Solve this overheat mystery, I will send you a beer!

[coves]

I am serious as a heart attack. This issue is driving me to drink and I will gladly share with you.<br /><br />Issue: '96 Mariner 225 EFI hanging on the back of 23' Hurricane Deck boat, having a bizarre overheating issue. This is a bit long to get you all the details, so bare with me.<br /><br />I know this boat. I purchased it from my friend and it has always run great, not a single problem. My friend moved away from where the boat lives and his sister and husband kept it for a couple of years. Did not use it much, but provided no TLC or maintenance. When I purchased a year ago I did the basics and had a great summer with her. Very rarely, and only around 3000 RPM the overheat alarm would go off. Shut her down, wait a few seconds, and back to skiing, playing, whatever. The only other thing that bugged me is that it would not hit WOT (5600 on the engine label). Well, late last season I happened to pull the cover off to just inspect and lo and behold the connector to the top port cylnder capacitor unit was hanging free. What the hell? How long has this been disconected? Well I connected, the boat ran unbelievably for about a minute and then burnt out one of the coils on the stator. Damn! When that got replaced I had the lower unit reworked, new impellar, etc. THis baby will fly now. Hits WOT, 50MPH, lovin it. BUT, over 2900 rpm or UNDER 5000, the overheat alarm goes off and if over 3000 the system retards back to 3000 just like it is suppose to. Stop the boat or even continue at 3000 and within a few seconds the alarm stops, the engine jumps back up to RPM and off I go until the next episode. I removed and rebuilt the poppet (new diaphram and then removed again and changed the spring, but not the internal rubber gasket, it looked fine). I put in two new thermostats, and even changed out the heat sensor located on the starboard side just under the top cylnder and it still misbehaves. Nice strong tell tale, and when this occurs I have stopped, pulled the cover and felt the heads. Maybe the starborad is hotter, not much, and I can still rest my hand on both heads for a short while before the blisters form! What is going on here? It sure acts like a stuck poppet. Is the water pressure at 5000 keeping it open and therefore no overheat? Can I just take the poppet out and throw overboard? Or are we now talking computer problems that were somehow aggravated by the stator problem from who knows how long ago running on only 5 cyls? Like I said. I know this boat from five years ago and it never did this before.<br /><br />I am serious. If you solve this mystery, give me your address and favorite brew, I'll send you more than one!<br /><br />Thanks guys!

 

[dingbat]

Re: Solve this overheat mystery, I will send you a beer!

The easy one first. WOT is a function of the prop. If not propped correctly you'll never get to WOT<br /><br />Seeing that you changed just about everything mechanically, I'd put good money on a problem with the alarm module. Waht is your water pressure at 5K rpm?

 

[jasonbailey]

Re: Solve this overheat mystery, I will send you a beer!

Blown head gasket perhaps? That was the cause of my overheat problem. Do a compression check.

 

[Qld_Kev]

Re: Solve this overheat mystery, I will send you a beer!

Maybe blocked water passages, wont let enough water flow for higher revs/speed

 

[jasonbailey]

Re: Solve this overheat mystery, I will send you a beer!

I read your post again, my o/b had the same exact symptoms. I put a new thermostat in it, changed the temp sensor, & impeller. I'd take it out and it would run all day at 1/2 throttle no problem. It started and idled with no problem. Once I went to 3/4 or WOT the buzzer would go off a minute later. I'd back off and it was fine. I told my dad about it, he is a master mechanic, I'm a dumb accountant, he told me to do a compression check and sure enough I had a cylinder that was low. Took the head off and the gasket was blown. I got a new gasket, put it back together and buzzer went off again. Back to the parts store got another gasket ($35.00) and this time I had the head machined. That cured my problem. That was 3 seasons ago and its still running like a champ.

 

[KCLOST]

Re: Solve this overheat mystery, I will send you a beer!

The water pressure is the key thing here to determine if the stats, poppet, and impeller/pump are working properly...<br /><br />Without that we are going to have to systematically go through all, one at a time...<br /><br />Do you have a water pressure guage?<br />Also, do you have a temp guage?<br /><br />Go ahead and check the compression on each cylinder..<br /><br />There are multiple causes of this problem you have... Could be from the poppet, impeller, water pump gasket, stats, head gaskets, water jacket cloggs.<br /><br />By the way just to confirm, is the alarm you are hearing a constant "beeeeeeeeeeeeeeeep', or and intermittent "beep,beep,beep"... This is also important to know..<br /><br />The reason I ask is that it sounds similar to problems that can occur with a faulty switchbox, if you get the intermittent alarm...

