Another overheat mystery . . .

Michael Algase · July 07, 2007, 02:10:29 PM

So the engine started overheating last weekend (M25. Boat is an 86, #91). Temp was reading ~185-190, 1900 RPM cruising at ~4.5 knots.

I first verified that I have raw water passing through the heat exchanger. Confirmed that. Seemed like a regular flow out the transom. I also verified that when I remove the hose from the freshwater pump to verify flow through the hot water heater, I get flow out the hose. That started me thinking it could be the thermostat. I picked up a replacement at the local Kubota dealer, and changed it today. Still overheating under load.

Next step was to remove the thermostat, and run the engine. It maintained ~165-170 on the gauge. That seems a little warm to me given the lake temperature is no higher than 70F. When I look inside the pressure cap, I see the coolant swirling in the exhaust manifold indicating freshwater (antifreeze) flow.

I did notice when the engine was running without the thermosatat, even though the temp sensor showed that number, I could place my hand on the exhaust manifold and pretty much keep it there. That would indicate being cooler than 165F.

I also noticed that with the thermostat removed, the heat exchanger was warm, but not hot, to the touch.

So I am struggling a little here. One thought is that the temp sending unit is bad, and I am getting a false positive on the overtemp alarm. The other is that the freshwater pump is going, and can't establish enough coolant flow through the engine. That seems unlikely, because I think that pump uses a rigid impeller, not a rubber one that could lose vanes. I am going from here to Torresen to check out the parts diagrams.

Any chance anyone has a part number for a temp sender that is Kubota, versus paying a "Marine" price for the sender?

Thoughts / comments welcomed and appreciated.

Michael

jmnpe · July 07, 2007, 10:43:52 PM

Hi Michael,

Check all the connections between the water temp sensor and the indicator, including the ground connection of your temp indicator. A higher than normal series resistance ( i.e. - "bad connection" ) in the temp sensor circuit will cause your temp indicator to read higher than normal.

Sounds like your temperature is really close to normal, so this would be the first thing to check.

John

Stu Jackson · July 08, 2007, 10:28:12 AM

Michael, when was the last time you replaced your raw water pump faceplate? We had what amounted to "reduced" flow which made our thermostat "creep" up ten degrees, and then the pump stopped working altogether. I replaced the faceplate and another new impeller (new impeller last done less than two months ago!) and all is well. We have the Oberdorfer raw water pump.

Jeff_McKinney · July 08, 2007, 02:31:44 PM

Michael,

In case you have not already looked at the following, some other causes might be:

1. Scale build-up in the heat exchanger. A coating of 1-2 mm thickness inside the little tubes will significantly impair the heat flow from the engine coolant to the raw water circulating in the jacket. There are numerous posts in here on cleaning it out. Worst case scenario is you'll have to pull it and get it "boiled out" professionally.

2. Look in the end of the exchanger where the zinc lives. An accumulation of residual anode crumbs, impeller pieces, or other debris will reduce the flow.

3. Check that the raw water intake strainer is clean.

Good luck.

Mike and Joanne Stimmler · July 08, 2007, 06:07:07 PM

The raw water intake strainer is the easiest thing to check first. Because of all the seawead and other stuff that floats around my dock, I check mine every time I go out. When I take out the strainer I open the thru hull valve to verify water flow to the strainer.

Mike

Stu Jackson · July 08, 2007, 07:41:18 PM

Michael, Jeff's ideas are pertinent, and I also suggest that you check the intake and discharge pipes in the HX. I have never found a problem with the tubes inside the HX, but have many times reported that the hose connections get "gunked up" with HEAVY DUTY scale and could be a cause of your issue.

Michael Algase · July 09, 2007, 10:48:46 AM

Thanks all for the suggestions. A couple of points / comments:

1) The engine has the 2" heat exchanger on it. I replaced the zinc this year, and found I had a fair amount of calcification on it (had trouble extracting it from the opening), but didn't really think much about it. Maybe it is time to pull the end caps off and see what gives in there. From the looks of it, the raw water flows around the fresh water in that exchanger. Maybe it still being warm without a thermostat in it indicates a restriction, so engine heat has more time to exchnage for sawater temperature rise . . .but:

2) I have a fair amount of water flow out the back end with the exhaust. I would be hard pressed to say it is less than normal, so I don't think it is the impeller, but will check that out as well. I have a sherwood water pump and don't know how much the cover plate could wear. I will open it and check for missing vanes and coverplate wear this weekend.

3) Missing impeller vanes or not, I think it may be time to remove the cooling system hoses one at a time and verify they are clear.

3) I still have the old wiring harness (I can hear you all cringing now), and actually broke the connection at the engine end this spring (first time in my ownership it has ever been opened, so probably the only time in the life of the vessel). Makes me think that there could be additional resistance in that connector as a result of a demate / mate. I am prepared to rewire to the control panel, so I think that may be this weekend's project.

