE smoke, your problem will be the coolant finding its way to into the intake/combustion/exhaust side of your application. The most likely cause, after reading your own diagnosis, will be that the head gasket is allowing coolant into the combustion chamber. Since the combustion chamber does not have OIL in it, other than the fuel shot, you are not going to see oil in the coolant, and since you can already see the coolant in the exhaust (white smoke) . If you really cooked it, you may have cracked the block, or warped the head or worse. I would recommend having a wet, AND DRY compression test on your engine. This is the only way of knowing what is wrong. Any thing short of those two sets of numbers for EACH cylinder, bill be a waste of your time and $. Brian.
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