Overheating at mid rpm 2005 150 optimax

[currentaddiction]

I have twin 150's and the starboard motor overheats when i'm trolling around 1200 rpm. It seems to overheat till i get over 2000 rpm. Under a thousand its fine, wide open is fine also. Usually around 140 deg.any ideas?

 

[Bondo]

Ayuh,.... Welcome Aboard,..... If ya haven't replaced the raw water impeller, That's the place to start,......

 

[Texasmark]

On the smaller hp Mercs and surely on yours also, you have a 2 stage cooling system. At engine speeds below the spec'd value of 2500 rpms, the thermostat(s)...one in each bank on your engine probably..is that way on the 150..at the top of the cylinder bank on the rear, control the block temp with an opening temp of 143F. Once above 2500 water pressure forces a larger valve called a pop-off open, which may be connected to a hose from the top of the block down to below the block on the rear center which allows for an increased water flow through the block and essentially bypasses the stat as the volume of water through the pop-off should be sufficient to keep the block temp below 143F.

As an aid to cooling, the lower unit should be designed with the water pickup holes positioned so that ram water pressure from your boat speed helps the impeller in your water pump supply the necessary volume of water to cool the engine.

I'd say your pop-off valve is stuck shut! However, how long since you serviced your water pump....aka changed the impeller as Bondo mentioned? If it's been awhile you might want to service it and the other engine while you are at it.

Mark

 

[Dukedog]

tha very first thing done with an "unknown" over heat is a complete (oem) water pump kit no mater how good tha parts "look".......

 

[Texasmark]

tha very first thing done with an "unknown" over heat is a complete (oem) water pump kit no mater how good tha parts "look".......

Opinion!!!!!!!!!!!

I bought a used 2002 90 hp in 2004 without a water test.....price was right. On my first outing, I had an overheat alarm at the lower rpms which cleared out above 2500 and returned when I slowed back down. My pop-off was working but my stat was stuck shut. New stat and no more problems until last year 2013) I decided to do an impeller as I hadn't done one since I owned the boat. No overheat problems of any kind, I just decided to do it since the OEM had over 10 years on it.

The impeller had a set but was still pumping water satisfactorily. No other deformation/damage to the impeller.

One reason I don't think it's his impeller is at the higher engine speeds, ram water pressure from the lower unit assists in the cooling effort, if not the majority of the cooling effort as the blades are said to be bent back at those rpms anyway. Course I have no history on his operation or servicing habits, prior maintenance or any of that. Besides he has another engine not experiencing the problem that has been subjected to the same environment etc.

I don't change things just to be changing them!

 

[Dukedog]

No water pressure gauge! No idea as ta whats goin' on! So yeah its an opinion but its a very experienced Merc V6 opinion...Not based on one motor that kin folks own...JAN, there's is never enough "ram" effect ta cool a V6, even with a cone with very large holes, much less a "stock" gearcase.....JMO!

 

[Texasmark]

No water pressure gauge! No idea as ta whats goin' on! So yeah its an opinion but its a very experienced Merc V6 opinion...Not based on one motor that kin folks own...JAN, there's is never enough "ram" effect ta cool a V6, even with a cone with very large holes, much less a "stock" gearcase.....JMO!

Makes sense on the gauge. Rest is what I have read as I have never been inside the water pump at WOT on a V6. Grin

 

[currentaddiction]

Ok, i haven't put it back in the water yet but yesterday i did some testing on the hose and this is what i got. The starboard engine got up to around 150 deg I'd say within 5 or 10min and stabilized at 151 deg and the water pressure was at 2.0psi and had a very strong pee. The port engine got up to 97 deg and stabilized there for 15 min and the water pressure was at .5psi and had a old man pee. I tested all the t stats and they were good. I'm suspecting the strainer or/and poppet. I did change the water pump on the starboard engine a year or two ago and never changed the port engine water pump. I bought the boat in 2011.

 

[currentaddiction]

And thank you for your responces!!!!

 

[currentaddiction]

I just checked the strainer and poppets on both engines and they were ok, the strainer on the port engine has some small shells. Also i haven't changed the waterpump on the port engine. BUT i don't understand why it only runs at 97degs?? Ambient is probably 75 or so. I don't know what the hose water temp is...

 

[Dukedog]

ta get a true reading (pressure and temp) they need ta be in tha water under a load.....ona hose, not so true...

both should run pretty cool at idle ona hose.......

 

[currentaddiction]

Just an update, i changed out the poppet and I'm getting the same temp and pressure as the other motor now. So all should be good. Real test will be when it's in the water..

 

[Texasmark]

Well that tells me that your poppet was stuck partially open allowing the cooling water in the block to leak out rather than be controlled by the stat which opens at 143 F. With it stuck like that, then one can only surmise that it didn't open fully when needed above 2500 rpm. Glad you found your smoking gun.

Mark

 

[currentaddiction]

another update, i planned a fishing trip a couple weeks ago and still had the same problem. i didnt metion this before but not only would the engine overheat but it would also steam. and water is coming out of the exhaust port. i decided to take to the shop and see if they could figure it out. i hope they resolve the issue and will definetly post up the cause and hope it helps out the next person.

 

[bpowdrell]

Hi currentaddiction (or anyone else who may have insights), I know this post is almost 5 years old, but I'm experiencing the same problem with my 1998 Mercury 200 EFI offshore outboard. I will definitely change the stats and poppet but before I do so I'm curious what the diagnosis and resolution was with your issue? I also have a lot steam (similar to what you listed in your last post). The professional mechanic couldn't reproduce the overheating problem when not under load so that wasn't much help. The entire lower unit was just replaced over the summer so the pump and impeller are brand new.

My problem is very similar. Fine engine temp at low RPM (<1,200), overheating at midrange RPM (2,000), then fine engine temp at high RPM (4,000) on a plane. I've included some pics of my temp gauge. Note that for the 2,000 rpm pic the engine temp would have just kept going up if I let it.

Also, on a side note my engine overheating alarm has never gone off even nearly 200 degrees. That is a bit alarming (pun intended). This likely means I do not have a functioning engine alarm.

Thanks in advance everyone!

-Brian

PS - Don't be too hard on me. I'm a newbie and this is my first post.

 

[GA_Boater]

Here is the problem with asking your questions in an old thread. currentaddiction last visited on 5/14/15 - He won't be returning to answer. The best thing is for you is to start a new thread so current members can assist.

Closed.