running hot at high rpms

dcfrank

Cadet
Joined
Mar 21, 2005
Messages
20
If water pump, thermostats, & poppet valve all replaced, what would make motor run hot at high rpms. Seals, tubing and everything in water circulating system seems to be working correctly, what next. Can I check sending units, but doubtful its that. The engine really seems hot, but runs and idles great?????
 

Boatwizard

Seaman
Joined
Feb 15, 2006
Messages
68
Re: running hot at high rpms

How long has it been runniong hot?<br />Is it on a jack plate?<br />How long does it run before it runs hot?
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,778
Re: running hot at high rpms

Don't know what you are running or in what kind of water. Could have the water tube or passages in the engine clogged with scale, pieces of impeller or weed type things.<br /><br />My engine has a bypass valve adjacent to the (used to be) thermostat whose job it is to insure adequate flow at the higher rpm's hence water pressures. If it doesn't open, she should run hot at high rpm's. What it also does is to bypass the thermostat so that you can get home if it sticks....which is what I did when it stuck. In this case it ran hot at slow rpm's and ok at high....where the bypass was open.<br /><br />Other thing is that if you (engine) are jacked way up you may be exposing some water pickup holes and this would allow water to leak back out before it made it's way up thru the cooling system.<br /><br />Mark
 

andy6374

Lieutenant Commander
Joined
Aug 4, 2005
Messages
1,617
Re: running hot at high rpms

You mentioned you replaced the poppet valve, did you replace the rest of the pressure relief assembly?<br /><br />What are your water pressures? And a what RPM does it overheat<br /><br />It could be many things really, I don't know the history of the motor. When the impeller was replaced was it fragmented, cause like Mark mentioned there could be impeller debris in the cooling system. You could have some lean carbs causing an overheat. Like I said many a thing could cause this, so work down the list and we'll go from here.
 

kegzzz

Recruit
Joined
Mar 19, 2006
Messages
5
Re: running hot at high rpms

hi i also own a blackmax 200hp 1987 with the same problem <br />as the speed increases so does the temp guage<br />you can run it flat out for about 3minutes before the over heat alarm comes on.<br />you pull the throttle of and you can see the temp guage come down.<br />i have had this problem for a year and no one can help me please help
 

CARLOS

Petty Officer 3rd Class
Joined
Feb 20, 2003
Messages
94
Re: running hot at high rpms

Hi,<br /><br />I´ve facing the same situation as above mentioned. I have 02 2.5L 1993...and both engines has the exactly same situation..."both" engines has similar overheat with new impellers...the gauges indicate the same overheat mark...but the alarm never turns on...<br />So, what is happening???? I can touch the cams with my finger for 2 or 3 seconds only during idle... Difficult to touch more than 5 seconds...Well, after many tries to repair this, I´ve been using these engines like this condition...<br />If anybody has any answer to this problem, please reply us.<br />Best Regards<br /><br />Carlos Eduardo<br />Venezuela
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,778
Re: running hot at high rpms

Ok, let's try something. Too many replies on the same engine indicate a design problem to me.<br /><br />Maybe the water passages aren't big enough, or the (high pressure/speed) bypass isn't bypassing properly.<br /><br />Your impeller in your water pump is out of the circuit at high rpm's so forget it....unless your removed an impeller with part of the blade missing.....but on 2 engines.....naw.<br /><br />My engine has a thermostat and pressure relief valve under the same cover on top of the head. Probably your's does also; [probably 2 because your's is a V6, one for each bank.<br /><br />If so, remove the cover and remove the thermostat and pressure relief valve and spring, and anything else that is under that cover.<br /><br />Put some RTV on the gasket (silicone sealant) and put it back on . Give it 24 hours to cure before you run.<br /><br />Try that and come back with your comments.<br /><br />Carlos, you might run your engines with the cowlings off so that when you get into this indicated overheat condition you (or someone else, very carefully) can reach back and feel the water jacket. <br /><br />If you can keep your hand-finger on the water jacket (by the spark plugs, then the side and top of the block) when the indicator says you are overheated, you are not overheated. If it is really hot, where you have to take your hand off after 15 seconds or so, you are running 140 degrees F or higher and you have a water restriction.<br /><br />You might want to run the hand-finger test before you remove any cooling components.<br /><br />Check back and flag me. I'm no expert, but I have a lot of troubleshooting experience and a gift of serious common sense. None of this Sh-- (caca....grin) happens; it's caused. Just have to find the mechanism that caused it.<br /><br />Mark
 

andy6374

Lieutenant Commander
Joined
Aug 4, 2005
Messages
1,617
Re: running hot at high rpms

Unfortunately the tstats, waterpump, or poppet assembly aren't always the cause of overheating. But 99% of the time they are. They should all be replaced every two seasons and if they haven't my money is them. <br /><br />The pressure relief valve (poppet assembly) is located on the starboard side of the motor right under the ignition plate (under the switchboxes). There is a tstat in each head above thethe #1 and #2 cylinders and of course a waterpump in the L/U<br /><br />However many others things can cause an overheat.<br />-The grommet fitting for the watertube where it enters the powerhead might need replacement or the watertube itself.<br />-An air leak between the waterpump housing and the gasket which it rest on.<br />-A blown headgasket can let exhaust gases into the cooling system.<br />-Blocked water passages.<br />-Carbs or fuel pumps that are running lean.<br /><br />All of these things will be bring your engine to a quick demise.
 

CARLOS

Petty Officer 3rd Class
Joined
Feb 20, 2003
Messages
94
Re: running hot at high rpms

Andy & Max,<br /><br />First, thanks for your time in answer us.<br />I bought this boat 1 year ago. This boat is same a Phoenix 29 with twin 2.5L 200HP...A/C, bathroom, 1000L of gas, 600L of water..etc,etc, very heavy boat... <br />These engines must to go everytime at 4200-4600 RPM...I´ve been using this boat 1 year, around 150 hours...<br />It seems, there is not evidence of overheating, because in both engines there is some plastic wires (Protectors) in touch with cams and they are OK!!!<br />Anyway, this weekend I´ll take some time to check for thermostats...<br />My personal opinion is that these engines maybe work at high temp as design...<br />Maybe I´ll remove the tstats.<br />Thanks again for your advise.<br />Best Regards<br />Carlos
 

andy6374

Lieutenant Commander
Joined
Aug 4, 2005
Messages
1,617
Re: running hot at high rpms

Don't remove the tstats, unless that is you are removing them for a clean or testing. If so they are cheap enough to replace and should be replaced every other season.<br /><br />V6 mercs from the 80's and early 90's run hot, just the nature of the beast. There are many talks about this topic if you do a search for it. Like "operating temperature".
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,778
Re: running hot at high rpms

Carlos, Andy speak the truth.<br /><br />I got a tstat and gasket set for $13 American form this site. You can't beat a deal like that.<br /><br />Mark
 

ronnieboy

Petty Officer 2nd Class
Joined
Feb 5, 2006
Messages
149
Re: running hot at high rpms

like andy, i think the top water tube seal is bad, it lets exaust take the place of cooling water. thats what happened to my 88 magll 150. andy knows, hahahah good luck ron
 
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