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Old 05-12-2020, 08:50 AM   #1
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Lehman 120 only gets to 160 degrees

Hi All,


Before writing this, I did do some research and really didn't find a good answer. After replacing the head gasket, I finally got her out for sea trials yesterday. Took my time getting to 1800 RPM, and the temp gauge only got to 160 deg F. We were out for almost 3 hours and the temp gauge never got over 160. Good oil pressure and alternator. I also have a temp gauge with it's own sender into the holding tank down in the engine room, it also said 160. Coolant level in the tank after the run was right where I filled it to... 1/2 way full. During the head gasket change, I did clean out the heat exchanger (real good) and have new coolers.


I pushed it all the way up, only got 2300 rpm, and the stinky black exhaust. The boat came with a nice 4 bladed prop that might be a pitch or two over propped (swag on my part).


Thermostat stuck open or partially open? Input welcome, thanks.


Dave
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Old 05-12-2020, 08:55 AM   #2
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Has the temperature been like that since you’ve owned the boat? When we had our engine rebuilt, the rebuilder put in a 158 degree F thermostat. I replaced it with a 180 F. I aslked about the lower thermostat and said it was fine since we did a lot of long distance cruising. He didn’t convince me. It’s easy to change.
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Old 05-12-2020, 09:00 AM   #3
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First buy or borrow a photo tach. You really want to know what your true RPM is. Alternator driven analog tachometers are notoriously inaccurate. Do not waste your time chasing an RPM issue using the dashboard tach. A new photo tach is $30-40.
Second. Buy or borrow a hand held infrared temperature gun to confirm the temperature readings. Take the reading no more than 3 inches away from the coolant sender.
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Old 05-12-2020, 09:11 AM   #4
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First buy or borrow a photo tach. You really want to know what your true RPM is. Alternator driven analog tachometers are notoriously inaccurate. Do not waste your time chasing an RPM issue using the dashboard tach. A new photo tach is $30-40.
Second. Buy or borrow a hand held infrared temperature gun to confirm the temperature readings. Take the reading no more than 3 inches away from the coolant sender.

I have a mechanical tach in the engine room and an electrical tach on the dash... they read identical. I have an infrared... forgot to try it.
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Old 05-12-2020, 09:29 AM   #5
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I'm in a similar position, but after this weekend I'm not as concerned. We have a completely rebuilt FL 80 hp, the 4 cyl. version of yours. It had 80 hours on it and had been back to the rebuild shop at 50 hours for oil change, head retorque, etc. I was concerned that the temperature was very slow to rise. I saw posted on here a note that one should not exceed certain rpms until the engine was warm. The rule of thumb given was that rpms should not exceed 10 times the temp in Fahrenheit. So I thought I would try that.

It could not be practically done on my engine. Using the rule, if I wanted 1,400 rpm, I would wait for the engine to heat to 140. It didn't work. It would not heat to 140 even at 1,400 rpm. Not anyway in a reasonable amount of time. After a half hour waiting for the engine to get up to 140, I went ahead and motored off, always rpms exceeding 10 x temp.

At 1,600 rpm, the temp would not get above 140. At 1,850 for three hours, the temp finally rose to 170 and stayed there. I assume that is the thermostat opening and it never went any higher. 1,850 is a little above where I would generally choose to cruise, but maybe that is required for this engine to operate at its best temp?

I did have the coolant circulation valves open to the hot water tank and the Red Dot heater, so they were both throwing off some heat in addition to the (newly cleaned and dipped) heat exchanger. Interesting that running at lower rpm with lower coolant temps (like 130F) means that the water heater takes quite a few hours to actually get hot water.

I am also apparently overpropped. Max rpm was 2,350 with a 4 blade prop (20x16, the same size and pitch as the original 3 blade, which is still onboard).

I just got a IR tach and need to use both it and my IR temp gun to really see what is going on. Both the tach and the temp gauge are new and came with the engine rebuild, so they are likely correct. I'm not sure that I have an issue and, if I do have one, where it fits on my list of things to do.
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Old 05-12-2020, 09:44 AM   #6
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When you did all that work did you replace the thermostat?
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Old 05-12-2020, 09:53 AM   #7
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When you did all that work did you replace the thermostat?

