'88 150hp - 18 month prob - o'heating

[mortey]

I have a 88 150hp merc that o'heats as revs do to the point that the buzzer comes on. I have checked temps with laser thermo No 4 is getting to 240 while the others are not far behind. i have replaced entire water pump, head gaskets, exhaust manifiold gaskets, power head gasket, poppet valve, removed t'stats, checked water passages especially around No4, checked timimg, it has 15+psi press. still seems to have performance, good flow out of both t'stat housings with hoses removed, No2 bore is scored slightly, compressions relativley even. Is anyone able to help with this prob before it becomes a boat anchor?

 

[andy6374]

Re: '88 150hp - 18 month prob - o'heating

I think the water pressure should be higher then 15 psi at high RPM's. Should be more like 20+, but I'm not sure if this is your problem though.<br />You have really done everything I would suggest.<br /><br />Have you tried to richen the jets in the carbs? A slight lean condition can heat things up. As can dirty carbs and bad fuel pumps.<br /><br />Inspect the watertube near where it enters the powerhead. Maybe run some water down the tstat housing and see if it leaks around the grommet for the water tube. Also make sure you use some sort of sealant between the waterpump housing and the baseplate, or there could be a slight air leak.<br /><br />Are you mixing 50:1?<br /><br />These engines run hot but obviously 240 is too hot. Let me know what you find.

 

[mortey]

Re: '88 150hp - 18 month prob - o'heating

Thanks very much for advice Andy as i am pulling my hair out!! My gauge only goes to 15psi and on high rpms it is at its limit. I havent tried to richen the mixture as it still performs well but will try that and have a good look around the rest of fuel.<br /><br />I checked the grommet when i had the power head off, have also jammed a hose up the tube and did not appear to leak.<br /><br />All was sealed up when i replaced the entire water pump a month ago. Oh yeah the oil pump is disabled and i am running 50:1 premix - actually a little more to try and keep it cool.<br /><br />thanks again Andy I will look at the fuel side of things. (the only other thing i can think of is a miss matched part such as a powerhead base plate or the like? though i will have to get a more trained mercury eye to check that for me)<br /><br />Appreciated

 

[KCLOST]

Re: '88 150hp - 18 month prob - o'heating

Does it overheat at High rpms only or at all rpms?

 

[mortey]

Re: '88 150hp - 18 month prob - o'heating

Hi KC, It only overheats at high rpm's - once up on the plane and under load, if i sit on 3100rpm it is hot but buzzer wont go off but anything over the buzzer goes off.<br />thanks

 

[Bt Doctur]

Re: '88 150hp - 18 month prob - o'heating

What about the water pump base and base gasket. might be sucking air.once the leg empties out on plane ,theres nothing but hot exaust in the leg.

 

[mortey]

Re: '88 150hp - 18 month prob - o'heating

Hi Bt, good idea but i replaced the entire water pump, fitting new gaskets and seals last month with no difference? <br />Thanks

 

[KCLOST]

Re: '88 150hp - 18 month prob - o'heating

That 3100 rpm number is very interesting.<br /><br />It is very possible that the pressure relief valve is not opening, and it should open at rpm's above 2500-3000....<br />Take it apart again and check the components and make sure there isn't any debri clogged in the exit port... Also make sure the grommet and carrier that is fixed to the block is not damaged (did you replace them also)?

 

[Texasmark]

Re: '88 150hp - 18 month prob - o'heating

Or just take the pressure relief valve out until you solve your problem. <br /><br />The only reason for it is to provide additional water volume when needed at higher rpm's because of the t-stat's small hole. Course you said your stats are out so that should give you significantly increased volume but still not what you would have coming thru the bypass valve.<br /><br />Other thing 3000 is barely on plane if at all. What are your WOT rpm's with the normal load? You may be lugging. My closest info is on '94 which says max recommended rpm's are 5600. Are you turning near that at WOT? If not, I'd bet you are overheating because you are loading the engine.....A prop pitch reduction would be in order at about 200 rpm/inch of pitch reduction.<br /><br />My 2c,<br /><br />Mark

 

[mortey]

Re: '88 150hp - 18 month prob - o'heating

thanks guys, WOT is somhwere over 5000 but have not stayed there long at all (too hot) the 150hp is on a 17ft runabout, doing around 20knts at around 3100-3200rpm (have a 19inch pitch prop). I have only replaced the diaprhram on the relief valve otherwise all looked ok. I tried cutting some coils from the spring but it seemed to make it worse? My understanding of how it works is that at high rpm the valve opens dumping water pressure to exhaust - Is this correct? I have the heads off at moment so when i have them back on i will try all that has been suggested on the water. <br />Thanks again.

