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Old 04-08-2018, 01:15 AM   #1
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Exhaust Manifold Overheating, Detroit D. 4-71N

My new to me boat is a 46 ft. steel FD trawler powered by a Gray Marine marinized Gray Marine Detroit Diesel 4-71N engine. The boat was purchased in Stockton, CA and while running it to Alaska I had problems keeping the exhaust manifold from overheating.

I have heard else where on this forum that there are 1,000's of ways to plumb the cooling system on these engines. It should be noted that the engine is keel cooled and the usual heat exchanger coils have been removed. Is this common on a keel cooled system?

One theory for the overheating of the exhaust manifold is that when the thermostats are open the water flow is not sufficient in the cooling loop with the exhaust manifold. The cooling water loop for the exhaust manifold is down stream from the cooling water manifold, thus when the thermostats open most cooling water flows back to the keel cooler leaving less water flowing in the parallel cooling loop to the exhaust manifold.

Looking for anyone that might have similar or differing thoughts or solutions.
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Old 04-08-2018, 01:26 AM   #2
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Welcome aboard. I don’t have a clue about keel cooled systems. Someone here will chime in, but in the mean time, welcome.
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Old 04-08-2018, 01:54 AM   #3
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Assuming it’s got a dry stack exhaust?
The hoses that disappear below the Racor are supply/return to the transmission cooler?
Get an IR thermometer and shoot all along the coolant path, looking for sudden spikes that could indicate clogs, stuck thermostat, bad pump, collapsed or kinked hose, etc. If the system has been reworked, make sure all hoses and fittings are compatible size, no choke down points.
The exhaust manifold inherently runs hot, it’s the nature of the beast.
Since you don’t know the history of the motor, you might want to have a look at that exhaust manifold, it may have been fed salt water at some point in its life, and be rusty inside?
It does still have paint on it, so it isn’t overheating too radically.
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Old 04-08-2018, 03:49 AM   #4
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Welcome and good luck with your new boat!

What were the indications that the exhaust manifold was overheating? What were the engine temps during the overheat condition?
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Old 04-08-2018, 06:29 AM   #5
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What's the temperature of the exhaust manifold?

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Old 04-08-2018, 08:14 AM   #6
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I would look at the rear hose on the exhaust cooler, it looks to be a hydraulic line which has a smaller I.d. Than a coolant line. At least make that the same size as the line in
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Old 04-08-2018, 08:16 AM   #7
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Also take the old valve gizmo out of the loop before the manifold
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Old 04-08-2018, 08:31 AM   #8
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What do you mean by "the usual heat exchanger coils have been removed."

Dig around the tech manual and see if you can determine the expected temp of the exhaust manifold. Call a local tech if you cant find your own tech manuals.

I had a keel cooler on my N46. Running wide open from LI to Ft L, the cooling water exhaust temp would rise beyond the acceptable temp. I would just reduce the RPMs for a bit, the temp would drop back into the normal range. You may have to run at a lower RPM until you get this sorted out.

The exhaust manifold reflect the temp of the exhaust gases. Don't touch with anything you down want burned. HOT HOT HOT VERY HOT.

Start small and work up.....
Replace the thermostat with one that opens at a lower temp.
Check the impeller of the cooling water pump too. Give consideration to replacing it but hold off on this until you try everything else.

Two things to try right now. Flush (boil) out the engine THEN remove the keel cooler to a radiator shop to be flushed and checked. Upon reassembly, replace the gasket with a new gasket, fill with new antifreeze mixed to the recommended ratio.

You may have a plugged up manifold or exhaust elbow.
I was reminded repeatedly that the exhaust elbows are suppose to be replace on a schedule.

You did not mention the ambient temp of the ocean.
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Old 04-09-2018, 12:41 PM   #9
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Quote:
Originally Posted by kapnd View Post
Assuming it’s got a dry stack exhaust?
The hoses that disappear below the Racor are supply/return to the transmission cooler?
Get an IR thermometer and shoot all along the coolant path, looking for sudden spikes that could indicate clogs, stuck thermostat, bad pump, collapsed or kinked hose, etc. If the system has been reworked, make sure all hoses and fittings are compatible size, no choke down points.
The exhaust manifold inherently runs hot, it’s the nature of the beast.
Since you don’t know the history of the motor, you might want to have a look at that exhaust manifold, it may have been fed salt water at some point in its life, and be rusty inside?
It does still have paint on it, so it isn’t overheating too radically.
To answer your questions as best as I can:
1. Yes, it's a dry stack exhaust.
2. No, the supply/return lines below the Racors are the supply/return lines for the cabin heater. Those are the longest lines in the coolant loop and I'm thinking of eliminating this heater to see what this does to the temps.
3. I have shot temps with an IR gun. The exhaust manifold has registered as high at 400 F. before backing off on engine RPM. The engine water temp. with IR gun was just above the thermostat temp. of 170 F. (Normal temp is 170-180 F.)
4. Yes it's possible that the exhaust manifold has some flow restriction. For now, I'm holding off on removing/testing pending checking out easier solutions.

