1999 200 OX66 --- Overheating Issues/Repairs

[Get'n Busy Liv'n]

I recently purchased a set of 1999 Yamaha 200 HP OX66 motors for my center console and this weekend I had the overheat alarm go off twice after extended running (15 minutes continous) at rpms above 4200. I removed the cowling and the head seemed pretty warm but the tell tale pee-stream seemed fine. The oil levels in both resevoirs was perfect so I'm pretty sure it was an overheating alarm/issue.

The strange thing is if I kept the rpms under 4000 I could run without the alarm sounding the entire trip. Plus I never received another alarm even after idilling for 6 hours straight.

The previous owner seemed to have taken excellent care of the engines (he gave me all the service records) so it appears that the impellers were changed during the last annual (roughly 25 hours ago).

Is this an impeller issue or am I dealing with something more serious? I was planning on doing the impellers within the next couple weeks anyway but I'm hoping you guys can help me troubleshoot this issue.

Thanks in advance!

[Get'n Busy Liv'n]

------------------ U P D A T E --------------------

Installed a new Water Pump Housing and Impellar Kit. The housing did show some small signs of damage and the Impellar definitely needed replacement.

Thinking I had the cause of the alarm licked ... I put the boat in the water and fired her up. The water flow from the tell tale still looked a little weaker than my port motor but when I increased the rpm a little it looked very strong.

I let the motor idle for about another 5 minutes while I cleaned up the boat some and then I got the overheat alarm again! This time I confirmed that my temp gauge showed the elevated temp. I immediately shut the motor down so as not to hurt anything.

Some people from another forum mentioned that the thermostat and/or temp switch could be bad also. I bought new thermostats and gaskets when I bought the Impellar Kit because I was planning on changing them anyway. I didn't look at the Thermoswitch yet ... but it seems I'll have to now to continue the process of elimination.

I saw in another post where people suggested the problem could be with the Poppet Valve (?sp) ... could this be my issue as well?

I'm quickly leaving my comfort level in trying to solve this issue .... any advice would be appreciated.

[Ken]

Get'n Busy,
I can't tell from your post which motor you are having overheat problem with. Whichever you suspect is giving overheat alarm, remove the thermo switches - there are two, one in each head assy on a v-6 block, test them in a pan of water on the stove with an accurate thermometer and voltmeter. The thermo switch should close around 185 degrees and open back up aroung 160. I have seen them close as soon as 130 and give a false overheat alarm. You need to eliminate this possibilty.
I currently am fighting a similiar overheat alarm on my 2000 C115 - I have a temp gauge with the sensor on the port cylinder assy. Above 4000 rpm, the alarm goes off, but the temp gauge reads normal (130). I removed the thermo switch on the port side and it tested good. The temp gauge sensor (thermistor) is located only about five inches from the thermo switch. How in the heck can there be that much difference in temp (130 vs. 185)? Well, there is a thermostat between the two - and maybe therein lies my problem (not yours since you replaced thermostats). Your problem (and maybe mine) may be clogged up water jacket in the head assy - that means removing the heads and cleaning. Hopefully, we won't have to do that.
Good luck, and let me know how you come out.
Ken K

[Get'n Busy Liv'n]

Ken,


Well after getting the overheat alarm at the dock at a fast idle I decided to remove and replace both Thermistats which appeared to be in pretty rough shape (scale buildup).

I also pulled the Thermoswitches from the block and I just finished the Continuity test oulined by the Service Manual and Clymers. Per the spec, each sensor gives continuity at a temperature around 180 degrees F.

I didn't have a chance to water test it with the new Thermistats in place .... I'm hoping that corrects the issue!

If it doesn't what are my other options .... the poppet valve? then cleaning the water jackets?

Look forward to hearing your sugestions.

