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Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15378
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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PLEASE HELP I have a 1999 MerCruiser 7.4 MPI Bravo 3 and I

Customer Question

PLEASE HELP
I have a 1999 MerCruiser 7.4 MPI Bravo 3 and I have a overheating issue. The engine is installed inside of a 1999 Bayliner 2855 Ciera sunbridge.
It started overheating last night so I did some research and found broken pieces of the impeller in the bottom of my oil cooler. I cleaned out all of the pieces that I assume had gotten cought in it a few years ago when I had to replace my water pump impeller on the starboard side of the engine running off the serpentine belt. I felt sure that was the problem considering I have good water flow when I remove the hose coming off the top of the oil cooler now but the only problem is when the thermostat closes at 175 degrees it causes my intake manifold to completely dry up and get hot. After the temperature gauge starts to come down and the thermostat opens back up you can hear the water rushing into the intake manifold like water in a hot frying pan. I do want to add that I did remove the thermostat housing to check for blockages and all was clear. The water located in the hole on top of the intake manifold was scorching hot. I also removed the small 90 degree hose that K'Nex the engines water pump housing to the intake manifold end it was clear and all of the ports were open. I know that when the thermostat closes the water cakes the path of the least resistance and that would be through the intake manifold but it obviously isn't flowing through the intake manifold correctly for some reason. I can not find a clear schematic on the cooling system for this engine including the intake manifold. I also know that there are sensors involved but I'm not aware of any valves they control that would not allow in the water to pass freely through the intake manifold and cause it to dry out when the thermostat closes. I am stranded at a marina right now and any help would be greatly appreciated!!!!! I removed the two wire sensor located under the thermostat housing on the front of the engine in order to bring the boat In to the marina to hopefully fix the problem. I have checked the power steering cooler and fuel pump vapor lock system I suppose by removing the hose coming out of the top of the oil cooler when I checked for the impellers pumping action. I don't know how to be any more clear about this issue butt please let me know if there is anything else I need to check and keep in mind all of this happens just as soon as the thermostat closes so it seems like there is a problem with some type of the bypass best circulates the freshwater through the engine while the thermostat is in the closed position in order to maintain proper engine temperatures for the multi-port injection system to work an optimal level.
Thank you very much!!!!
***** *****
Submitted: 9 months ago.
Category: Boat
Expert:  Jason replied 9 months ago.

Hi John, my name is Jason.

All Mercruisers are cataloged by serial number. Can I start by getting that number.

Next, does it overheat at low speeds, high speeds or all speeds?

Expert:  Jason replied 9 months ago.

Are you still with me John?

Customer: replied 9 months ago.
Hello Jason I spoke with you a few years ago concerning the shifting cable problem and you told me to remove the outdrive to manipulate the cable and if the cable was free then my problem was located in the top of the outdrive. I have been wanting to tell you for quite some time that I took what you said to heart but instead of removing the shifter cable I pulled the boat out of the water notice that the shifter cable ballast was missing a zip tie so I took some Lucas Fuel Injector Cleaner and cut the tip of the Cone on the top of the bottom just enough to where it would fit tightly over the shifter cable and pressed the Lucas fuel injector cleaner down the shifter cable through the transom as hard as I could by squeezing the bottle until I had noticed 2 fluid ounces headband emptied out. I then reconnected the shifter cable and shifted it from forward to reverse about 3 times and it shifted so easy I had first stop the cable had broken but come to find out it was seized just like you said in the Lucas Fuel Injector Cleaner was just what it needed in order to Break Free the corrosion that she'd been through that missing zip tie. I installed a new zip tie heavy duty this time not like the last mechanic installed and it's been smooth sailing every since shifting like a brand new boat straight off the showroom floor!!!:). Lol. I know that's not on topic but I really wanted to tell you every since we spoke that day and removing the outdrive was going to be my only option in order to diagnose the problem correctly. Always told myself if I ever spoke with you again by some chance that I would tell you what ended up happening and figured you would really get a kick out of hearing that.Now for the problem at hand it overheats all the time it has nothing to do with the engine under a load or just at an idle it's not affiliated with RPMs whatsoever from what I can tell it has a 100% common denominator and that is the thermostat closing causing the intake manifold to completely dry up and then as soon as the engine starts to cool you can hear the water rushing into the intake manifold as the thermostat opens back up and that's when you hear the boiling water effect from it entering the draft intake manifold through that hole located right under the thermostat housing.Now as far as the engine serial number I'm going to give you my best opinion of what this number is ***** to the fact that someone has taken a clear sticker and stuck a different serial number on top of the original serial number that was printed on the sticker located on the intake manifold plastic covers.The number from the best I can make it out is 2 A 0 1 2 4 8 0. I'm not exactly sure if it is a ZERO, or the letter O but I can tell you that they are both identical so that has to help narrow it down a bit. Please let me know if you need me to try to translate the number located under that serial number? I would assume small modifications were made to the engine for this particular boat application and that is why they changed the serial number unless you can think of another reason.Thank you so much Jason for getting back with me so quickly!!!
Customer: replied 9 months ago.
I sure am. did you not get my last response? It shows that it was sent. You are the only person I'm interested in communicating with right now because like I said I am stranded at a marina on the other side of the lake from my lake house.
Customer: replied 9 months ago.
Picture of the entire cover.
Expert:  Jason replied 9 months ago.