No...
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Old 05-12-2020, 10:47 AM   #8
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Quote:
Originally Posted by Marco Flamingo View Post
Both the tach and the temp gauge are new and came with the engine rebuild, so they are likely correct. I'm not sure that I have an issue and, if I do have one, where it fits on my list of things to do.
Depends on if the sender was changed with the gauge. They need to be at least the same manufacturer designed to be used together. They are not calibrated, precision instruments. What they are is repeatable and cheap.
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Old 05-12-2020, 11:18 AM   #9
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Just assuming the temp is correct. You want more heat in your engine. Change the thermostat. Running too cool is probably the reason for the black smoke.

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Old 05-12-2020, 06:14 PM   #10
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Running too cool, long time to warm up, temp gauge ok, is almost always the thermostat stuck in a opened position (unless you're in Glacier Bay next to a berg).
If you run a bus heater that's too big can also effect warm up and operating temp because it bypasses the thermostat.
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Old 05-13-2020, 09:07 AM   #11
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Running too cool, long time to warm up, temp gauge ok, is almost always the thermostat stuck in a opened position...
Yep. Even at idle speed, if your thermostat is working correctly, the engine should eventually get up to normal, operating temperature.


A lot of people have the mistaken notion that the cooler your engine runs, the better it is. Not at all true! I once knew a guy (who THOUGHT he knew a lot about engines) who said that you should just remove the thermostat, so the engine can run as cool as possible. Dumb, dumb, dumb!
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Old 05-13-2020, 10:05 AM   #12
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My twin FL120s always read around 160 on all four gauges at cruise RPM of 1600-1750. IR thermometer at the senders read about 180. Never bothered going through the process of getting new gauges or senders in the possibly futile effort to wring 180 degrees indicated out of the system and just assumed a 20-degree error.
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Old 05-13-2020, 10:29 AM   #13
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I changed sender and gages so mine would read correctly. I also found gages that had a 180F mark. Not many do and I didn't want to guesstimate.
My Lehman also has the Bob Smith suggested manual gage off the expansion tank and that reads about 172 like he said it should.
The electric gages read right at 180 during normal cruising. And that agrees with the IR temp gun taken at the sender.
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Old 05-13-2020, 02:25 PM   #14
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Too cold, gauge or thermostat. Black smoke, loaded engine, unburnt fuel, way overpropped.
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Old 05-14-2020, 10:56 AM   #15
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I bought a new thermostat and cap, to be installed this weekend... let you know what happens.
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Old 05-25-2020, 09:25 PM   #16
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New Thermostat

Hi All,

I replaced the thermostat, took her out for a run, and as before the gauges (helm and bridge) read 160 deg F. I shot the infrared heat gun at the thermostat and it read 178. So now I'm satisfied with the actual engine temp.

Which brings me to my next thought. I replaced all the gauges in the helm and on the bridge with Siera heavy duty gauges. After doing some research, I see there is such a thing as a dual gauge sending unit. I didn't know that and wired the gauges as I found them. If the sending unit I have is for a single gauge and I piggy backed the bridge gauge off the helm gauge, would it possibly read low?

I am going to replace the sender with a Siera dual gauge unit advertised for the gauge set I have. Same goes for the oil pressure gauges... Thoughts?

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Old 05-25-2020, 09:39 PM   #17
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According to my op manual, normal range is 175-190 F, but Optimum temp is 195f! Both my gauges go about 185 or so. Last summer after running for an hour, I noticed temp creeping up. I remembered that we had gone thru some kelp just south of Anacortes in the channel. So I stopped and backed up for a ways, then went back to the go ahead. Temp went back to normal.
I admit to being a gauge watcher..
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Old 05-25-2020, 10:10 PM   #18
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As long as you know what the gauge reading represents, why replace them? All you're really looking for in a gauge is if the temperature is going up.
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Old 05-25-2020, 10:13 PM   #19
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As long as you know what the gauge reading represents, why replace them? All you're really looking for in a gauge is if the temperature is going up.
Spent too many years as a flight engineer looking at analog gauges... what they read was accurate. Old habits perhaps?
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Old 05-26-2020, 06:32 AM   #20
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Spent too many years as a flight engineer looking at analog gauges... what they read was accurate. Old habits perhaps?
Yes you want to know your gages are accurate. No need to second guess. Everything is not always linear.
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