[Ken]

Get'n Busy,
Hopefully replacing thermostats will cure the problem. When you say "poppet valve", do you mean the pressure relief valve?
I personally have not had the experience of having to replacing the PRV, but from what I have read on other posts, it is just a spring loaded water valve that opens when the pressure in the cooling system gets too great. It can also get clogged with scale/deposits and stick open, and that will cause a lot of the cooling water from the impeller to bypass the water jackets and dump out the relief holes that are just below the lower cowling. You can test this with the earmuffs and a garden hose. With the motor at or a little above idle, if the PRV is stuck open, there will be quite a bit of water coming out the relief holes. There should not be but a few drops using the motor flusher on the driveway if the PRV is not stuck open.
There still could be the possibility of the water jackets of the heads being clogged up - that will be the last thing I check. Hopefully, we will solve our overheat problem before we get to that.
Good luck,
Ken K

[fishthis10]

I am having the exact same problem as Get'n Busy. I can run all day at 4000 rpms or less, but if I open her up the alarm will start sounding on my starboard engine. Keep it shut off for a while and restart and it works fine unless I go over 4k rpms. I am going to start by replacing water pumps and thermostats. BTW, I have twin '92 200 HP Yamahas.

[Ken]

fishthis,
My fishing buddy has the same C115 that I do, except his is 96 while mine is 2000. He was getting the same alarm we are above 4000 rpm - I decided to experiment with his, and I pulled both thermostats and tested them - both tested good. I figured he had a weak water pump, even though the pee-stream looked strong, so we just left the thermostats out and went fishing the next day with no problem. My fishing buddy knows only two speeds - idle and WFO! I figured with the thermostats removed, there was less restriction and therefore more water flow into the heads.
Bottom line - you might want to try this as a temporary measure until off season. That's what I am going to do. A nice winter time project - water pump repair.
Good luck,
Ken K

[Mike E]

I have a 1994 C115 Yamaha, I had a problem with an overheat alarm at 5000 RPM and up. Run all day at slow speeds. I had 25 psi water pressure at WOT. Pulled the thermostat's and found trash in one of them. Popit valve was clean. This went on for several months. Finally pulled the heads off last week, the head gaskets were in BAD shape. Holes all the way through in some areas around the water jackets. Got it back together tonight, ran it on the hose at idle for 15 min. Ready to take it back to lake. I am sure the Head Gaskets were the problem. Check the heads, make sure they are not warped, and make sure you bolts are in good condition. 6 of my bolts were correded and I was afraid to use them. I bought my new ones from Boats.net, no problems.

Hope this helps someone

Mike..

[Ken]

Mike,
Thanks for the input - how did you check for warpage on the heads? Also, did you use any kind of sealer on the head gaskets, or just put them on dry? Has your outboard ever been used in salt water? What made you suspect head gaskets? Did you do a compression test?
Thanks,
Ken K

[fishthis10]

If I remove the thermostats, will the overheat alarm still work if they do overheat? What are the repercussions of removing the thermostats.

[Ken]

fishthis,
No problem - the thermoswitches which close at around 185 degrees and sound the alarm are totally independent of the thermostats. The only downside of running without thermostats is that in cold water, say below 80 degrees, your motor may not come up all the way to operating temp. Not going to hurt you down in Florida in the summer time. I'm going to run mine and my fishing buddy is going to run his without thermostats until this winter off season.
Good luck,
Ken K

[Mike E]

The moter came from Alabama or somewhere down South. Shows some use in Salt or Brackish water.
Tried everything else, run fine with no thermostat's but not recommended. Without thermostat's in the water pressure was less that 5lbs at WOT. Talked to a mechanic I trust, he said pull the heads and look for deposits in the water jackets. I used a straight edge,(Starrett scale)and a feeler gauge to check the heads, I work in a machine shop. Cleaned everything good, used gasket's only with blue loctite on the bolts, Borrowed a torque wrench, a must. I now have 5lbs water pressure at idle. Check the Popit valve for trash.

Later Mike

I havr 99 model did same thing . pull cover off. looking at the back of motor,down at the bottom of the block theres a water line,pull off theres SCREEN FILTER clean it ,you should be ready to go, good luck

[Tucker]

Ok, WTF with the screen filter?? I sure missed it on mine when I replaced the PRV. Where is it in relation to the PRV and the hose that clamps to the housing?

[makomagic]

Ox66 overheat

I have the identical problem. 1999 Ox66 overheats over 4000 rpm. New water pump assembly. New stats. New temp switches. New PRV updated version. Runs all day under 4000 rpms. Temp alarm goes off, motor shuts down to 1200 rpm. Let cool for 1 minute. No alarm. Used laser temp gun. Can find anywhere on motor over 150 degrees. Where is this screen located by the PRV? Next heads come off. Appreciate your help.