Hi John,

Got the serial. Lets start with a water volume output test. Locate the output hose of the seawater pump. This will run from the pump to the thermostat housing. Disconnect the hose at the housing. If you have a bucket this will work best. Start the engine up, bring the rpms up to 1500. Aim the hose at a bucket for 10 seconds, and then at the end of those 10 seconds aim the hose away, and quickly shut the engine off. Let me know how much water you get approximately in gallons.

Customer: replied 9 months ago.
10-4!!!!! I'm doing it now and I will be back with you in approximately 5 to 10 minutes max!!! I'm using a 5 gallon bucket because I can't imagine using anything smaller but I could be wrong. I'll be right back with you thank you very much!!!!!
Customer: replied 9 months ago.
I did it exactly for 10 seconds at 1500 RPMs and it looks like approximately three and a half gallons because this 5 gallon bucket is actually a little larger than 5 gallons I believe. Going to send you some pics I hope the one I took it still attached because I use the hose coming out of the top of the oil cooler to fill the bucket
Customer: replied 9 months ago.
This is the line I used
Customer: replied 9 months ago.
Thought I would send a picture of the entire engine
Expert:  Jason replied 9 months ago.

Got it. Wait for the engine to cool a bit. Do the same thing but this time snap the throttle in neutral a few times. Let me know if the water volume decreases or increases when you snap the throttle

Customer: replied 9 months ago.
I was in neutral when I did it and as far as snapping the throttle I wasn't sure if you were talking about snapping it back into the center position while holding the button in the center to keep it from engaging the out drive at the doc but anyway when I pumped the throttle, and increased the RPM's it increased the water significantly. There is a light flow of water at an idle and it increases about five to six times more GPM when I pump the gas I just wasn't exactly sure how far you want to me to pump it as far as to what RPM so I just tried to stay under about 2 to 2500. When the motor was idling and I pulled the shifter handle to the locked position there was Zero change in the water GPM. Like I said before I wasn't sure if you were talking about conducting that test at the dock door running in gear. Waiting on your response
Expert:  Jason replied 9 months ago.

What I meant was disconnecting the throttle cable from the engine, and then just touching the throttle linkage by hand and snapping the throttle. Chances are you have bravoitis but i'm just trying to confirm. When was the last time the bellows were changed on the boat?

Customer: replied 9 months ago.
Okay I got you same difference I was able to clearly see the water flow from the position I was in when applying the throttle and my girlfriend was also staring straight at it at the same time to confirm. When you speak of bellows are you talking about the outdrive bellows around the universal joint and exhaust or the Bellows that are located on the back of the exhaust manifolds? They are all in excellent condition to the best of my knowledge and are approximately 2 - 3 years old. Remember the first time I contacted you I told you I had just purchased a brand new Bravo 3 outdrive from MerCruiser not rebuilt but brand new. At that time all of the bellows were replaced. And what in the world is brovoitis???? In your next reply could you please do one thing for me and that is tell me where the water escapes from the intake manifold after entering through the bottom of the thermostat?????? that has been my biggest question all night and up till now today. Is there some type of a crossover valve or some type of a sending unit that directs the water other than the thermostat working off of a thermocoupler?? I removed that sensor that has the Rubber seal around it and has two prongs for a two wire connection and is I presume made out of some type of copper or bronze although it was black when I removed it until I send it it down enough to remove the corrosion it's screwed into the front of the engine right below the thermostat housing. When I did that it appeared to keep the intake manifold cool it just sprayed water on to the firewall and my bilge pump pump it out but the exhaust manifold remained cool and that's how I made it to the marina in order to dock the boat and connect to shore power temporarily. When doing this does it cause the back of the exhaust manifold to overheat and not get a good circulation of cool freshwater?? If it is ok to drive it like that at least I could take it back to my lake house and park it at my job instead of leaving it here at the marina but I still don't know what that word means that you just said and it is possible that my impeller could need replacing because it has been a few years and I have driven the boat quite a bit just not this summer. Only been out less than a dozen times this summer
Customer: replied 9 months ago.
I meant to ask you does it require a large amount of pressure not GPM to push water through the intake manifold??
Expert:  Jason replied 9 months ago.

