mysterious overheating OB

[rstrange]

Forgive me if this is a long post, but I want to get all the detail in. Bought a '94 Johnson 120 (20" shaft, V6 gear case, 15x17 SST prop) and 19'Hydraskiff new. Every year I trailer to south FL from TN for two weeks of intensive SW fishing. The first few years everything was fine. About three years ago, I began to get occasional overheating. Cruising at 4000-4500, I get the horn and come down off plane. At idle horn goes off and usually does not immediately sound again when I start off. Then a day or two later it goes off again. Nerve wracking and not confidence building. This happens 3-6 time in two weeks. In the last three years it has happened once in TN.<br /><br />The first time it happened I changed out the water pump, when I got back (I have a chrome w.p. housing and have changed the impellers several times though they always look good and pressure is consistent, 24 psi at 4300 rpm). It did it again the next year. This year I decide to get serious about troubleshooting it before I go down next month. I put in a Teleflex temp gauge, two sensors and switch so I can check both heads under way. I haven’t calibrated the gauge, but according to the range Teleflex gives, the second line should be 115, the middle line 150, and the upper line 185, there is a redline above 185 and below 115. I’m figuring it is a balky thermostat or damaged restricter under head (or maybe sand there). My Clymer manual says my engine should run about 120 with the horn at about 200 (if I’m reading it right).<br /><br />I take out boat and the right head runs hotter (about 160-170) than the left (about120). So I replace the right thermostat and test again. The right side still is running hotter, but not quite as much (about 150), left still 120. OK, today I replace the left thermostat, now the LEFT side runs hotter (about 160-180) and the right side still about 150. I guess I have at least ruled out restricter problem as the I changed which head was hotter with new thermostats. But it seems to be all around too hot, but I really don’t know w/o calibrating the gauge. IDEAS???

 

[clanton]

Re: mysterious overheating OB

You can verify the engine temp with a melt stick from NAPA, 163 degrees I think, check the book, or borrow from your mechanic his Raytek laser temp checker. Engine temp ok check alarm system, and temp sensors included the blocking diode, which should be 19 3/4 inches from the large red connector in the engine harness. Do you use inpeller with o-ring on bottom side of inpeller, and change the o-ring under the inpeller cup. O-ring under cup needs to be glued in, insure tiny air bleed hole under o-ring is open.

 

[jegervais]

Re: mysterious overheating OB

Here's the spec's from the factory service manual:<br /><br />Run engine (in gear - preferably with correct test prop - in lake or test tank) @ 3000 rpm for 5 minutes. Reduce rpm to 900 rpm for 5 more minutes. Check temp at top of t-stat pocket.<br />Engine temp should be 125 degrees (F) to 150 degrees (F). If temp is too high or to low troubleshoot cooling system to correct problem. <br /><br />Run engine at 5000 rpm, temp should not exceed 120 degrees (F). If temp is over 120, troubleshoot cooling system for overheat problem.<br /><br />Hope this helps.<br /><br />-John

 

[Paddling]

Re: mysterious overheating OB

Hi rstrange, I have had this very same problem on my 150 hp evinrude. You may find that the water deflectors in the cooling water passages have swollen up to obstruct the water flow. You probably have salt build up in the passages also. This situation will cause over temp at upper revs but not at idle, until the passages become totally blocked. According to local mech. this is a common problem on older salt water motors that are not flushed for 30 minutes or more after salt water use. I had to pull head covers and heads to remove salt and replace deflectors.

 

[rstrange]

Re: mysterious overheating OB

Paddling, that sure sounds like my problem, cool at low rpms and too hot at higher rpms. I was hoping to avoid pulling the heads, but now I will. Any precautions? I never pulled the heads on a big motor before.

 

[jegervais]

Re: mysterious overheating OB

Hold it there sports fans!!! The 120 looper does not use water deflectors. Think again before you pull those cylinder heads.<br /><br />-John

 

[rstrange]

Re: mysterious overheating OB

Good point, John! The manual confirms this. However, what about salt build up and sand blockage (I sucked up some serious sand early on) under the heads? If it is not the water pump or the thermostats and it is not the alarm (as the temp gauge shows it too hot)- wouldn't pulling the head be the next step? I don't want to do it but the fact is my engine runs cool up to about 2500 rpm and then heats up. The looper 4 is supposed to run around 120 all the time.