 

[KCLOST]

Re: Solve this overheat mystery, I will send you a beer!

And what is the overheat sensor measuring? Metal from the cylinder head or actual water in the jackets...?

 

[AMD Rules]

Re: Solve this overheat mystery, I will send you a beer!

Have a peek at this thread, on another site:<br /><br />87 150 Black Max running HOT!<br /><br />In that instance i is a smaller older motor, it was the spark advance module but the thread goes through a lot of troubleshooting techniques in general. Food for thought.

 

[coves]

Re: Solve this overheat mystery, I will send you a beer!

Originally posted by KCLOST:<br /> The water pressure is the key thing here to determine if the stats, poppet, and impeller/pump are working properly...<br /><br />Without that we are going to have to systematically go through all, one at a time...<br /><br />Do you have a water pressure guage?<br />Also, do you have a temp guage?<br /><br />Go ahead and check the compression on each cylinder..<br /><br />There are multiple causes of this problem you have... Could be from the poppet, impeller, water pump gasket, stats, head gaskets, water jacket cloggs.<br /><br />By the way just to confirm, is the alarm you are hearing a constant "beeeeeeeeeeeeeeeep', or and intermittent "beep,beep,beep"... This is also important to know..<br /><br />The reason I ask is that it sounds similar to problems that can occur with a faulty switchbox, if you get the intermittent alarm...

 

[coves]

Re: Solve this overheat mystery, I will send you a beer!

Thanks for taking the time to review this mystery! I have no h20 pressure guage or temp guage. The sensor on this engine bolts into a hole in the head casting just below the top starboard cyl. It is metal to metal and I just replaced it. The beep warning signal is "beeeeeeeeeeeeeep" and not "beep, beep, beep".<br />This is all so wierd as I can run at 5K and all is well. I can run at 2900 and all is well. But in between and the alarm sounds, the computer cuts back the RPM to 3K and I can even let it continue to run, not touching anything (except the steering wheel!) and in about a minute or so the alarm stops and the boat comes back up to RPM. What! I'll have to see where to install an H2O pressure guage and it will be a week before I get back to the boat to check compression, but if there is a problem with this, would this puppy run like a champ at WOT?<br />When I "rebuilt" the poppet i did not change out the gromet that fits into the engine where the actual valve seats, but it looked fine when I removed and inspected. That one is a pain to get to because you have to remove the lower cowl to access it. As a reminder, if you can stand reading my novel again with the original post, last season I was running only five cyls as the top PORT cyl was getting no fire due to a bad winding in the stator. During that time, only rarely would the alarm go off and always right at 3000 and never at any other RPM. I guess I am whizing directly into the wind until I verify that compression is OK on all six. Thanks for helping me think thru this puzzle!<br /><br />Chris

 

[verojoe27]

Re: Solve this overheat mystery, I will send you a beer!

Try checking out the two switchboxes. Abnormal voltage from the switchbox will cause a continous beep from the alarm. I had a problem where the engine (1992 Merc 150) would run fine, then the continous beep would start and I could run at idle speed. A few minutes later, the alarm would go off and all would be fine. Good luck, Joe.

 

[jasonbailey]

Re: Solve this overheat mystery, I will send you a beer!

Mine ran just fine with the blown head gasket. Compression was low on one cylinder. I think I had 130 or so on the top 2 and the bottom one was at 95. I always thought a blown head gasket was a really big deal in terms of engine performance, but my lack of "real" experience led me down the wrong path until I asked my Dad about it. Fortunately for me my Dad has worked on everything from helicopters, tanks, bulldozers, lawnmowers to some outboards along the way.

 

[coves]

Re: Solve this overheat mystery, I will send you a beer!

Thanks CPA! I won't get back to the boat for a week but obvioulsy my next test is compression. So far the experiment has been pretty inexpensive in parts, head gasket sounds like way more $. Yikes.<br />Happy boating!<br />CC

 

[achris]

Re: Solve this overheat mystery, I will send you a beer!

