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

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.

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.

High Wire said:

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.

Click to expand...

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.

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.

When you did all that work did you replace the thermostat?

bayview said:

When you did all that work did you replace the thermostat?

Click to expand...

No...

Marco Flamingo said:

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.

Click to expand...

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.

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.

pete

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.

Lepke said:

Running too cool, long time to warm up, temp gauge ok, is almost always the thermostat stuck in a opened position...

Click to expand...

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!

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.

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.

Too cold, gauge or thermostat. Black smoke, loaded engine, unburnt fuel, way overpropped.

I bought a new thermostat and cap, to be installed this weekend... let you know what happens.

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?

Dave

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..

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.

Slowmo said:

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.

Click to expand...

Spent too many years as a flight engineer looking at analog gauges... what they read was accurate. Old habits perhaps?

Dave_E said:

Spent too many years as a flight engineer looking at analog gauges... what they read was accurate. Old habits perhaps?

Click to expand...

Yes you want to know your gages are accurate. No need to second guess. Everything is not always linear.

How about adding a set of mechanical gauges in the engine room? I didn’t trust the panel gauges at the helm after we rebuilt the engine. It took awhile to find gauges in the range and the display (simple/basic) that I wanted; oil 0-80 psi and temperature 100-220 degrees F. Farm tractor type companies usually had a good selection. Steiner Tractor Parts is who I ordered the gauges from.

https://www.steinertractor.com/

If your sending unit is designed and made for one gauge you will definitely get a wrong reading if it is powering two gauges. If there was always two gauges it is probably a dual unit, unless it has been changed out.

BTW, you will find there are two sending units on the engine. I have been told that the one on the port side is the sender for the high temp alarm. The one on the starboard side is the sender for the gauges. Again, they may have been switched out. Ground the wire on the starboard side sender to the block, the gauges should "peg".

I have also been told you want matching gauges, designed for dual installation.

pete

Dave_E said:

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?

Dave

Click to expand...

Using a single station sender with two gauges wired in series will produce an indication error.

The gauges work by producing a known voltage and applying this voltage to the sender. The sender has a variable resistance that is proportional to the temperature of the coolant in the engine. The variable resistance produces a variable current which drives the needle on the meter.

When you hook two gauges up to the same sender, you are introducing a bit more resistance into the circuit and skewing the display of the meter by a small amount.

The dual station sender has a reduced resistance built in to accommodate the extra gauge and should indicate closer to the actual temperature of the coolant.

sbman said:

Using a single station sender with two gauges wired in series will produce an indication error.

The gauges work by producing a known voltage and applying this voltage to the sender. The sender has a variable resistance that is proportional to the temperature of the coolant in the engine. The variable resistance produces a variable current which drives the needle on the meter.

When you hook two gauges up to the same sender, you are introducing a bit more resistance into the circuit and skewing the display of the meter by a small amount.

The dual station sender has a reduced resistance built in to accommodate the extra gauge and should indicate closer to the actual temperature of the coolant.

Click to expand...

Thanks! This was the good technical answer I was looking for!

Slowmo said:

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.

Click to expand...

I agree with this. After many hours of operation I have now put a dob of white out on the guage at the operating spot- oil pressure and temp so my glance at the instruments is not at the reading but if the indicator is under the dot.
I have also done it to the tachs - at the 1700 rpm sweet spot - that's to remind me and is an age related memory device, not a engine safety thing

Brisyboy said:

I agree with this. After many hours of operation I have now put a dob of white out on the guage at the operating spot- oil pressure and temp so my glance at the instruments is not at the reading but if the indicator is under the dot.
I have also done it to the tachs - at the 1700 rpm sweet spot - that's to remind me and is an age related memory device, not a engine safety thing

Click to expand...

This also makes it easy for someone that isn’t as familiar with the boat to check if things are normal. Just tell them to check if the needles line up with the dots then everything is ok.

You can check your sending unit with an ohmmeter. Disconnect the sender lead. At 100 degrees you might read 450 ohms. At 175 degrees you might read 100 ohms. Dual station senders should have about half the resistance of a single station sender.

Also make sure everything is US standard. (33-240) Lot of European gauges (10-180) installed in error. I am assuming your sender range is 0-240F. You could install a 0-300F which would a different gauge.

I've checked mine with IR. They're close enough.

Sorry guys the cooling system is to keep the engine at a efficient operating temp. To cold excess wear etc. Lehmans like 175-185 I believe . There's your goal , that's what a thermostat is for. Get a decent temp gun and check the thermostat housing temp. You can also pull the thermostat drop it in a pan of water put it on to boil and check the temp when it opens ergo.......

Ford Le

Dave_E said:

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

Click to expand...

The Ford Lehman 120 should run at a steady 180. Check your gauge, sender or thermostat.

FL engine manual says 175-190 and an optimum temp of 195F.
optimum= most favorable point