 

[jegervais]

Re: mysterious overheating OB

Does your motor still have the aspirator valve on it?? If so, you should consider removing it, and all related hoses and changing the t-stats, springs and covers to the 1992 vintage p/n's. It's more reliable than the aspirator and an old or plugged aspirator valve can cause hi rpm overheat problems.<br /><br />Granted, IF your cooling system is plugged up by sand, salt, whatever you may have to start taking things apart. If you want to go that route first, well, ok, head gaskets are only about $20 each. You could also try back-flushing the cylinder heads. Remove the t-stat covers and hardware and stick the garden hose in there and turn the water on (engine not running). <br /><br />You can also try purging debris by running the engine in a tank or barrel (about 5 - 50 minutes) with those same components removed - keep running speed under 1500 and keep an eye on cylinder head temp and an ear open for the alarm horn. <br /><br />By the way, have you ruled out a defective or loose temp switch?<br /><br />-John

 

[rstrange]

Re: mysterious overheating OB

John, I know it is not just the alarm, because I installed a pair of teleflex temp senders and gauge (BTW, I just calibrated the gauge and senders against a thermometer and found that at lower marks it was hotter than I thought. I am running about 145 at low rpms and around 170 at high). <br /><br />I don't fully understand your first paragraph. The aspirator is the thing that sucks up water that gets under the cowling? I should remove that? Seemed like it would be good to have. "changing the t-stats, springs and covers to the 1992 vintage p/n's" I just installed new thermostats that the dealer looked up on his computer...I presume they were 1994 p/n's. Replacing the t-stat springs might help?<br /><br />I ordered head gaskets to be in Tues, so I guess I'll pull the heads...I'm kinda curious. Besides following the torque sequence, is there anything I should watch? My Clymer says to use gasket cement on head gaskets, would you?<br /><br />Thanks<br />Richard

 

[rstrange]

Re: mysterious overheating OB

Is the aspirator valve have anything to do with that tube that comes out of the t-stat cover on some loopers? Mine doesn't have that.

 

[jegervais]

Re: mysterious overheating OB

The aspirator valve does have a bit of "extra plumbing" that goes to the t-stat cover. The valve itself is connected to the throttle lever and as the throttle lever is advanced, it diverts water flow, allowing the t-stat to unseat. It can be a source of problems, but apparently not yours... Somebody must have removed it previously. <br /><br />There were some factory "approved" changes to that cooling system, including installing the black "capsule" t-stat assemblies, a different spring and cover (same used in 1992). The springs are "lighter" - combined with the older style t-stat cover, this allows the t-stat to unseat sooner and farther = lots more cooling water thru the block. A sharp parts guy who is familiar with the product line might know this.<br /><br />Where are your temp gauges sending unit(s) mounted? Are they on the back face of the cylinder head?? If so, you WILL see higher temps than the motor is considered to be running (compared to the service manual test spec's which take the engines temp at the top of the t-stat pocket). This is why I think outboard temp gauges are a waste. <br /><br />Through-out this exchange I've gone back and completely re-read your original post several times. Based on what you say, I do believe you MAY have an over heat issue going on, but I need to ask 2 questions: 1. In your original post, when you quoted your temperature reading on both cylinder heads, what did you use to measure the temp and where did you measure it from?? 2. Have you verified the temp switch is functioning at the correct temp and is not loose??<br /><br />-John<br /><br />BTW - Use the OMC gasket sealing compound on the cylinder head gaskets (both sides).

 

[rstrange]

Re: mysterious overheating OB

John, I measured the temp with a Teleflex OB temp gauge and two senders with a switch. The temp gauge senders are mounted in the threaded holes opposite the temp switch at the top of both heads. They would be reading essentially the same temp as the temp switch (I think). Since I calibrated the gauge and senders against a thermometer yesterday, I would have to revise the lower temps upward: idle to 2500-about 140-150, 4000 to 4500 about 160-180.<br /><br />My engine was purchased new in the summer of 1994 and never had the "extra plumbing" on the t-stat covers. I don't know about the springs, but they are original.<br /><br />I am not sure whether to pull the heads or not, but I am running out of ideas.<br /><br />I really appreciate your time and interest...thanks so much, it would be great great if I could solve this riddle that has bugged me for several years.<br /><br />Richard

 

[rstrange]

Re: mysterious overheating OB

I tested both temp switches this morning, they are close to spec, closing at 205 (right) and 215 (left, dual temp/quickstart switch) and opening again before 150.