I'll probably still order a temp sensor, perhaps an end cap for the heat exchanger, a new coverplate for the water pump, and maybe the hoses if not too expensive. I figure the parts are less than the cost of labor to have a marina doing the work anyhow . . .

Again, thanks to all for the feedback. New ideas and fresh perspectives always helps.

Michael

Gary · July 09, 2007, 11:05:48 AM

Hi Michael,

You engine temp " was reading ~185-190, 1900 RPM cruising at ~4.5 knots." From that first post the temperature (185-190) is what an older 2" heat exchanger will produce, usually after an hours running time.

If you change to the 3" heat exchanger the temperature will run at a steady 160 degrees regardless of running time.

Gary

Michael Algase · July 16, 2007, 08:44:10 AM

I think I got it licked this weekend. Here's the final deal - and it links directly to the thread posted for electrical problems at http://c34.org/bbs/index.php?topic=3794.0.

After much though about the valuable responses here to my original symptoms, I took a two pronged approach. Not sure which one fixed the issue, but neither was overly expensive.

First, I did my equivalent of the harness upgrade. I read what "jmnpe" wrote above, and it made a lot of sense. I pulled the motor this spring to do a transmission, and opened up the harness connection at the engine end. When I read the post about contact resistance, the bells went off. Never touched them in the past, but did open one for thefrst time in 21 years this spring. Then I looked at the wiring diagram, and one of the 8 wires is the temp sensor lead. Taking Ron Hill's approach (thanks Ron) I replaced everything with #10 wires (had them from another project anyhow). Where necessary, I used 1/4 in. spade lugs and receivers to adapt #10 to #18 (i.e. at the purple alarm wire at the panel, and on the oil pressure sensor at the engine).

Next, I installed a new temp sensor. I had purchased one from Torresen, and even if the wiring did the trick, I would have paid return shipping and a restocking charge if I did send it back. (One drawback of being 1.5 hours from the boat - buy all the parts you can think you would need, rather than losing days of time ordering parts for the next trip).

Started up the enigne, and let it get up to temp. No issues. Right in the slip (blowing stink and not the right day to go out and motor to test it), I put it in gear, and ran it at 2500 RPM for about 10 minutes. I watched the temp vary between 170 & 175 as the thermostat opened and closed, but it never went to 180 degrees (160 degree thermostat, but a 2" heat exchanger).

Bottom line - I should have done the harness upgrade much sooner, and if I had intact connections the whole time, I may not have lost two weekends sailing to engine troubleshooting and repairs.

Michael

Norris Johnson · July 16, 2007, 10:25:55 AM

My last heating problem was solved by removing the HE and pulling the end plates. About 1/3 of the tubes were plugged, not fouled, but plugged. I took a 1/8 welding rod, removed the flux, flatted the end just enough so it would fit in the tube. Put the welding rod in a variable speed drill and drilled out each tube from both ends of the H/E because the welding rod isn't as long as the H/E. The last cleaning step was to install one end plate and fill the tubes with Lime Away and leaving it until the chemical reaction quit (about 3 to 5 mins). Then I repeated the process with the shell side. Not much reaction there because it is the treated H2O side.

This job is fairly easy after you remove the 'dog house' from the back of the engine. It is held in place with 7 screws. I went back with 4 screws.

Before this, I couldn't run the engine over 3/4 throttle. Now I can run it wide open and the temp is never over 165 F with Matagorda Bay H2O temp over 80 F.

Ron Hill · July 16, 2007, 05:36:12 PM

Morris : Alot easier to clean out the HX with Lime-a-way (in place) every couple of years - than taking off the end caps or removing it completely.

Norris Johnson · July 16, 2007, 08:38:56 PM

Ron,

Had to pull it because about a third of the tubes were completely plugged, but you are right if it was done on a regular schedule it could be done it place. My point is, it is very easy to get to the H/E if you remove the "dog house".

Ron Hill · July 17, 2007, 02:46:12 PM

Norris : FYI, The 50% ethyleneglychol(sp?) coolant mixture is what flows thru the tubes. The raw water is what's flowing on the outside of the tubes and that's usually where the real sediment occurs.
I always mix distilled water with my Prestone to insure no sediment can occur inside the tubes and change it out every 3 or 4 years.

John Sheehan · July 17, 2007, 07:10:16 PM

Ron,

On our M-35B the raw water goes through the tubes and the antifreeze solution goes on the outside of the tubes. If you look at the position of the zinc in the heat exchanger the raw water goes around it at the end of the tubes.

John

jmnpe · July 17, 2007, 08:02:49 PM

Hey Ron,

You have it backwards. Look on p 16 of the Universal Operator's manual. The raw water goes through the copper tubes which are submerged in the engine coolant.

On my old Perkins 4-154 in the Stamas 44, you could see the guts of the HX ( i.e. - bundle of copper tubes ) looking down through the radiator cap into the Prestone...

John