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.

Welcome aboard. I don’t have a clue about keel cooled systems. Someone here will chime in, but in the mean time, welcome.

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.

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?

What's the temperature of the exhaust manifold?

Conall

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

Also take the old valve gizmo out of the loop before the manifold

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.

kapnd said:

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.

Click to expand...

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.

GregBrannon said:

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?

Click to expand...

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.

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.

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.

Conall63 said:

What's the temperature of the exhaust manifold?

Conall

Click to expand...

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!

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.

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.

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.

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.

Ski in NC said:

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.

Click to expand...

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!

Lepke said:

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.

Click to expand...

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.

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.

The air lock might very well have been the problem for over heating. Maybe a 'slight' leak out or air leak in, that over time allows the over heating? The coolant system is a closed system, no air, no air locks in the system!! With a properly operating coolant system, manual adjustment should be totally unnecessary, once it is set up to faculty specs.

From what I read I believe your keel cooling is not set up properly.

While cooling temperatures can be controlled by a valve , the valve will need adjustment with any major engine load change. PIA!

A better system used a coolant bypass thermostat .

This is installed between the line TO the keel cooler and the return FROM the KK.

The thermostat is usually 160F or so , which is a constant , the engines existing thermostats will raise the coolant at 180F, before it goes off to the KK. .


KK are usually way oversized to make up for tropical operation or inches of fouling .

The bypass thermostat allows the engine to operate with known input water temps .

Try Sendure or a KK mfg to purchase , fine in iron as the coolant is rust protected.

www.sen-dure.com/Sen-DureCatalog.pdfcall 954.973.1260. ORIGINAL EQUIPMENT MANUFACTURERS

www.fernstrum.com/‎

www.waltergear.com/‎

some tips from a pro.....

."....they need to do is look at the manual ... DD 6-71 Manual page 2 and page 53 to see how the exhaust manifold is designed to be cooled.

If that is too difficult, maybe a photo would provide a clue ...(attached)

(he) doesn't have enough water flowing through the thing to cool it. There are probably other issues caused by the amateur plumbing job but since all that seems to bother him is the manifold (which doesn't appear to be burning the paint off) he should start by plumbing it the way it supposed to be."

psneeld said:

some tips from a pro.....

."....they need to do is look at the manual ... DD 6-71 Manual page 2 and page 53 to see how the exhaust manifold is designed to be cooled.

If that is too difficult, maybe a photo would provide a clue ...(attached)

(he) doesn't have enough water flowing through the thing to cool it. There are probably other issues caused by the amateur plumbing job but since all that seems to bother him is the manifold (which doesn't appear to be burning the paint off) he should start by plumbing it the way it supposed to be."

Click to expand...

Yup, start all over from factory specs.
You will find you miss the engine domestic hot water heating while underway. When you reinstall it, put a ball valves on the in and out from the small coils on the engine, just in case you need to isolate the hot water heater.

OldDan1943 said:

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.

Click to expand...

And in addition to that, there are two basic types of ball valve. So-called standard port, and full port. Standard port ball valves have a considerably smaller port through the ball, while full port valves do not restrict the flow, at least not any more than any full-bore straight fitting (negligible). Full port valves have a larger body than a standard valve with the same connections, and should present less flow restriction than an oversized valve with reducers (another option), though at low flow rates relative to the valve size, that may not matter.

I don't know keel cooler systems, but a thermostatic bypass valve is normal for hydraulic coolers, since the circulation pumps are usually positive displacement types, and can't just be choked off like a centrifugal coolant pump can when the thermostat is closed or restricted.

We have a 6-71 in LUCY with KK (Keel Kooling) and dry stack.

A "bus heater" ( 60,000BTU) is plumbed in as well as a heat coil for the FW hot water heater.

When fall cruising a small circ pump will circulate engine heat , after securing the engine , for a couple of hours.

3000 lbs of cast iron is a fine heat source for cool evenings .

I really seems like you have a hack-job of of a cooling system installation. I would suggest stepping back and looking at how I SHOULD be set up, then figure a plan to get there from where you are today. If coolant flow through the keel cooler is manually rather than thermostatically controlled, something is fundamentally wrong in the installation.

FF said:

From what I read I believe your keel cooling is not set up properly.

While cooling temperatures can be controlled by a valve , the valve will need adjustment with any major engine load change. PIA!

Click to expand...

PIA, yes this is what I have had to do to maintain the proper engine coolant temp. Also, the sea water temp also plays into the need for constant valve adjustments.

FF said:

A better system used a coolant bypass thermostat .

Click to expand...

Help me out here, I'm not sure how to plumb such a bypass.

FF said:

This is installed between the line TO the keel cooler and the return FROM the KK.

Click to expand...

The way I interpret this is shown in fig. 1. Seems that the water is just mixing between the keel cooler "IN and OUT". I may have misunderstood your statement.

FF said:

The thermostat is usually 160F or so , which is a constant , the engines existing thermostats will raise the coolant at 180F, before it goes off to the KK. .

Click to expand...

Would something like fig. 2 be the way to make things work properly? Just thinking? I assume that the engine water pump is build to handle pushing the water up against a closed thermostat. I assume car pumps do this all the time?

When trolling at 600 RPM the engine coolant never gets to operating temp and yet at over 1500 RPM the exhaust manifold overheats. As other have said much of this problem is due to the current keel cooler plumbing not being built in such a way as to thermostatically regulate the coolant temperature.

Thanks to those who have offered suggestions. Great Forum!

If the cabin heater is in parallel with the manifold flow path, it may be shunting off too much flow from the manifold. In this case enlarging the hose /fittings will only make the problem worse. If the cabin heater is in series with the manifold flow then enlarging the hose/fittings/heater will allow more flow and help the problem.
The fact that there are valves to cut the flow to the heater would seem to indicate a parallel path.

"The way I interpret this is shown in fig. 1. Seems that the water is just mixing between the keel cooler "IN and OUT".."

Yes the thermostat is mixing the engine coolant output with the return coolant , to create a constant temperature.

This constant return temp is far easier on the engine than slugs of cold water.

The installed thermostats from DD stop most water circulation ,until they open, so the engine warms up faster on start.

Closing the water circulation , then re opening is difficult for the engine because the untempered KK water can be really cold.

So there is little circulation till it gets warm in the engine block.

On an air cooled (road or genset) engine the return water is held about 20deg F below the engine output to stop large temperature variations.

This is what the bypass thermostat does for the KK cooled engines.