Coves,<br /><br />Don't stress about the $$ for the head gasket, they are cheap. Just make sure the job is done right. If you're doing it yourself, GET A MANUAL!!! if you don't already have one. And check to make sure the head is in tolerance, warp and flatness. <br /><br />As an aside, I was working on a friends V6 which was overheating and it turned out to be the spring behind the poppet, too strong. We cut a couple of loops off it and it hasn't so much as burped since.<br /><br />Good Luck,<br /><br />Chris...........

 

[coves]

Re: Solve this overheat mystery, I will send you a beer!

Somewhere else the suggestion of clipping the spring was mentioned and that sounded too wierd. But, we are working on boat engines here! After I confirm or deny compression issues I may try that next. My understanding of the poppet is that it releases water pressure as RPM go up. Is it not true that the entire engine is being cooled at low RPMs and then as pressure rises the poppet just keeps flow and pressure at proper running levels?

 

[achris]

Re: Solve this overheat mystery, I will send you a beer!

Coves,<br /><br />Almost but not quite. <br /><br />At water pressure lower than will open the poppet (low engine speed) the engine temperature is controlled by the thermostats in the heads, designed to keep the engine running hotter at lower speeds, to reduce cold-fouling on the spark plugs. As the water pressure rises to the poppet spring pressure the poppet lifts off its seat and allows full water flow through the entire block and heads. The flow will be restricted by the poppet spring pressure, so if it take 12 psi to lift the poppet, and the water pump will only produce 14psi, then you would only have 2 psi of pressure to push the water around the block (and heads). <br /><br />Your problem is only occuring at 3000rpm. It could be that at that speed the water flow through the thermostats isn't sufficent to keep things cool enough, but the poppet hasn't lifted enough for the fulll flow to be effective. Above 3000 the flow through the poppet is sufficent for adaquite cooling, and below 3000 the flow though the thermo is adaquite. You may well have one that just doesn't like being at 3000. Once you verify things with a pressure gauge, you'll be able to pass on enough info for more informed advise. You could just go out and buy a new spring (or use one of the ones you took out), cut one coil off and see how that goes. It's not going to cause any other problems to do that.<br /><br />Chris............

 

[coves]

Re: Solve this overheat mystery, I will send you a beer!

How ever wierd this sounds, I find this all rather interesting. I am getting conflicting descriptions on the function of the poppet. It has been stated that it only controls and maintains water pressue. You state that it actually restricts water flow to the engine until it opens. I would love to see a cut-a-way schematic of this engine to really visualize the water flow through this beast. What an interesting science designing these things. What I really don't get are the new 4-cycle OB"s. How the hell did they jam the valve train and all the parts into the same size box? Anyway, i hope to get to the boat this weekend and verify compression and go from there. Thanks for all the feed back!<br />Chris

 

[heybaylor]

Re: Solve this overheat mystery, I will send you a beer!

it does not restrict water flow to the engine ..<br />it allows the low rpm to have enought pressure to get it to the heads ...<br />after it pops..at a higher rpm it allows more "flow throught the block,and heads than the thermo's would ..simple simple .....

 

[achris]

Re: Solve this overheat mystery, I will send you a beer!

Coves,<br /><br />I'll send you a water flow diagram and full description when I get home (currently in Russia). The diagram and description are from Merc's own training manual. <br /><br />As Heybaylor says, the poppet doesn't restrict flow, it redirects it to the thermostats, allowing them to control engine temperature until the water pressure is high enough to open it. <br /><br />Chris.............

 

[achris]

Re: Solve this overheat mystery, I will send you a beer!

P.S. I have just read the link that 'AMD Rules' posted. Be VERY careful following this guys advise.<br /><br />1. 'take out the thermostats' This will lead to an overheat faster than you can blink. The system needs the thermostats to create the necessary restriction in the heads for full water flow to work properly.<br /><br />2. 'use a bigger prop to achieve the same speed but at lower revs' If you plan on doing a rebuild in the very near future, by all means! This will again lead to the engine getting hotter quicker. It's like driving your car around in the next highest gear, seriously bad idea!!! If you really want to try messing with props, use a smaller prop. This will reduce the load on the engine, and thus create less heat.<br /><br />On the link thread - It sounds like the poppet is not opening at all, hence the high block water pressure.<br /><br />Coves, get yourself a water pressure gauge and let us know what the results are. <br /><br /><br />Chris............