When I get back to the boat in Alaska in May I will try to reduce the length of the cooling loop starting with the cabin heater and increasing the size of some of the hoses to see what effect this might have. When you have a cooling loop inside a cooling loop your never quite sure how flows are affected, i.e. when the 2 thermostats open how much water then is sent back to the keel cooler and how much less is then going to the exhaust manifold before returning to the keel cooler?

Thanks, for your suggestions.
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Old 04-09-2018, 12:50 PM   #10
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Welcome and good luck with your new boat!

What were the indications that the exhaust manifold was overheating? What were the engine temps during the overheat condition?
The cabin heater off the engine started blowing cold air. A check of the engine temps. with and IR gun showed 400 F. on the exhaust manifold and normal 170 F on the engine block and coolant tank. I needed to add a gallon of coolant to bring it up to the level at startup. This may seem like a lot, but considering that this coolant is also in the keel cooler, this is a small percentage of the total coolant.
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Old 04-09-2018, 12:54 PM   #11
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First, how do you know if the manifold is overheating? It seems that it is missing its insulating jacket. It will therefore radiate a LOT of heat into the engine compartment. This is why you see the rest of the stack wrapped.
Second, the heater loops will help cool the engine, not cause it to heat up.
If the jacket is not heating up, the engine does not run hot, you do not have a problem. Just wrap the exhaust to keep the heat in.

There is no water pump except the main water pump on the engine, that should pump the coolant around the cooler. I am pretty sure that the exhaust manifold is cooled by the engine coolant and therefore if you have the complete circuit like my installation, all the coolant circulates through the keel cooler when the thermostat opens.

I just overlapped your last post, do you have a large coolant reservoir? You need a big one for a keel cooler, the normal engine size is too small. I believe the one I bought was 5 gallons, it is a nice welded tank for fuelling a dragster with a big opening cap and AN fittings. Do you think you are losing coolant somewhere? Different issue if so.
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Old 04-09-2018, 12:59 PM   #12
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Sounds like the cabin heater and exh manifold share a flow path in series. Is that right? Also the hoses for the exh manifold are too small. Want at least 3/4" id hose there.

Exh manifold should be supplied from block, then return to head tank or circ pump suction. Should not be in series with cabin heater. May be able to use the same taps, but put cabin heater in parallel vs series.

Ports of exh manifold will go over 400F as they are not jacketed. Area with water jacket (painted) should not go over about 200F.
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Old 04-09-2018, 01:00 PM   #13
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What's the temperature of the exhaust manifold?

Conall
400 F. Caught it early on and reduced the RPM to 1500 and also opened the keel cooler flow valves to maintain the required engine operating temperature at 170 F. Problem then becomes that if operating at a trolling RPM of 700 the engine never reaches operating temp. Between a rock and a hard spot!
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Old 04-09-2018, 01:24 PM   #14
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The "keel cooler control valves?" That's strange, you want the coolant flow to be diverted to the keel cooler by the thermostats. Then the temperature is controlled and automatic. Think of the identical cooling method used by a car (or truck, if you prefer) radiator. Entirely passive. The keel cooler is just a water-cooled radiator.

I think you need to do some plumbing. Once the engine is up to temp and you reduce the power for trolling, the thermostats should close and maintain the heat. Having said that, diesel engines are very efficient and do not produce a lot of excess heat, my old VW diesel used to cool off to the temperature stop when stuck in traffic in Montreal when it was cold.
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Old 04-09-2018, 01:40 PM   #15
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A valve before the keel cooler?? never heard of it.
Is there are valve on the return from the keel cooler? If so I doubt if they are defined as throttle valves but rather isolation valves ex, opened or closed .....

