7.4L Bravo 2 Merc Overheating

[colburn3]

Nebbie post, I apologize if its been answered in an old post...

I have a 7.4L 454 Bravo 2 Merc (raw water cooling system) that overheated when I took it out of slumber yesterday. I went thru my checks on land with some muffs. Wasn't sure how much water was suppose to suck up thru the hose but things seemed normal.

After I "confirmed" or lack there of everything was fine I dropped it and idled down the channel keeping an eye on the temp guage. Things, again, seemed to be operating normal. I got out on the lake and gave it some juice and the temp guage went from 120 to 180-190 quickly. The alarm sounded, I shut everything down and popped the engine hatch. There was mild smoke, which seemed to be the water steaming from exhaust outlets. I didn't want to push the potential overheating damage so I didn't try to fire her back up.

Questions I have are the following...

1) 96' 250CR Crownline (eng specs above) - is there a pump in the outdrive or is the only pump for water cooling front-mounted on the engine?
2) If this is the only pump (front-mounted on engine) does the inlet-hosing run under the engine all the way out the exhaust bellow and down to the outdrive water inlet slits (where muffs would go)?
2) Does this sound like a water pump issue or are there other potential causes for such overheating issues?
3) I'm somewhat hands on but is this one left for the professionals?
4) What are the typical costs for impellar replacement? Full pump replacement (what are the odds the actual pump goes bad as opposed to just impeller)?
5) Where exactly is the exhaust / exhaust water being dumped at?
6) Should I be looking anywhere else (cooling system) to trouble shoot?

Any help would be great! I'm trying to learn everything I can via-books but talking to someone who has experience is worth its weight in GOLD!

JC
Lake St. Clair, MI

 

[26aftcab454]

Re: 7.4L Bravo 2 Merc Overheating

sounds like your impeller is shot---it's on the front of motor--the exhaust manifolds are water cooled-- the water runs out thru the exhaust.

big blocks can get real hot real fast--- get this taken care of before there is damage done.

impellers themselves are cheap(the only thing cheap on a boat)-labor would be 1 hr- about $95

 

[dubs283]

Re: 7.4L Bravo 2 Merc Overheating

there are two water pumps mounted on the engine - one to draw sea water to supply the cooling system and one to circulate the water though the engine - the second one is the one you refer to in your post

the first step in your troubleshoot is to inspect the sea water pump, mounted on the lower half STBD side of the enigne - if the engine is carb'd the fuel pump will be mounted to the water pump and you will remove the whole assy for repair

if you determine the sea water pump is the culprit there are other steps to take depending on what you find

 

[HT32BSX115]

Re: 7.4L Bravo 2 Merc Overheating

1) 96' 250CR Crownline (eng specs above) - is there a pump in the outdrive or is the only pump for water cooling front-mounted on the engine?

Front starboard side of engine

2) If this is the only pump (front-mounted on engine) does the inlet-hosing run under the engine all the way out the exhaust bellow and down to the outdrive water inlet slits (where muffs would go)?

It runs back along the starboard side to the transom fitting and to where the "muffs" would connect

2) Does this sound like a water pump issue or are there other potential causes for such overheating issues?

START at the raw water pump. It's usually the culprit

3) I'm somewhat hands on but is this one left for the professionals?

You can do it easily

4) What are the typical costs for impellar replacement? Full pump replacement (what are the odds the actual pump goes bad as opposed to just impeller)?

depends on how badly the plastic housing is damaged.....here's a good desription of how to do it.... http://www.sterndrives.com/rebuild_bravo_raw_waterpump.html

Here's an example of a kit http://www.marinepartssource.com/pa...2=Water Pump Kits&subcategory3=&mfgno=18-3150

If you pay someone to do it it can be a couple of hours of labor + parts....

5) Where exactly is the exhaust / exhaust water being dumped at?

with thru-transom, it goes out with the exhaust otherwise it's out through the prop via a Y-pipe in the back of the engine......

6) Should I be looking anywhere else (cooling system) to trouble shoot?

With a Bravo you'll have an oil cooler and a power steering cooler. (or a combined cooler) You NEED to ensure that it is clear. the discharge from the raw water pump goes there first usually.....if it's blocked by debris or pieces of a previous raw water pump failure, replacing your impeller won't help.....

Your problem is usually not caused by the circulating pump failure. (that's the cast iron pump mounted to the front of the engine.......they usually don't "fail".....they just start leaking)

Your first overheat source is usually the raw water pump or blockage of water flow. If you didn't get it hot enough to smoke (rubber parts in the exhaust system burning) then a raw water pump replacement usually fixes it.

If it doesn't, then it may be other things that you can check AFTER you ensure that your raw water pump is good and the lines are clear.....


Cheers, and Welcome to iboats!


Rick

 

[Don S]

Re: 7.4L Bravo 2 Merc Overheating

Your raw water pump also has the fuel pump attached to it, see link below.

http://www.boatfix.com/merc/Servmanl/16/16A6R2.PDF

 

[colburn3]

Re: 7.4L Bravo 2 Merc Overheating

Great thanks again!

What is a good operating temp range for the big block?

 

[Don S]

Re: 7.4L Bravo 2 Merc Overheating

What is a good operating temp range for the big block?

With a 160? thermostat installed as per Merc, you should be seeing in the range of 170 to 175?. Maybe up to 180? if in very warm lake water.

 

[colburn3]

Re: 7.4L Bravo 2 Merc Overheating

This site is great! I'm so glad I bumped into it, an enormous amount of info.

Now all I have to do is fold myself up and get into the engine compartment!

JC
Lake St. Clair, MI

 

[HT32BSX115]

Re: 7.4L Bravo 2 Merc Overheating

Now all I have to do is fold myself up and get into the engine compartment!

Yeah....I just changed mine. I STILL hurt!

 

[colburn3]

Re: 7.4L Bravo 2 Merc Overheating

And the final question in regards to my overheating issues...

*Does the outdrive (at transom) contain an intake water hose which is pulling water up to the water pump (front-mounted engine pump STBD side) in question?

*Does the outdrive also contain hosing from the exhaust manifolds (after they Y-together)? If so, where physically does the exhaust exit the outdrive?

*Do both hoses run inside the exhaust bellows or are these in two different places at the transom? I imagine intake / exhaust wouldn't rest inside the same bellows.

Still feeling my way around this vessel after 2-years.

Much thanks!

JC
Lake St. Clair, MI