Chasing a BF75A overheating problem at WOT

[bryyce62]

I’ve been having an overheating problem on my 1999 or 2000 MY Honda BF75A and can’t find the reason for it.

A little background first. In 2011 the head was changed. Water was getting into the crankcase from a failure on a cam journal. So the engine has a 2011 head on an otherwise great running ’99 or ’00 MY outboard.

Late last year while under WOT for about 20 mins or so, I got a overheat alarm. Pulled back to idle, looked for water coming out of the hole at engine cover, pee hole, I guess, and everything looked normal. Powered back up and it ran fine. Figuring that I hadn’t reconditioned the water pump for a couple years, I assumed that the water pump was beginning to fade. Rebuilt the water pump. All new parts except the stainless steel cup that the impeller spins in and the plastic housing that the SS cup and impeller fits into.

Next time out, startup went fine, water flowing nicely from the pee hole. Got an overheat alarm after just a couple minutes of WOT. Ok, something is going on here….

Next, I picked up a new thermostat and thermoswitch. Got online here and started looking for other similar problems. Checked my pressure relief valve, and while it was a little grungy, not much, I cleaned it up and reinstalled.

Back out on the water. Again an overheat alarm after about 90 seconds at WOT. I can run at about 60-75% throttle all day with no alarm. The engine sounds and seems to run the same as it has since I bought it new in 2000.

This time, I bought new pressure relief valve parts. Spring, plunger and rubber grommet thingy that it fits into. I also back flushed the block by removing the lower unit, & removing the t-stat. Using a spray nozzle, I sprayed some pretty high pressure water in through the t-stat housing and water came out of the bottom about as fast as I sprayed into the top. I filled up 3 buckets. A little bit of sand came out, but by the 3[SUP]rd[/SUP] bucket, it was just water.

Buttoned ‘er back up and had it out again this last weekend. This time I brought a handheld IR thermometer with me. Once again, after just a minute or two of wide open, the overheat alarm came on again. I stopped right away and hit the head with the thermometer were the thermoswitch is mounted (near sparkplug #1) and read about 195 degrees.I also measured the head temp near where sparkplug #4 is and the temp at that point was about 165 degrees.

One final thing I tried was to remove the t-stat to see what would happen. Popped it out real quick and took it for a spin. Ran it for about 10 minutes as hard as I could and it ran fine. No alarm. Nothing but a full power drive. Stopped and popped off the engine cover & hit the head with the thermometer. The temp was about 160F at the thermoswitch, and about 145F at sparkplug #4.

Couple observations; This is a brand new t-stat. When I removed it, the housing was HOT, but the t-stat itself was not even warm. The old t-stat that was removed at the beginning of this story, checked out good in a pot of water on the stove. So I don’t think the t-stat is my problem. Also, the head temp variation between sparkplug #1 & 4 was about a 20F+ degrees difference.
That doesn’t seem right either.

So If you've read this far, you can probably figure out that I’m out of ideas. Is there anything else that I can/should do, short of removing the head and looking inside for something? This motor runs great. Always has. Even when I was getting water in the oil back in 2010 it never missed a beat.

HELP!!!

gw

 

[hondadude]

When you pulled the new thermostat when it overheated....you said it was not hot. Was it open?

Try running the engine without the thermostat and without the thermostat cover on. You can do this on a hose if it is easier.

Observe the flow of the water out of the thermostat housing. Even at idle, there should be a really good flow of water shooting out. At higher rpms it should shoot out really far. If it just dribbles, then you may have a blockage in the passages around the cylinder walls. Hopefully, when the head was replaced, they did not use any sealant on the gasket.(that could have oozed into the passage and partially blocked it.

One last thing. You said you did not replace the cup in the water pump....was there any scoring at all in the cup or was it in "perfect" shape?

Mike

 

[bryyce62]

Thanks for the help, Mike
To answer your first question, the thermostat was not open.
Second, the impeller cup has 15 years of wear, so perfect may not be an accurate description, but there is nothing that I would consider scoring or grooving in it.

I'm off to watch some preseason MLB in Phoenix this weekend, but by next week I'll be able to check the flow at the t-stat and report back.

gw

P.S. base-ball is a dirty word on this forum??

 

[jgmo]

Yes, it most likely is....just as the phrase "nake-d eye" is here. Had I spelled it normally, you would see a series of asterisks instead of the letters in the first word. The admin guys explained that it is because of some sort of spam type "filtering". No disrespect or prudishness intended.

Have fun at spring training!

 

[bryyce62]

Was back at things yesterday. No Progress. Dropped the boat in the water yesterday and tried a couple more things. First thing, at the dock, I ran the motor with the thermostat housing off and got real good water flow out of the engine there. Revving the engine got a stream shooting maybe 6' out with good volume. Definitely not a dribble. Back together, headed out for a run. Throttled up to 4K rpm for 10 mins or so, no problem. Pushed it up to 5K and within 20-30 seconds, overheat alarm.
Removed the cover and got the thermostat removed in about 30 seconds. It either had already closed by that time, or never opened. Ran the engine for about another 30 minutes, wide open without the t-stat and it ran great.

The t-stat opens in a pot of water starting at ~140F. I've tested it twice. WTH else could it be??

I've read on other forums about an exhaust leak of some kind sometimes causing overheating issues on outboards. I don't understand how that could cause my problem, but is that a possibility? Could I be getting exhaust gas into the cooling system?

I have a feeling that a trip to a mechanic is in my near future. I'm stumped.

One more thing, I replaced the 2 parts of the water pump, not previously replaced. The cup that the impeller fits into and the housing. Every component of the water pump has been replaced.

 

[hondadude]

I am scratching my balding head....

Looking back over all that you did....If it did not overheat before the head repair....what changed when the head was reinstalled? Did they use a new head gasket? Did they use rtv or sealant (where they should not have used it) and it is partially blocking a passage?

I know there is no answer to those questions...

How about.....when you cleaned the bypass parts, did you also check to be sure that the bypass hose was clear and water will pass through the hose and out where it is supposed to go out. Maybe try flushing with a garden hose.

If the bypass was partially blocked, taking out the thermostat might have compensated for that blockage.

How's that for pulling one out of my........pocket?

Mike

 

[hondadude]

I am scratching my balding head....

Looking back over all that you did....If it did not overheat before the head repair....what changed when the head was reinstalled? Did they use a new head gasket? Did they use rtv or sealant (where they should not have used it) and it is partially blocking a passage?

I know there is no answer to those questions...

How about.....when you cleaned the bypass parts, did you also check to be sure that the bypass hose was clear and water will pass through the hose and out where it is supposed to go out. Maybe try flushing with a garden hose.

If the bypass hose or lower was partially blocked, taking out the thermostat might have compensated for that blockage.

How's that for pulling one out of my........pocket?

Mike

 

[bryyce62]

Thanks again Mike for the help.
I've disassembled, checked & cleaned pretty much everything that has to do with the cooling system short of removing the cylinder head.

At this point, I might be ready to have someone else look at it. A leak down test at the very least. Or I may run it without a t-stat for the time being.

I've got a business trip all of next week, so nothing is going to happen for at least a couple 3 weeks.

I'll be back.

Thanks again,

 

[Dtersch]

What happened at the end? I have same issue!

 

[jgmo]

Hi,

Welcome to the forum. Not trying to be rude but...

10 year old post. Most members aren't following this and many won't bother reading if they see the date.

If you really want the best chance at getting the most help, start your own thread and include model and year along with YOUR observations and ANYTHING you've tried so far to check/correct the problem.

Good luck.

 

[beto_chamorro]

Perhaps you can check out this.