Any time you put a valve in any system, the valve even fully open still reduces the flow a bit even if they are ball valves.
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Old 04-09-2018, 02:16 PM   #16
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Your cooling setup looks to be a home-made conversion of a normal heat exchanger cooled engine. The oil is normally cooled by engine coolant directly after the coolant is cooled. You could be cooling your oil too much. A keel cooler usually has a separate thermostat that regulates the flow to the keel cooler. The flow thru the engine remains the same and more or less water is sent to the cooler and then cold excess water is returned to the engine w/o passing thru the cooler. Usually the engine thermostats aren't used in keel cooling.
The normal flow thru the exhaust manifold is back to front with the coolant coming from the temperature sender housing, thru the manifold, and out the top front, under the expansion tank and to the front of the circulation pump. That circuit bypasses the thermostats to ensure the engine always has some circulation and is where heaters are supplied from. It should always be open. You should be able to have a heater circuit without engine heating problems. I have a pair of 671s heat exchanger cooled and did have a troller keel cooled. My 671 current exhaust manifolds run normally and the engines are usually run at 1800 (max hp rating) but have been run at times at 2100 w/o problems. I use the bypass circuit to heat my boiler when the mains are running w/o problems. I also do it in reverse, when the boiler is hot, I circulate hot water thru the engines before starting using the boiler circulation pump.
There are 2 basic circulation pumps. One has more blades on the impeller, a higher flow, and is made for turbo engines. That pump could solve some of your problem.
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Old 04-09-2018, 03:52 PM   #17
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Interesting alternative. My keel cooler has no pump and no heat exchangers, it did have a heat exchanger with a loop to heat the hot water tank but I removed it for simplicity. The coolant also cools the oil and the transmission (and the turbo). The exhaust elbow is not cooled but it has a blanket, as does the turbo (off now for maintenance). I think that there is no reason why an engine driven coolant pump cannot circulate the keel cooler coolant which is identical but isolated by thermostats, from the engine. A truck radiator does not have an extra pump.

Go for simplicity.

And once again, because I had this problem, ensure you have a large reservoir/overflow tank.
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Old 04-09-2018, 05:01 PM   #18
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Originally Posted by Ski in NC View Post
Sounds like the cabin heater and exh manifold share a flow path in series. Is that right? Also the hoses for the exh manifold are too small. Want at least 3/4" id hose there.

Exh manifold should be supplied from block, then return to head tank or circ pump suction. Should not be in series with cabin heater. May be able to use the same taps, but put cabin heater in parallel vs series.

Ports of exh manifold will go over 400F as they are not jacketed. Area with water jacket (painted) should not go over about 200F.
Your right-on about the sharing path. IMO the coolant path is called upon to do too much as currently configured.

It seems to me that the existing flow path is as follows: The engine driven pump sucks coolant from the keel cooler and pushes it into the engine block. After flowing through the block it enters the water manifold. From the water manifold the flow goes two directions, assuming thermostats are open, to the coolant tank and oil cooler back to the keel cooler. The other flow path out of the water manifold is a series path consisting of the following: 1. engine mounted air compressor, 2. exhaust manifold, and 3. cabin heater. From the cabin heater the water enters the oil cooler where it mixes with the coolant from the engine tank and returns to the keel cooler.

You have recommended that the exhaust manifold cooling come directly from the block. I will have to check to see if there are any ports in the block to tap into. I don't recall any, as I think they are only available on the water manifold. I will check when I get to Alaska next month.

OK, so I think I have some plans:

Plan of Action: Increase hose sizes as suggested. I'm also thinking of a tee at the exit from the water manifold and add a parallel flow route directly to the exhaust manifold by installing another tee at the entry into the exhaust to mix the water coming from the air compressor.

Back-up Plan of Action: Since there are 2 keel coolers, one port and the other starboard I'm thinking of adding a dedicated second water pump/expansion tank to cool only the loop which includes the exhaust manifold and cabin heater. There is sufficient cooling capacity for the main engine using only one keel cooler.

I Welcome any and all thoughts on this. Thanks to all who have provided help to this point. There is a great wealth of knowledge/experience on this forum. Happy boating to all!
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Old 04-09-2018, 05:32 PM   #19
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Partial Quote: A keel cooler usually has a separate thermostat that regulates the flow to the keel cooler. The flow thru the engine remains the same and more or less water is sent to the cooler and then cold excess water is returned to the engine w/o passing thru the cooler. Usually the engine thermostats aren't used in keel cooling.
Interesting, I have no way of regulating the keel cooler flow other than gate valves on the keel cooler inlet and outlet pipes. Also, no way to send water back into the engine if it doesn't flow back thru the keel cooler. My engine has 2 thermostats in the water manifold. The thermostats do have a couple of small holes which allow some water flow prior to opening.

I will be considering flow control on the keel cooler as a replacement for the engine mounted thermostats. Seems this should prevent over cooling of the engine oil and better control engine coolant temp over the RPM range of operation. Thank You.
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Old 04-10-2018, 12:00 AM   #20
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Your cabin heater went cold because it ran out of coolant.

You said you added a gallon of coolant. I bet that fixed the cabin heater. Could be a vapour lock in the cabin heater though after running dry. Low coolant volume might have lead to a hot manifold too. When did you add the gallon?

Three ways to make sure it is set up correctly: 1) check the manual. 2) do it the way the manufacturers manual says to. 3) read the manufacturers manual and set it up the way they say.

I hope this helps.
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