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Old 02-28-2011, 01:54 PM   #1
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High RPM = Hot Engine

Just bought boat, it's been sitting for about four years.* It has a LF120; starts great.* Temp guage stays around 180 all the way up to around 1600 or so RPM.* If I try 1800-2000* the temp goes to 200+ quick.* Now, after reading the post about 3 vs. 4 blade props I'm thinking the prop may be the trouble; 4 blade.* BUT, the oringinal owner (passed away) was an engineer, loved his boat, an had four props (two 3s & two 4s).* The boat; '79 41ft, came with a 3 blade on it.* SO, is my problem the engine or prop?* Oh yea, I only tried opening it all the way once; topped out at 2100 RPM with a lot of black smoke.
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Old 02-28-2011, 02:01 PM   #2
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RE: High RPM = Hot Engine

A complete servicing of the HXs, water pumps maybe replace all hoses etc is in order. Do the same to the genset.

Once you are 100% sure of the cooling system, then you can start worrying about the prop size - if it is even an issue. Validate that the temperature gauge ( check with IR gun) and tach are accurate (use a photo tach)
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Old 02-28-2011, 02:28 PM   #3
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RE: High RPM = Hot Engine

Make sure the thru-hulls are clear. If they have any kind of grating on the outside, check inside the grating for obstructions. Multiple coats of antifouling can partially block the gratings too.
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Old 02-28-2011, 02:42 PM   #4
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High RPM = Hot Engine

My guess is that the impeller is shot plus the coolers and exchangers may have to be boiled out.

Be sure to find all the pieces of this impeller if it is damaged and all the pieces of previous impellers that may still be in there.

-- Edited by Doc on Monday 28th of February 2011 03:43:58 PM
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Old 02-28-2011, 03:06 PM   #5
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RE: High RPM = Hot Engine

Tom White (Sunchaser) is precisely correct.
You have a cooling problem, not a prop problem.

You didnt say where you or the boat is. Suggest you start by calling American Diesel, the foremost experts on the Ford Lehman engines, and consult with them FOR FREE. You will be glad you did, as so many of us Lehman owners have.

When your cooling system is passing water correctly, your engine should be able to be run at full throttle and never heat up much past 190 deg.

Secondly, you need to verify your tachometer with a infrared tach so you know the REAL RPMs you are doing when the heat changes.

Dont even worry about props. Thats a different discussion. After you get the engine cooling correct, come back and start a thread about props.

R.
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Old 02-28-2011, 03:32 PM   #6
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RE: High RPM = Hot Engine

"Temp guage stays around 180 all the way up to around 1600 or so RPM.* If I try 1800-2000* the temp goes to 200+ quick."

That could be related to blocked or dirty heat exchangers or*a damaged raw water impeller.*

Do you have what appears to be a good flow of*raw*water coming from the exhaust pipe while running? Does there appear to be*"steam" coming out with the exhaust when the temperature rises? Are there hot spots on*the*exhaust pipe downstream of the spray ring? Is the exhaust pipe discolored*around the manifold or spray ring?*Is the heat exchanger housing too hot to touch?

What condition is your thermostat?

****
"I only tried opening it all the way once; topped out at 2100 RPM with a lot of black smoke."

Was there much smoke at lower rpm or did it suddenly appear black when at or approaching 2100?

How accurate is your tachometer? Does your transmission run unusually hot or noisily? How tight is*the stuffing box? Does it run hot?

Is the exhaust clear until it reaches the point where the temperature begins to rise?

What pitch are the props you got with the boat? What pitch is installed?

I suspect the PO may have subscribed to the "cruise prop" method of reducing diesel engine life but*there are many more questions to be answered before reaching a conclusion ...
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Old 02-28-2011, 04:53 PM   #7
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RE: High RPM = Hot Engine

The first thing I do with any new to me boat is go thru the raw water side 100% as discussed above.
Take NOTHING for granted.
Everything comes apart including the intake hose from the thru hull so you inspect for blockage.
Then remove everything and clean it out, minimum a new impeller but you might need a complete pump.

Remember that the raw water system is the engine's lifeblood. Don't skimp.

If I were you I would also consider buying or borrowing a IR heat gun and "shoot" the header tank (should be 170 F) and gage senders to verify the gage readings are close. They very well may not be.