Before I answer all those questions. If you hook the water line back up, and simply run it at idle, does it still overheat or does it run a stead 160F or so?

Customer: replied 9 months ago.
Okay I hooked everything back up and screwed that sensor back in and plugged it back up tightened all of the hose clamps down with the engine completely cold. I crank the engine up and monitored the engine as it idled this boat always as far as the gauge shows runs at exactly 175 degrees which is possibly not exactly accurate but never the less it always operates it exactly 175 degrees by the Gage. I'll let it run for quite a while and it's slowly but surely blind from 100 degrees up to 175 degrees and both right and left exhaust manifold stayed cool along with the top of the thermostat housing. Everything felt as it normally does very cool until it reached 175 degrees and then it slowly creep above 175 degrees and that's when it slowly started to rise up to approximately 200 or a little over and then kept slowly Rising butt the exhaust manifold continue to stay cool and I could feel the top of the thermostat housing getting warmer and warmer of course until I shut it off at approximately boiling 212 degrees or so. I cut the engine off and after a couple of minutes it appears that the thermostat closes again but I'm not exactly sure but that's what I told you about that noise I hear like hot water boiling butt it wasn't near as bad this time because the motor wasn't under any type of strain I suppose so the temperature on the entire manifold wasn't AZ Xtreme but I cannot be sure of that I just know it makes a lot louder noise as far as the gushing sound or something that sounds like the thermostat closes and boiling water enters and area maybe it's just the other hoses can't say for sure but to answer your question yes it does still run hot at an idle but it very slowly goes up in temperature like there is some type of restriction only intake manifold when the thermostat is in the open position as strange as that may sound but maybe not. I have had to replace the impeller before as I mentioned to you earlier and although I tried to catch it before any pieces broke off I was unsuccessful. It had a couple of the rubber points missing but not all. I'm going to send you a picture of what I cleaned out of the honeycomb oil cooler for the first time which I should have probably sent you that originally but I know that some of these pieces have to be from the previous impeller but it is possible that some are from the impeller that I replaced a few years ago along with a brand new housing. It wouldn't surprise me if the impeller is bad although I do keep the boat in the water at the dock at my lake house but that's why I keep going back to the question concerning pressure and if it can cause this type of problem because we all know I wore out impeller will cause a motor to overheat I would just like to know if there is anywhere else I need to check for debris such as an exit point for the intake manifold or does it just drop down into Ports through the Engine. From what I've told you doesn't it up here also that the thermostat is operating correctly?The included picture is the debris that I removed from the honeycomb oil cooler Grill on the bottom side coming from the engine mounted Serpentine driven impeller housing. This is after it passed through the power steering cooler and fuel pump anti vapor mechanism.
Customer: replied 9 months ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Customer: replied 9 months ago.
Sorry to bother you Jason.I have my boat parked at a very dangerous location as far as theft is concerned. I have a lot of very noticeable stereo speakers and custom rifles that can easily be stolen from this public dock.Can I remove the sensor located under the thermostat housing and allow it to drain into the Village area just 2 putt at approximately 1500 RPMs back to my dock at my lake house without hurting anything are overheating the rear portion of my Engine????? I have family court in the morning concerning being the guardian for my nephew to keep him out of foster care and there is no possible way I can pull the boat out of the water tonight. I requested a phone conversation just to ask you if you thought it would be risky to try and move my boat to a safer location. This is obviously a lot more complicated that I had ever anticipated it being or I would have mentioned this much earlier. I haven't heard back from you so I just wanted to try to make one last-ditch effort to get your opinion before I moved the Boat.Please let me know if you have any suggestions and if it is risky to drive the boat with that sensor removed and a low RPM?? I know it's not good for fuel economy butt I wanted to get a professional opinion as far as this temporary measure to keep my boat safe.Thank you!!!!***** *****
Customer: replied 9 months ago.
I meant to say custom peripherals not custom rifles. My speech recognition isn't working well today. Thank you
Expert:  Jason replied 9 months ago.

Good morning and sorry for the delay. When you replied last night I was already off to bed. As long as the engine isn't overheating at 1500 rpms then you can certainly run the engine without fear of harm. But if the engine is overheating at that rpm, then you would want to tow it, not drive it.

