steam

[wshelton]

I have DD 671Ns in a 80 mod. 43 DC. The engine temps have always been around 175 to 180 when RPMs around 1500 and above. Lately I have one engine creeping beyond 180 to 190 when running at 2300 which I generaly do not but sometimes will to clean bottom etc. I did notice that exhaust did seem a bit whiter in color so I thought steam and though the strainer/impeller might be tired. Replaced both and backflushed thru hull. Same problem, but engine will not climb much over 190 unless you go to WOT, which I did but did not let it get any hotter and not sure it would have. I did a test with infered thermonater in the slip. Temp on gauge showed 180 and test shot showed 159. I was shooting at a fitting on top of the engine with a wire attached to it and assum that is the sender. Shot sender on other engine gauge at 175. Both senders were a few degree apart 155 to 159 at idle in slip. Anti freeze good. Whats up?

 

[Nonchalant1]

You may have a clog somewhere in the raw water path. Mine did this and it turned out to be gunk built up in the intercooler tubes, which is the first thing after the the raw water pump on my engines. Since you have N's you don't have intercoolers to worry about, but as Karl (Genesis) says: "the next thing upstream from your sea strainer is ALWAYS the "auxiliary sea strainer". Bet on having to clean that regularly".

Maybe there's stuff stuck in your heat exchanger, trans cooler or oil cooler? WHere does the raw water go first on your engines?

Doug

 

[Trojan]

When was your heat exchanger cleaned? I would start there if my strainer was clean and my impeller was new.

BILL

 

[Dan Mapes]

Hmm good question. The thing to remember about the IR thermometers is that they are generally off by 10 degrees. At least that is what I have heard. I always take a peek in the bilge while running hard to double check things like the output shaft of the trannies and the tops of the oil filters. Checking the temps at the dock idling doesn't cut it imho.
Detroits cool off when idling, and I can't get ours up to temp unless I take the boat for a ride. Consider that when looking at the temps while idling at the doc. They may heat up a few degrees when idling after a hard run, but sitting at the dock you won't get a good representation of the temps.
As I type, we have the port side exchanger out of the boat being cleaned and painted. For kicks I pulled the plate off the exchanger on the input side and indeed there were a few of the tubes plugged up. The starboard side went out the week before. All in all a pretty easy job to pull them and send them out.
On our boat, after the strainers (at least on our boat) there is a tranny oil cooler, the intercooler, and then the exchanger. But in spite of that, the exchanger still had a few tubes plugged!
So check the input side of all the coolers in the system. While you may have good clean strainers that doesn't mean something didn't work its way past them.

 

[luckydave215]

Don't forget the fresh water side. Mine were full of zinc mud.

 

[Nonchalant1]

671Ns don't have an intercooler or aftercooler. Those are only on turbo engines. Just follow the raw water path and see if you find anything plugged up. I agree that it's reasonably straight foreward to pull your heat exchangers and take them to a radiator shop or plunge into an acid bath per Detroit's specifications. That will also clean part of the fresh water side. Get all new gaskets before you start, use lots of rust buster or PB Blaster on every bolt a day before you try to remove them and hit every bolt head hard a few times with a hammer (straight into bolt) before you put a wrench on it. Use a torque wrench to put bolts back in correctly.

Doug

 

[MikeP]

Re temp readings - if the gauges to which you refer are the helm (electrical) gauges, pay no attention to them as far as an actual reading is concerned. They are only useful for noting relative temp changes.

Install mechanical temp gauges in the engine room and monitor those occasionally for accurate temp readings. IR temp gauges are very useful but they CANNOT measure coolant temperature because they are not measuring the coolant, they are measuring the surface of some other material that coolant is running through. Their readings change based on the material (steel, brass, rubber, aluminum) and the reflectivity of the surface (chrome, flat paint, glossy paint, etc.) There is a wide difference range that the IR gun will show based on the material itself. In other words, a brass object and a steel object both heated to exactly the same temperature in an oven will read different temps on an IR gun. The distance and angle also changes the reading so there are a lot of variables. In a nutshell, don't shoot a temp with the IR gun and assume it's the actual engine coolant temp. It's not.

 

[TedZ]

There must be differences in IR guns.

When i shoot my thermostat housings i get 180 plus or minus a degree. My electric gauges read 180 degrees exactly (VDO a few years old). My old gauges were all over the place so it certainly does occur.

White smoke, which i assume is different from the cooler engine, should disappear after engines are warmed up enough to get rid of condensate in the engine oil. If it doesn't, that deserves checking.

No matter what the IR gun reads, you should be able to follow the path of the cooling water to get comparison readings on the engine itself and in the same locations on the other engine. However, if you shoot a spot and come back to it 20 seconds later and get a different reading on the engine that is cooling properly, get another IR gun. They all do NOT do that.

Ted

 

[Wild Bill]

I had the same problem on my starboard engine. I too run 6-71N's. I found an abundant growth in the intake screen. I back flushed through the hull, but it did not dislodge it. I had to haul the boat and srape the buggars out. We have tube worms here. They are a real pain in the intake. Two to three weeks and there back. They attach to every piece of metal in the water regardless of paint.
These tube worms will block the intake so that the overheating only begins at higher RPM when you need the most water flow. Mine didn't begin to get hot until I exceeded 2300.