1999 7.4L MPI Inboard steaming and overheating

"I have Twin 1999 Mercruiser 7.4L inboards one engine will start to overheat above 2500 rpm.
I've installed new exhaust manifolds and elbows (no shutters) new thermostat, new raw water pump, just removed the heat exchanger end caps and the exchanger looks very clean. It doesn't overheat at idle...I have no idea what to do next! Any suggestions?"

"Pull the long hose off the t-

"Pull the long hose off the t-stat housing that comes out the transom and hold it up in the air, with water going to the drive or boat in the water start the boat and see if you have good water going to the t-stat housing.

The water should shoot 12" or more into the air."

"A common problem with the Bra

"A common problem with the Bravo water intake systems is,

At the transom where the hose goes into the transom from the motor side it is held in the hole by a PINCH method.

The surrounding material becomes swelled up due to oxidation and squeezes the hose down and restricts water flow to the pump.


Here are some pics of my suggested source of the issue.


"

"The engines are straight inbo

"The engines are straight inboards not sterndrive models, sorry for the confusion, I don't see another topic area for strictly inboards.

I have Velvet drive 5000 series transmissions. The outflow of water at idle seems the same at both exhaust ports (port and starboard) It really blows alot of steam above the 1500 RPM or so. Can I still pull the thermostat hose and verify good water flow?"

Take a picture of your motor s

Take a picture of your motor showing water hose placements and post it.

Also are you sure you have the incomming water hose going to the correct water pump inlet connection and not the outlet???? If they are incorrectly positioned then that would definatley reduce the water flow!!

"Dusky:

Assuming that you c

"Dusky:

Assuming that you correctly bled the air out of the closed side and have adequate raw water flow, I'd suspect the oil cooler(s). The usual location is on either side of the oil pan.

Probably easier to measure the raw water flow from the pump first. If ok, proceed to the oil cooler inspection.

Follow the hose from the raw water pump and you will find them. I'd expect the first one you come to is most likely to be the culprit."

"Maybe you have mussles or som

"Maybe you have mussles or some other obstruction, blocking the intake, that is why i asked you to see if you have good water going to the t-stat, where it branch's off.

See good water to the t-stat then the obstruction is further down the " pipe line "


And i understand the steam problem, and it scares you.

First good water to t-stat.

Then if good water to t-stat do a vacuum test.

here is the links.

http://www.secondchancegarage.com/public/186.cfm

http://www.users.bigpond.com/ergoff/vac1.htm

Keep us posted after checking the water flow and doing the vacuum test we can go from there.

I don't want to scare you anymore with what it could be. Just conjecture at this point.

Good luck !"

"I believe the water connectio

"I believe the water connections are correct as I did not change any hoses only off and on. Here's some pictures I found I already had, kind of hard to see the hose connections. I seem to have good flow out the exhaust, maybe the circulating pump?
I will have to put the exchanger back on after the new end gaskets arrive, in the mean time I will check the flow through the oil cooler. I didn't bleed any air out of the system, just filled with anti-freeze. It runs right at 160 degrees at idle after warmed up, only at speed does it begin to overheat. I'll look into doing the vacuum test.engine"">engine 2

Is it me or is there something

Is it me or is there something missing??

Where is the raw water that should be going thru the elbows??? What is cooling the exhaust?

I do not have a big block manual here so i can look at the water flow diagram........

"I am looking at manual 16 and

"I am looking at manual 16 and there is a "second type of closed cooling system that must be yours.

here is a diagram of it.

Looks like raw water comes in thru FUEL FILTER COOLER and then into pump, from pump it goes to exchanger, from echanger it flows to the bottom of the exhaust manifolds and out the elbows, similar to a raw water only system.



"

"That appears to be the coolin

"That appears to be the cooling system I have.
I just don't understand how it over heats when I seem to have very good flow, at least I see good volume at idle. anyway I'll keep digging! Thanks for everyone's help, I'll keep you all informed if/when I find the problem."

"Well when idle is good and hi

"Well when idle is good and high speed is bad then you either have a incoming flow issue or a outgoung flow issue.

Follow the clear arrows in the diagram and that shows the RAW water path.

You have a fuel cooler in line before the pump, you also have a transmission cooler after the pump.

Either of those could have something STUCK inside...............It does not take a lot of raw water to cool an engine at idle.

Above idle is a different story..........any restriction will cause a reduction in flow."

If the boat is kept in the wat

If the boat is kept in the water you could have marine growth in the raw water intake. Sounds like some kind of raw water inlet flow restriction.

how long have you had this pro

how long have you had this problem? ever since you've owned the boat or did it start recently?

"I've had this problem sin

"I've had this problem since late last summer, (the boat is always in the water, it is 8 tons)the first thing we replaced was the raw water pump, and cleaned all the debris out of the system. Then the exhaust manifolds and elbows.
Yesterday I removed the first cooler in line with the raw water pump, the transmission cooler, and I found a 1/2" sized, what looks like part of the raw water impeller. Other wise the cooler holes looked fairly clear, I then moved to the Mercruiser "Cool Fuel" cooler and there is definitely not a blockage there it is just a straight through pipe, I next pulled the oil cooler tube and found a bit more, what looked like sea weed in it, not much, but looked to be enough to imped flow a little. That's as far as I got at this point. After seeing the impeller piece in the trans cooler I will again pull the raw water pump later this week and inspect it again. I'm waiting on seals for the heat exchanger, in the mean time I'll clean the cooler tubes, trans, oil, and exhanger. I'm going to also pull the raw water pump and verify it is still in good shape.

