91 Yamaha 150 TXRP Overheat Alarm

[Cuz]

O.K. here's my story. Been trying to figure out an overheat alarm problem for several months now and thought I had it fixed, but it came back. Have done all the steps as suggested on this site and by the book, pulled both t-stats and checked good, both temp. sensors check good, replaced H2O pump. Found and repaired a bad ground in boat wiring harness as well as a shorted battery. All ran fine until last weekend when alarm went off again with RPM reduction. Checked all of the above once more with addition of PRV. found that water was coming from two back ports at high idle. About 900 RPM on the problem motor, but not on the other motor. Removed PRV, which appears good, no damage or visible cause for sticking, so I "shimmed" it with a couple of hose washers to increase tension on the spring and ran it. Now I get no alarm, but also even at cruising speed, 4000 RPM, see very little water coming from relief ports. Ordered parts to replace PRV, spring, gasket and grommet. Does anyone think this could be the problem or possibly something worse? Any helpful advice appreciated.

[Ken]

Cuz,
Would really help if you had a water pressure gauge installed. If all components of the water cooling system are working properly, then you should have around 5 psi at idle and 20 above 3000 rpm.
Since you increased the tension on the prv spring, and that took care of the overheat alarm, then you either have a weak spring, a weak water pump, or the head water jacket is clogged with mineral buildup.
That prv is designed to open at around 20 psi to relieve overpressure in the cooling system on the load side of the pump. When it opens, water will dump out the relief holes. There should be very little water coming out the relief holes at anything below 3000 rpm.
Hope this helps [img]smile.gif[/img] ,
Ken K

[Cuz]

If I wanted to install a water pressure guage, where would it go? Second, if I'm not so lucky as to solve the problem with the new PRV and spring I've ordered, how do you go about cleaning deposits from the head? Thanks in advance for any shared advice.

[Ken]

Cuz,
For your 150, the fitting for the pressure gauge tubing screws into a hole in the top of the port cylinder bank. That hole currently should have a 14mm stainless threaded plug in it.
You attach the tubing to the fitting and run it from the motor, under the deck, and up to the console gauge. It's a little bit of work, but well worth the effort because the water pressure gauge is the best indicator of how well your water pump is doing.
To clean out the deposits in the heads, they have to be removed from the cylinder assemblies. They can be cleaned out by high pressure water or by soaking in something like WD-40 and cleaned out by brush (not metal bristles).
I currently have this problem with my little C115, and since I would rather fish than tear down my motor, I have put off the project until this winter. I did a water pump repair thinking that would solve the over heat problem, but noooo .
Then I installed a water pressure gauge because I wanted to make sure the pump was doing its job - ok there - 5 psi at idle and 20 above 3000 rpm. Still overheat. Tested the thermostats per the shop manual - good. Tested the thermoswitches per the manual - good. Checked the PRV - good. The only thing left was head water jackets - removed the stats and looked into the jackets, and yep, a lot of mineral deposit buildup - yuck !
So, I removed the stats and installed stainless washers in their place. This gave me more water flow than wide open stats, yet enough restriction to completely fill the cylinder water jackets. Some owners just "gut" the stats - cut out the center and leave just the rubber grommet. Does the same thing as the washers.
Ok, I have told you more than you want to know [img]smile.gif[/img] ,
Hope this helps,
Ken K

[Cuz]

Thanks Ken, actually you've told me exactly what I need to know. That maybe there's a possibility I can bypass the thermostats and get by the rest of this season and pull the head this winter if the new PRV and spring don't cure my ills. BTW, when I did have the thermostats out to check them, I didn't think it looked too bad in the water jackets, but I don't know how sensitive these things are to buildup. There was definitely some, but it sure didn't seem like enough to cause this condition. How difficult is it to pull the head? Any chance of broken bolts? Thanks again, you're a great help.

[Ken]

Cuz,
I gotta be honest with you - I have never pulled a head on an outboard - next winter will be my first.
As long as a head bolt doesn't seize, the job shouldn't be too bad. If one of my head bolts refuses to come out , well, that's when I go to my trusted marine mechanic (the one I go to when I am in over my head) and ask for advice.
I read back over your first post and you said that you checked the thermo switches - you need to test them in a pan of heated water and thermometer to confirm that they close at approx 180 degrees f. and re-open at approx 160. I am emphasizing this because I have personally experienced one closing at 130 degrees and giving me a false overheat alarm. You want to make sure your motor is overheating before you go to the trouble of pulling the heads - and, yes, you need to pull both of them.
When you pull the heads, in addition to replacing the head gaskets, you need to also put in new sacrficial anodes that are pressed in to the cylinder block just under the heads. This is something that should be done about every 500 hours, but I don't think most owners do this unless there is another reason to pull the heads.
When you look into the water jacket behind the stats, you are actually looking into the cylinder jacket, not the head jacket. I think the head jacket will clog up before the cylinder jacket because it has a lot of small passages.
Good luck, and let us know how you come out [img]smile.gif[/img] ,
Ken K

[Cuz]

Ken,
I plan to install the new PRV and spring this week and we'll see how that works. If that doesn't do the trick, I'll probaby try by passing the t-stats until I pull the boat out of the water. It's a 1991 23' Grady Gulfstream w/twin 150's which we keep on a lift. It's trailerable, but it's work. I'll let you know how it goes.

[jb123]

heads and jackets are quite simple and ive never had a bolt seized, 2 hrs total.

[Cuz]

Thanks jb, good to hear. I guess the worse part of it is properly torquing them back down.

[Ken]

Hey, JB,
Do you use any kind of sealer on those head gaskets? Or just put the new ones on dry?
What about thread sealer on the head bolts?
Ken K [img]smile.gif[/img]

[jb123]

ive never used any on the surfaces or bolts. i would just clean/wire brush any corrosion if present, surfaces and bolts. ive never seen any work loose, but they may, as ive only owned/worked on 9 yammies.

[Cuz]

Had the opportunity to run our boat on Saturday after installing the new popit (PRV). By the look of the tattletale, the pressue looks good and ran a total of about 20 miles with no overheat alarm, but there is very little water coming from the relief ports on this motor as compared to the port motor. I'm not convinced I solved the problem and wondered if this could still be because the heads need to be removed and cleaned. As I said, I didn't get and alarm and would like to run the rest of the summer it it will keep working, but am curious why this motor doesn't "dump" as much water at high RPM. (4000+) Thanks for all the help, this site is the best source of information available. I have a Yamaha manual and Clymer manual and can't find this stuff in either.