Customer: replied 9 months ago.
I had to Leave the Boat there. Could you please read the long comment I wrote Above that explained how I kept the engine from overheating by removing the sensor located under the thermostat housing allowing the water to spray straight into the firewall through the serpentine Belt. That's the only way I could keep the engine from overheating. To make a very long story short I just wanted to know where the water exits the intake manifold after it enters under the thermostat housing??? Is there some type of crossover valve where does the water just get pushed reports in the engine block from the front to the rear under the intake manifold??
Customer: replied 9 months ago.
The exhaust manifolds are not overheating so there is water flow through the exhaust are you concerned about the exit hole located in the Trans Am where water spills out when the boat is trailered and connected to a hose pipe and a set of ears??
Expert:  Jason replied 9 months ago.

Good morning. Let me ask you another question here. Is the engine actually overheating? Did you take the engines temperature manually in order to rule out a bad gauge or sending unit giving a faulty reading?

Customer: replied 9 months ago.
IT IS!!! Like I said originally when the engine reaches 175 degrees at normal operating temperature and the thermostat opens up that's when the problems start and the intake manifold starts to overheat. After I let it start to cool down from approximately 225 degrees because that's where I cut the engine off at I go back and sit down beside the motor and listen AZ the temperature slowly drops the thermostat closes right at approximately 175 degrees and when that happens you can hear water gushing into sounds like the intake manifold like water in a hot frying pan. My main concern was whether or not debris had entered the intake manifold and clogged Deportes that I have researched myself located towards the rear of the engine where the water leaves the intake manifold cooling area. I'm pretty sure that with the Impaler being 3 years old it's probably missing some of the tips off of it causing the Impaler 2 steel pump water but lack pressure so the GPM is about where it should be considering the elevation is just a little bit higher than the water level outside of the boat but does it have enough pressure to push enough water through the engine when the thermostat is in the open position due too too much resistance when it comes to pushing the water through the entire engine block. When the thermostat is closed I can see that it simply seems the water straight to the exhaust manifolds but when the thermostat is open it must push the water through the engine and the exhaust manifolds which would require proper GPM and a sufficient amount of pressure which a wore-out impeller would not be able to provide. I was just trying to get a clear answer from you when it comes to if you have ever heard of a wore-out impeller causing such a issue with this particular motor and not being able to supply sufficient water although it appears to be pumping enough water but not under a closed pressurized condition such as when the thermostat opens??? My father was a mechanical structual electrical Plumbing and nucular engineer. He never gave me the answer to anything but would describe how it worked so that's how I approach fixing things in general. If you could answer some of my questions like the gentleman I found on a form I could have figured out this problem on my own butt you are used to talking to women people that obviously don't have a clue of how things work as far as mechanically or fluid dynamics so you keep telling me to conduct these tests and will not allow me to approach the problem at Hand by understanding how the system works and every nook and cranny the water travels after it enters the engine. A gentleman for free on a farm explained in great detail exactly how the system works so now I understand where the water goes when it enters the intake manifold without having to disassemble the entire engine. I don't know if you were just too busy because it was a holiday weekend or if this is some type of a protocol for keeping the customer as dumb as possible so they have to keep asking questions and never really understand anything. I would have really appreciated a simple description of the Highway the water tanks once it enters the engine the first afternoon we spoke. Thank God this gentleman on a forum that is a certified Mercruiser mechanic sent me a detailed diagram of my engine's sea water cooling system from the entry point through the transom all the way through the block and out the exhaust. By giving me that simple but hard to find diagram that required me to give him my serial number he allowed me to figure out what was going on in just two simple E-mails. Are you even reading what I'm writing? I know it's a lot butt making things short and sweet is not how you fix a problem that could possibly be very complex. When you asked me if the engine is even overheating or is my gauge not working properly there is no way in this world you have read what I have written because you would have seen that I stated I could clearly hear boiling water just like it hit a very hot frying pan every time the engine overheats and then the thermostat closes as it cools. Engines that are not overheating do not sound like a bucket of water hitting a bonfire. I did not think you were reading what I had written and now I know it. My girlfriend tried to explain to me how it was a holiday and you were probably trying to fit this into your schedule. I told her that when you are on call as an expert you stop what you're doing and give it your undivided attention and I feel sure I paid extra for that service. I could be wrong so I will double check. I'm sorry I interrupted your busy schedule and I do mean that sincerely ***** ***** don't know what's going on in your life just like you don't know what's going on in mine.
Expert:  Jason replied 9 months ago.

Do you have enough room to pull the water line and also the fitting for the water line off of the inner transom plate?

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