And check the tach as mentioned above.
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Old 02-28-2011, 07:01 PM   #8
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RE: High RPM = Hot Engine

"I plan to simply run a wooden dowel through each tube, but how do I complete the job correctly"

There are lots of ways, but I take everything apart and rod the tubes with a .22 cal bore brush.
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Old 02-28-2011, 08:31 PM   #9
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RE: High RPM = Hot Engine

Regarding David's accurate comment on max RPM, until you get the cooling system up to snuff, do not attempt a max RPM test in gear. At the dock in neutral yes, which would be a good test of governor, cables and linkages all being OK.
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Old 02-28-2011, 08:49 PM   #10
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RE: High RPM = Hot Engine

Quote:
Old Stone wrote:

Again, I am amazed with what I learn here. I agree with everyones' comments, and come away with two things this time.........the final decision to buy an infrared temp. guage, they are ridiculously affordable. But second, and this is one of those "why the h*** did that not occur to me before", is from Rick this time: "Is the heat exchanger housing too hot to touch?" So simple, so straightforward, so dang logical!
Thanks Rick

So while we're on the subject, is it safe to flush the exchangers with muriatic acid? I plan to simply run a wooden dowel through each tube, but how do I complete the job correctly
On Volunteer I would take both end caps off the heat exchanger and run a gas brazing rod through each passage, I "soften " the square edge of the brazing rod with sand paper so it cannot gouge the inner side of the tubes...
HOLLYWOOD

*
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Old 03-01-2011, 12:55 AM   #11
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RE: High RPM = Hot Engine

Quote:
DavidM wrote:

The brazing rod or 22 cal bore brush will open up the tubes, but will leave a layer of scale. I wouldn't use muriatic acid as it is too aggressive and somewhat dangerous. But take the heat exchanger, close one end and set it upright. Fill it and drain it a couple of times with hot water to heat up the metal. Then pour a hot, 50/50 mixture of Rydlyme or CLR in the top until it is full. Let stand for a couple of hours and then flush with lots of water. You will then have sparkling tubes

David
For those inclined to have others do this work, every 6 years the heat exchanger goes to a radiator shop where it is flushed, cleaned, pressure tested, new end caps installed if needed,*new end cap gaskets, and painted to match the engine color.*

About $100.

*
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Old 03-01-2011, 04:23 AM   #12
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RE: High RPM = Hot Engine

The black smoke at full RPM is the giveaway. Overload.

The engine is probably overproped , a smart move on a slow boat, esp if the PO was an engineer.

Cruise at 300 rpm below the 2100, or where the black smoke stops, and spend the bucks for an EGT to operate the boat as set up.

The speed difference of the reduced rpm should be minor.

I would guess 1600-1700 was the desired cruise rpm , smooth , quiet and efficient.
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Old 03-01-2011, 12:22 PM   #13
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RE: High RPM = Hot Engine

Quote:
DavidM wrote:

While I agree 100% with the comments about completely servicing your cooling system, you do have a prop problem or maybe a bottom problem. It may be as simple as a fouled prop. The FL 120 should reach 2400 rpm at wot. If you never, ever run it above 1,800 rpm you can get by with that much overprop (assuming the prop and bottom are clean). But if you want to push it any higher I would recommend taking a couple of inches of pitch out to get up to 2,400. Usually one inch is good for 200 rpm.

David
The Lehman operators manual, page 9, SPECIFICATIONS, indicates 2500 RPM is the max RPM under load. (not 2400). Must use a calibrated tach to make this measurement, and you MUST first measure the NO LOAD max RPM which should be 2650 RPM.* If you cant get 2650 under NO LOAD then you will never get to 2500 under load (or Wide Open Throttle WOT).

R.

*
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Old 03-01-2011, 12:29 PM   #14
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RE: High RPM = Hot Engine

"I would guess 1600-1700 was the desired cruise rpm , smooth , quiet and efficient."

Bob Smith recommends cruise speed of 1800 RPM for the Lehman 120 engine.

No guessing about it.

R.
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Old 03-01-2011, 12:42 PM   #15
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RE: High RPM = Hot Engine

Quote:
ralphyost wrote:


The Lehman operators manual, page 9, SPECIFICATIONS, indicates 2500 RPM is the max RPM under load. (not 2400). Must use a calibrated tach to make this measurement, and you MUST first measure the NO LOAD max RPM which should be 2650 RPM.* If you cant get 2650 under NO LOAD then you will never get to 2500 under load (or Wide Open Throttle WOT).

