Over heating when trimmed out on plane at 4800 RPM. Is my engine mounted to high?

[mlevee87]

Over heating only when trimed out at over 4800 RPM. Maybe my motor is mounted high?

I get an over heating alarm on my 1989 100hp Mariner outboard only when I'm at full throttle or anytime I pass 4800 RPM which is usually after I get up on plane and trim out my motor to push out some extra power for top end speed. To avoid an overheating alarm I can run my engine at about 4600-4700 RPM with the engine trimmed all the way down. I've been chasing this over heating issue all winter and last summer. I have had water pump replaced, bell housing, thermostat, poppet valve and I still have the same problem. I even moved my transducer away from the lower end thinking maybe it was creating air pockets near the water intakes on the engine. Either my engine is sitting too high on the transom or maybe an over sensitive temp sending unit?

Please see these photos and let me know what you think. I do have two spaces to go if i want to lower the engine on the transom bracket which would drop the AV fin by maybe an inch and a half or so.

 

[mlevee87]

I have an 1989 100HP Mariner. I get a solid over heating alarm when I am up on plane after reaching 4800 RPM or higher. Normal operating range RPM is 4800 to 5200. In order to reach optimal RPM I have to trim out the motor to push out some more power for top end. I don't over trim out otherwise I'd notice when the boat starts to porpus. I can run the boat on plane with no alarm at about 3/4 throttle running at 4600 to 4700 RPM and that also trimmed up a little. As long as I keep it below 4800 RPM im good. I want the max performance out of my engine and I know its capable of doing more, so I've been chasing this issue with no success. I've replaced the water pump and bell housing (good solid stream of water coming out of engine), I've replaced the thermostat and poppet valve, and have also moved my transducer away from the lower end on the back of the transom. I'm at a loss at this point, except I noticed something the other day. Is my motor mounted too high on my transom? I have two spots to go if I want to drop the engine about an inch and a half. What do you all think? Also if anyone was wondering about my prop size its a 16 pitch. The boat is a heavier 17ft fiber glass bay boat (VIP 180CC).

Below are pictures of what I'm talking about.

 

[Alumarine]

It's hard to say from the pictures but it could be high.
If it's easy to adjust why not move it down and see what happens?

What speed are you getting out of it?

 

[steelespike]

It does look like it could be touchy at that height. Drop it down a hole see how it does.
Do you have any ventilation(cavitation) issues when turning at speed or from wave action?

 

[mlevee87]

Im able to run trimmed out full throttle for maybe 1 minute before the over heating alarm goes off and I've had it up to 37/38 mph. Its not too easy to adjust. The engine sitting too high on the trailer for a cherry picker or engine hoist. Was thinking of maybe getting two buddies over to basically support the engine while I un bolt it to drop it down a hole or two.

 

[mlevee87]

I don't seem to be having any issues with cavitation when turning. Ive talked to several marine techs including calling Mercury itself and after everything I've done, they seem to think that I might be sucking air into the water intakes due to cavitation caused by something. My transducer was almost inline with my skegg so I moved my transducer about 18inches away from the skegg, took the boat out last Sunday and still getting the same over heating issues. One thing I have not done yet was replace the temp sending unit but Mercury techs don't think that that is the issue. Again they seem to think im sucking in air. I figured this issue only occurs at full throttle AFTER I trim the engine out for peak performance. Im wondering if the engine is sitting too high, thus when I trim the motor out the water pick ups are in a spot to the point where the hull is creating cavitation or air bubbles which are getting sucked into my cooling system... I know that my AV plate should be almost in line with the bottom of my hull and its clearly 2 to 3 inches above the bottom of my hull.

 

[mlevee87]

Cavatation Issues? Over heating when trimmed out on plane! NEED HELP/Advice

I have a 17ft VIP 180cc with a 1989 Mariner 100hp with a 16pitch prop. Operating RPMs is 4800 to 5200. When ever I get on plane and trimmed out for peak performance (RPM over 4800) I get an over heating alarm after about one minute at that RPM cruising at about 36 MPH. I have replaced water pump and bell housing (good water flow pissing out of engine), replaced thermostat and poppet valve, and moved transduce which used to be almost in line with my skegg. Still over heating at 4800 RPM or higher. With the engine all the way trimmed DOWN and 3/4 throttle (4600-4700 RPM) theres no alarm, only when I push it and trim it out do I get an alarm. Is my engine sitting too high on the transom? My water pick ups seem too high to the point where maybe the hull is getting in the way or creating cavitation to the point that I am sucking in air bubbles into the cooling system. Below are some pictures, what do you think?

 

[smokeonthewater]

Yep drop the engine down

 

[Chris1956]

You can have someone observe the water intakes at high speed. They need to be careful and hang on tight. You can also plug the top hole on the water intake, with a screw/bolt. That will help you determine if you are sucking air. Do you really need that Whale Tail? Maybe that plays a part in this?

 

[GA_Boater]

Merged your three threads, No need to repeat the question in three forums.

 

[mlevee87]

Sorry I figured it fit in multiple topics and wasn't sure about which would be the best place to post it. Was just looking to maximize my responses to get other peoples point of views or advice.

 

[mr 88]

Did you recently purcahse this boat/motor setup or have you owned it for awhile ? Since you mentioned maybe the temp sensor was going bad and your not to sure about it why dont you buy a infared temperature gun? They are cheap enough at Sears / Harbor Frt/ ebay etc.. Something that can be used around engines all the time so it's a good investmant for your own piece of mind. I have seen motors mounted higher than yours and the pickup tubes still work.If it was me before I went any further I would hit it with temp gun to see excactly whats going on or even briefly touch the head to see if it is really overheating.

 

[mlevee87]

I bought it a year ago and its been doing the same thing ever since. This experience has for sure taught me a lesson when buying a used boat... I thought about doing that except I have to run the motor with the cowl off. Theres a sticker under the cowl that says "DO NOT RUN OVER 2500RPM WITH COWL OFF." Not sure if the cowl serves a purpose other than keeping over spray (water) away from the carb intakes or not? I could very easily take the cowl off, run it till it alarms and check the temp with one of those temp guns. What temperature should that engine be running at when under full load? I mean I understand whats over heating for a car or truck but what temps are normal range for outboards?

 

[Faztbullet]

It is to high as these motors have the pickup raised and to the front of unit, drop it till AV plate is 1" above bottom, Also repeated overheats will damage engine...

 

[BigSkyMercGuy]

I'm a little impressed that your prop wasn't ventilating like an egg beater!

 

[emckelvy]

You might want to invest in a water pressure gauge, it's a pretty reliable indication of water flowing thru the powerhead..............ed

 

[quicktach]

Sounds like an air/water mix from turbulent flow under the hull. Also, when you're trimmed up, there's not much of your hull left touching the water....maybe 6"? And it's not all water hitting your intakes. Keeping more prop in the water, with a pitch to "grab" more water to get your speed up should work out. Experiment a bit..