 

[jegervais]

Re: mysterious overheating OB

Richard,<br /><br />Thanks for that info. I've got the day off today, so I don't have access to our service manuals... If I remember correctly, the temp switch spec is 205 +/- so many degrees (I think 6 degrees). Also, while I was looking thru the books yesterday, I didn't see where any '94 model didn't have the aspirator valve - perhaps you have a different model year than you believe??? What's the model number???<br /><br />You are correct, your gauge's sending units are essentially reading the same temp as the switches. Keep in mind, cylinder head temps will be higher than what the manuals tell you when you temp check an engine.<br /><br />Based on everything we've discussed so far, if I were working on your engine here's what I would do - I would install a test prop, and run the engine at the rpm where the problem occurs. This could be done in a test tank or tied to a dock. I have an infra-red temp gun and would use it to monitor engine temp at the top of the t-stat pocket. At any time if I saw over 120 degrees I would then shut down and pull the heads. If the alarm sounded and I did not see in excess of 120 degrees, I would dig into the alarm system.<br /><br />I'll probably be a little scarce the rest of the week-end, but I'll check back tonite and tomorrow nite. Keep us posted on what you're doing and anything you find.<br /><br />-John

 

[rstrange]

Re: mysterious overheating OB

John, the model is J120TXARC, serial is G03726253. Since I don't have a heat gun or test wheel, what I could do is get a temp stick for say 130, mark the top of the t-stat pocket and run it down the lake. I'll call a few auto parts stores and see if I can get one. <br /><br />Remember, in all the years I have had the problem, I've only gotten the alarm once in TN, but I get it every 5-10 hours down in FL. The water is warm (80), but no warmer than summer in TN. If it is a alarm thing maybe the salt has something to do with it, like I say, it is a mystery.<br /><br />Let you know what I find out.<br /><br />Richard

 

[rstrange]

Re: mysterious overheating OB

Well, auto parts stores don't carry thermomelt sticks, I discovered. I might be able to get them from a welding supply on Mon. The Clymer recommends using the 125 and the 163 when testing for overheating the 125 is supposed to melt at idle and the 163 is never supposed to. 163 seems awful hot for a engine that is supposed to cruise at 120, maybe a 138 or something might be more reasonable than a 163. <br /><br />John, if you see this tonight let me know if this melt stick approach is worth pursuing, if it is not, I'll dive into the heads tommorrow. If it is, I'll see if I can find the necessary sticks next week.<br /><br />Thanks!

 

[Franki]

Re: mysterious overheating OB

My old outboard has little in common with yours.. apart from being OMC...<br /><br />but when I first got it, it ran hot, very hot, so much so that I turned it off after 30 seconds of running..<br /><br />What I did, was drop off the lower unit,, take out the thermostat and flush inside with a hose.. then put the cover back on without the thermostat..<br />then attach the hose to the brass pipe that would normally plug into your water pump..<br />(in my case, it stretched over and I put a hose clamp on it..)<br /><br />then I turned the water on slowly.. at first I got a tiny trickle,, so I turned the tap up harder progressivly and about 10 seconds after I had the tap on full, all manner of crap, mud, white stuff and bits of gunk blew out of the cooling channels.. and the water did flow.. <br /><br />from that point on, things have been great, and I didn't have to pull the head covers or anything.<br /><br />maybe its something you should consider?<br /><br />rgds<br /><br />Frank

 

[jegervais]

Re: mysterious overheating OB

Follow the books recommendation.<br /><br />-John

 

[rstrange]

Re: mysterious overheating OB

Well, I couldn't find the Tehrmomelt stick locally, so I ordered one, when it comes in I'll do the melt stick test according to the book and post the results.<br /><br />Richard

 

[clanton]

Re: mysterious overheating OB

2 other things that cause warning horn problems, if it is a false signal is high/ erratic voltage and loose electrical connections.