R.
**exactumundo



*
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Old 03-01-2011, 12:49 PM   #16
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RE: High RPM = Hot Engine

I just looked again at the SPEC page of the Lehman Operators Manual......


...the TURBO version of the IN LINE 6 Lehman engine (the I-363 6yl)
does specify 2400 RPM as Max RPM under load...but I believe this fellow has a normal aspirated Lehman 120, which means 2500 RPM applies.
R.
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Old 03-01-2011, 01:04 PM   #17
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High RPM = Hot Engine

Quote:
ralphyost wrote:If you cant get 2650 under NO LOAD then you will never get to 2500 under load (or Wide Open Throttle WOT).
*Not necessarily so.

2650 rpm is "high idle" or the maximum rpm setting. It is determined by the position of the governor flyballs and beyond that speed the "speed stop" prevents more fuel from entering the injector pump.

2500 rpm is the rpm at which maximum rated horsepower is produced. If the boat is fitted with a "well matched" propeller it is also the rpm at which the "fuel*stop"*prevents further*travel of the fuel control rod and no additional fuel*can be*delivered to the injectors.

These are three separate and adjustable settings. Max rpm can be obtained in neutral with very little fuel, that setpoint is called "high idle."

Maximum*power available *is controlled*by limiting the maximum amount of fuel that can be delivered, that setpoint is called the "fuel stop." It used to be called the "smoke stop" because any fuel beyond that amount will only cause black smoke and engine damage from overheating.

Idle is set by limiting the minimum amount of fuel that can be injected.


-- Edited by RickB on Tuesday 1st of March 2011 02:09:59 PM
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Old 03-01-2011, 01:09 PM   #18
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RE: High RPM = Hot Engine

Quote:
RickB wrote:

*
*

Maximum*power available *is controlled*by limiting the maximum amount of fuel that can be delivered, that setpoint is called the "fuel stop." It used to be called the "smoke stop" because any fuel beyond that amount will only cause black smoke and engine damage from overheating.



would this also be the Maximum governed RPM?

SD
*
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Old 03-01-2011, 01:12 PM   #19
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RE: High RPM = Hot Engine

"would this also be the Maximum governed RPM?"

No. Max governed rpm is the high idle speed. It is the highest rpm allowed in any circumstance.
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Old 03-01-2011, 02:28 PM   #20
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RE: High RPM = Hot Engine

Quote:
RickB wrote:

*
ralphyost wrote:If you cant get 2650 under NO LOAD then you will never get to 2500 under load (or Wide Open Throttle WOT).
Not necessarily so.

2650 rpm is "high idle" or the maximum rpm setting. It is determined by the position of the governor flyballs and beyond that speed the "speed stop" prevents more fuel from entering the injector pump.

2500 rpm is the rpm at which maximum rated horsepower is produced. If the boat is fitted with a "well matched" propeller it is also the rpm at which the "fuel*stop"*prevents further*travel of the fuel control rod and no additional fuel*can be*delivered to the injectors.

These are three separate and adjustable settings. Max rpm can be obtained in neutral with very little fuel, that setpoint is called "high idle."

Maximum*power available *is controlled*by limiting the maximum amount of fuel that can be delivered, that setpoint is called the "fuel stop." It used to be called the "smoke stop" because any fuel beyond that amount will only cause black smoke and engine damage from overheating.

Idle is set by limiting the minimum amount of fuel that can be injected.


-- Edited by RickB on Tuesday 1st of March 2011 02:09:59 PM
*

Rick
Good explanation...... and a much more precise explanation.

However, I still dont get it when you say "not necessarily" to my statement that if the engine cannot achieve 2650 RPM (when the fuel injector pump is at the speed stop), then it will never be able to deliver the max RPM at full load.

If the speed stop is incorrectly set, and reduces the max RPM (NO LOAD), then the throttle travel on the fuel injector pump is not moving far enough to allow the correct amount of fuel to the injector system.

If the injector pump is not allowed to provide the correct MAX amount of fuel (based upon throttle travel), then how will the boat ever be able to produce the max power available to get to MAX RPM UNDER LOAD ?

???

thanks
R,

*
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