The above cooling system drawing shows the fuel cooler before the pump, it is actually between the transmission cooler and the oil cooler, very tough to get at on the lower port side under the motor mount and exhaust manifold and very near the port fuel tank.

I don't really see these minor blockages as the problem, but I'll find out as soon as I clean them all and put it back together again. Any suggestions on how to clean these cooling tubes?
I have some small engine cleaning brushes but they are a bit too tight to push through the small holes.
Thanks for your suggestions, work will continue, as it's another overcast weekend here in Tampabay area, no boating in this weather."

""""""I do

""""""I don't really see these minor blockages as the problem""""


You would be surprised what a little blockage here and a little blockage there adds up to in total blockage.

Just remember that your system is not overheating? correct? Just steam correct?

Well restricted flow at higher rpms will definately cause this. Especially a piece of the impeller.

Be dillagent and look real good at all points of cloggage, ie: where ever a transition of hose to hardware, that could be where another piece of impeller could be hiding........

good luck!"

Use a rifle barrell bore brush

Use a rifle barrell bore brush and rod for the small holes.

"Here's pictures of the de

"Here's pictures of the debris I noticed in the transmission cooler, and next the oil cooler, and upon standing the trans cooler on end and tapping on the bench more came out (final picture). I'm now believing that after seeing all this, it may be the problem. I'll clean all these coolers out, as well as the exchanger, also inspect the raw water impeller and put it back together. Thanks....I'll update all later.
Transmission cooler"">oil cooler"">debris

"Don't be surprised if you

"Don't be surprised if you get more stuff out of the holes. You may want to pressure test the coolers while you have them out, they have been at work for over a decade. Not certain what their expected service life is but would venture a guess saying not much more is left. I think they are fairly reasonable, commpared to most things with a 'marine' label."

"Good find!!

I would agree,

"Good find!!

I would agree, that much of a old impeller will definatley cause water flow problems...........

May want to take to a radiator shop and see if they can clean them for you.

Again good find!!"

"Update on overheating problem

"Update on overheating problem....I got everything back together today, heat exchanger, trans and oil cooler, checked the raw water pump impeller etc......The engine still produces tons of steam at anything above 1500 RPM and begins to overheat. I did notice that the port exhaust elbow was much hotter than the starboard one. I pulled the inlet water hose from the starboard manifold to verify water flow, and it seemed to have a very strong flow. I then pulled the port hose, reconnecting the starboard and verifed approximately the same amount of flow....
I don't believe there is a problem with this, in fact I wanted to do something, so I swapped raw water pumps from my starboard engine to the port engine with the same result.
Could I have a cracked head? An exhaust leak into the exhaust water? Remember, the exhaust and elbow are brand new, this is what I was told was originally wrong. I'm totally lost.
Will taking a compression check on the port head tell me anything?
I'm very sure that I have ample flow through the water cooling system.
I believe there is something wrong inside the engine.
I just don't know what to do next. Any suggestions?
Thanks all."

"Get a cooling system pressure

"Get a cooling system pressure tester and run the series of tests it can perform. It will find what you suspect, if that is the cause."

"Makomark,

I dont think thi

"Makomark,

I dont think this will help any, He has a closed cooling system and the issue is the fresh water side of the system.

The engine is or should be maintaining ~ 160-180 degrees when fully warmed up.

Duskey,

If you follow the FRESH water flow of the diagram I posted the next place for fresh water to be inhibitad is the HEAT EXCHANGER.

Have you removed that yet and looked at it in detail to see if the fresh water passages are free and clear.

The fresh water passes thru it and then goes thru the exhaust manifolds then up thru the elbows....

Some how fresh water is being restricted from exiting the manifols/elbows.....

That is my only suggestion at this point..."

"Hold up. Did you ever do a va

"Hold up. Did you ever do a vacuum test. Cheap and easy. Here is 2 links.

http://www.secondchancegarage.com/public/186.cfm

http://www.users.bigpond.com/ergoff/vac1.htm

The vacumm test will reveal how the internals are working, me i would stop everything and perform it ."

"I dont think that is necessar

"I dont think that is necessary,

If he had a head gasket leak the antifreeze would be burning off and he have a loss of it and would be overheating as a result of NO antifeeze.

Remember this is a CLOSED cooling system.

The steam is a result of HOT fresh water vapor.

Look closely at the diagram in a earlier post."

"Ghost how do we know for sure

"Ghost how do we know for sure. Takes all of 15 minutes to perform. Costs nothing, and we are running around chasing our tails here.

You want to help, same as me.

Do the test. He says it's now only the port side, that side always runs hotter anyway."

"""Ghost how do we kno

"""Ghost how do we know for sure.""