 

[KCLOST]

Re: '88 150hp - 18 month prob - o'heating

mortey,<br /><br />You are correct on the operation... But may not understand how it works and why completely...<br /><br />It does open at higher rpms and dumps water into the exhaust. The "main" reason for this is to increase the "flow" of water through the entire cooling system, which is needed to reduce the heat buildup at high rpms. It also drops the pressure obviously and you can see when this happens if you have a water pressure guage that is connected to the water jackets somewhere.<br /><br />But do not do what Texasmark suggests!!! If you remove the relief valve, you will be dumping excess water at all rpm levels. So when you are idling or slow running in the no wake zone, you will not have enough water to fully circulate the block/heads etc. You probably won't even see any water exiting the tell-tail! <br /><br />My guess is, that either the poppet is stuck too firmly in the grommet/carrier or you have debri somwhere that is preventing the release of the water, if it is actually opening. <br /><br />One way to check if it is opening, is if you have a water pressure guage, watch it as you slowly accelerate. The pressure will consistenly climb with increased acceleration (rpm) until you get up to around 2500-3000 or so. At that point you will see the pressure drop very quickly as much as 5 to 10 psi. With continued acceleration it will rise steadly again up to 20-23 psi at WOT. When you decellerate the opposite will happen. <br />Another way to check is when running above 3000rpms, have a buddy "carefully" take a look at the upper exhaust ports at the back of the engine. If water is exiting the ports with the exhaust it is comming from the release out of the poppet valve. That is why all of us with Black Max motors, see steam coming from the upper exhaust ports just after we power down from a WOT run. Any water left in the exhaust is being evaporated. No water means that the poppet isn't working. <br /><br />But checking this way can be difficult and dangerous so don't do anything stupid....

 

[Texasmark]

Re: '88 150hp - 18 month prob - o'heating

And just why not?

 

[Texasmark]

Re: '88 150hp - 18 month prob - o'heating

Totally disagree on your assessment. What are your cridentials for your assessment? Lest you forget that for many 10's of years (like for 50) water flowed freely thru the engine to cool it.<br /><br />Don't need to walk you thru the basics of thermal conductivity and aluminum specific heat. Having done it for 36 years in the aircraft industry I am very well versed.<br /><br />Mark

 

[mortey]

Re: '88 150hp - 18 month prob - o'heating

Thanks guys for all this info,<br /><br />thought i'd mention how quick the temp does go up! Virtually you can watch the gauge go up with the tacho! (and its not an electrical prob as you might first think, as i did)<br /><br />Cheers

 

[ronnieboy]

Re: '88 150hp - 18 month prob - o'heating

bet its the poppet, just fixed a 5 year problem with mine, turned out to be a scratched upper water tube seal and a weak ( 4# ) poppet spring, and a 4year old water pump. poppet has to sealblock to at least 3000 rpm. needs thermostats too. drill 1/16th in hole in thermo plate beside pellet to let air out, heads will get vaporlocked without these things, took me 5 years and lots of parts. oh yeah, make sure to use merc replacement parts, seal that failed wasnt merc. good luck ron

 

[KCLOST]

Re: '88 150hp - 18 month prob - o'heating

Texas,<br /><br />My credentials are so that you better actually do what you suggest on the type of motor that Mortey has before you suggest it...<br /><br />Get yourself a blackmax with a poppet valve and remove the darn thing, and run it at idle, or no-wake-it for a while... I guarantee that if you push it you will overheat... GUARANTEE IT... I might even guess that you have seen that happen based off of one of your other postings <br />What you will have is an open port that will be the path of least resistance so much that you will hardly see any water exiting the t-stats/tell tail...<br />Take a look at a schematic of the water cooling system for that engine, and you will understand. <br /><br />If you think you can run that engine normaly without it, you are completely wrong...<br /><br />We are not talking about a 9.9 horse. This is a V-6 with t-stats, relief valve, and capable speeds that make pressures, water flow, to have different designs necessary. <br />I know what I am talking about....

 

[ronnieboy]

Re: '88 150hp - 18 month prob - o'heating

kclost, i'm a believer! my magll has to have it! had to stiffen the spring to 5# preload on poppet, unseats bout 3000+ rpm, just so happy to finaly have mine fixed, thanks to this forum and you guys , thanks ron

 

[KCLOST]

Re: '88 150hp - 18 month prob - o'heating

And Mortey,<br /><br />These engines do run hot... You will reach temps at the heads (just under the #1 or #2 cylinder) if that is where you temp sensor is located, up to 180F at idle/no-wake rpms... And up to 200-220F at WOT... Now keep in mind, that your engine, if normally outfitted, will be measuring metal temps on the head!!! It never touches water... The sensor has a rubber seal that goes into the water jacket but is seated in a sealed off area from water entry. It touches the head and measures temperatures that are near the combustion chamber... <br /><br />But if you are having temps reaching 240+, you have a problem somewhere and I'd like you to check that poppet/port area...<br /><br />BTW, Can you run it at idle for an extended period of time with no overheat problems???

 

[KCLOST]

Re: '88 150hp - 18 month prob - o'heating

Here is a link to a schematic that shows the cooling system..And it says 1989 and older! <br /><br />Take a look at what happens when the water enters the cylinder area and goes into the heads...<br /><br />If the poppet is open (at lower rpms) you have an open port to dump the water out before it can reach the t-stats...<br /><br />They may open with high temperatures, but it's all about the path of least resistance...<br /><br /> Schematic

 

[andy6374]

Re: '88 150hp - 18 month prob - o'heating

Never, ever remove the poppet. Like KCLOST has been trying to get across, you'll cook the block at idle speeds.<br /><br />mortey-<br />I recommend replacing everything in the poppet assembly. We can't rule it out until you do. <br /><br />Is there any debris enlodged in the water pickups in the L/U?<br /><br />By the way I suggest you drop the pitch of your prop by 2 inches. You are lugging your motor with 5000 RPMs @ WOT. This also makes me think you have some severe carbon buildup on your rings/pistons/cylinder walls. This could cause you to overheat, easily. Get some cans of Seafoam "Deep Creep" or Mercury Power Tune or OMC Engine tuner and Decarb that engine pronto.