We will know if he responds and says he is NOT loosing any antifreeze.

I still think he has a obstruction in his incomming water lines. Fuel cooler, oil coolers, or heat exchanger....

I feel it is a blockage of Fresh water either comming in or going out."

"Thanks for the responses afte

"Thanks for the responses after yesterday’s trials, I decided the flows into the manifolds from the heat exchanger were not the same, so I went to the store and spent $30 on 1" tubing to swap the exhaust inlet sides, I did notice one elbow was cool the other hot. I wanted to see if I could swap the outputs of the heat exchanger and make the other side cool. This would tell me if the flows were enough and that internally the engine was OK. So I crossed the hoses and sealed them so the left hose out of the heat exchanger was now cooling the right exhaust, which was the "hot" one, and the right output to the left exhaust.......and now the left one got hot. This tells me the exhaust manifold that gets the first outlet from the heat exchanger stays cool. So I pulled the exchanger off to see that it is positively clean. I now believe, as some of you suggested before, that I have a flow problem in the pick-up or sea strainer. I know the sea strainer basket looks clean, but I need to dig deeper. I have someone routinely clean the hull, but I need to ask him to pay special attention to the pickup area of the port engine.
I'm glad I did the heat exchanger outlet swap as this tells me there is nothing wrong inside the engine. (I would have lost anti-freeze if I would of had a blown head gasket, but the fresh water side always seemed to circulate and never loose coolant or get over pressurized, so I didn't spend time messing with the fresh water (anti-freeze) side).
I need to examine the pick-up and sea strainer with more attention....I'm fairly sure after today’s test it's in front of the pump now. (remember I swapped the pumps yesterday, and the starboard engine still remains cool without the steam). I feel the $30 for hose was worth it as now I’m confident there are no internal engine issues.
ghost and chief have suggested all along that I have an inlet/pickup obstruction. After all I’ve done I believe this to be the case. The water is stinking cold now so I’m not getting in with my dive gear to look at this, but I can have the bottom cleaner take a closer look. I’ll examine the sea strainer better. Any other suggestions? Is there anything I can get at and check for inside the hull? I’m feeling better that I'm getting close to the root of this issue.
Thanks"

"Based on the flow diagram wha

"Based on the flow diagram what you are saying is the exact opposite of what should be seen.

The fresh cold water enters the heat exchanger on the right bottom side and exits the top right side to right manifold and the excess water flows thru the heat exchanger to the left and out to the left side manifold.

With and if this is the way your hoses are routed than the one on the left should be the warmest as the water would be warmer after flowing thru the heat exchanger.

So if the right one is getting hot and the left one is cool (original hose configuration) then there may be some restriction still affecting the right side.

After your hose swap and transfering the hot to the left tells me that the output from the heat exchanger still may have a flow issue.

I also agree it just may be a overall incomming water flow problem and the right may infact get the lesser amount of water.

I stiil at this point feel along with a slight incomming water restriction, there still may be something causing flow restriction in the heat exchanger."

"Inside the heat exchanger the

"Inside the heat exchanger the water does come in at the right side and flows through the cooling tubes directly to the left side exhaust manifold first, then back and forth twice until at the end it flows out to the right side exhaust manifold last. Ample flow goes directly to the left heat exchanger outlet. So which ever manifold is connected to the left output will remain cool. After flowing through the rest of the heat exchanger the final output of salt water (the boat is actually in an intracoastal marina, Bay Pines (St. Pete) to be exact) is surely the hottest and is the right manifold. My wife and I both have assessed the situation and we surely agree now that we just don't seem to have enough flow to remain cool by the time the raw water reaches the end of the line, the right exhaust manifold.
I guess I could actually swap heat exchangers from the cool running engine (starboard) to this engine, but we think it is pointless as we both feel there is just not enough flow as the majority flows directly to the first heat exchanger outlet. The design of the heat exchanger is just too simple we can see clearly that there are no obstructions in the tubing and end chambers.
Picture 1 shows the exchanger at the far left the curved inlet is the raw (saltwater) water input which when installed on the engine comes directly out of the oil cooler and is on the right side of the engine as it is now upside down in the picture. The straight down “outlet” beside the curved one is actually the right side exhaust manifold outlet. The deceiving part is that the raw water inlet (curved one) doesn’t flow right out the outlet beside it. The water flows directly across the bottom through the bottom tubes to the left side (straight down pipe on the right side of the first picture). This is the highest flow at first, so whichever side of the exhausts I have connected to this outlet remains cool.
I took a couple pictures of the ends of the heat exchanger and you cannot see in the picture, but I can see directly through all these tubes and see they are not blocked, but I also pushed a small brush through each hole to remove any small build-up of slime. But after doing half of them decided it was not really removing anything, and decided not to risk damaging any of the tubes.
The pictures of the ends of the exchanger with the end caps removed show all to be quite clear(in our opinion). With this second time looking at the exchanger we feel confident we should look elsewhere….to the pick-up and sea strainer. I certainly do appreciate your responses as it makes me re-evaluate each next step….thanks again.
I not sure how to examine the pick-up or sea strainer closer, but will next investigate this process.
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