seawater pump failure on merc I/O 7.4 - more damage?

[oldbuck]

Have a '96 mercruiser 7.4 B1 in my Cobalt 220. Pulling a skier the other day and heard an 'off note' exhaust sound - moments later the overheat alarm sounded.

Paddled the boat back to shore & pulled the seawater pump apart - all the rubber blades were shredded from the hub - replaced it, but motor wouldn't crank. Pulled spark plugs and #2 cylinder was full of water, while the #1, #4, & #5 had some water in them. Oil on the dipstick appears clean.

I'm not a huge gear head, but I also don't have hundred dollar bills falling out of my wallet, so I thought I'd at least diagnose the problem further before getting too nervous about what might have happened...

Is the next step a compression test on ea cyclinder? Is the compression I'm looking for about 150? (I searched but wasn't able to locate a spec)

Thanks in advance for any insight & support, she's a pretty boat - hate to think I may have cracked the block...
-d

 

[Bt Doctur]

Re: seawater pump failure on merc I/O 7.4 - more damage?

If the boat was stopped quickly it may just be some baskwash that went up the exaust.
get it dried and fired up as soon as possable .

 

[oldbuck]

Re: seawater pump failure on merc I/O 7.4 - more damage?

thx doctur - Is there a way to tell if the new pump is working (other than staring at the temp guage)

Can I also tell by running it if there is more damage to the motor? - I guess if I have a head gasket that failed or cracked block it's gonna run real bad immediately...

 

[windsors03cobra]

Re: seawater pump failure on merc I/O 7.4 - more damage?

Whatever you do get some motor oil, kroil, marvel or any oil sprayed into those waterlogged cylinders.
If the waters been there a while (more than 24hrs) the piston rings may be rusted to the cylinder wall a little, let the oil soak and loosen and soften the corrosion before you go turning the motor.

 

[oldbuck]

Re: seawater pump failure on merc I/O 7.4 - more damage?

good point windsor, thx - it's been a week - I did spray some WD40 (it was all I had a the time) but I should probably hit it with some marvel before I crank it again.

 

[HT32BSX115]

Re: seawater pump failure on merc I/O 7.4 - more damage?

I did spray some WD40 (it was all I had a the time) but I should probably hit it with some marvel before I crank it again.

Howdy,


If you "shredded" your raw water pump, you WILL have pump remnants in your oil/power-steering heat exchangers......Possibly enough to clog them sufficiently to cause an overheat of the engine and exhaust system. (all the cooling water from the raw pump goes thru them before going to the t-stat housing)

If you have ever had a similar (previous) impeller disintegration, you'll have additional fragments clogging them if you didn't clean them out.....


They MUST be removed if you don't want to have more problems.


If you allowed salt water to sit in cylinders, you may have caused permanent damage.

WD-40 or any other snake oil like MMO, Seafoam, etc, will just float on top of water in the cyls.

You MUST remove water immediately...... or within 24hrs or so by removing the plugs, cranking the engine, replacing the plugs and running it to completely clear it of water.

If it's fresh water, you can leave it longer but I would want it out of there immediately too....


Good luck.


Rick

 

[oldbuck]

Re: seawater pump failure on merc I/O 7.4 - more damage?

Great points Rick - I definitely shredded the impeller - bits and pieces jammed all through the pump housing.

How do I flush the oil cooler? backwash it with a garden hose somehow?

I was in fresh water, and I did crank the motor quite a bit whith the plugs out until I was getting what I thought was just a fuel charge coming out the cylinders - then I sprayed the WD40 in.

 

[HT32BSX115]

Re: seawater pump failure on merc I/O 7.4 - more damage?

You probably "saved" the engine doing what you did!

"Backflushing" won't always remove impeller fragments.


When I got my current 454/Bravo, I asked the previous owner if he had replaced the impeller. He said yes, last year etc.


When I looked at it it was not very new looking so I went on ahead and changed it and the rather worn looking plastic housing. (the kit is not all that $$)

Since I had the oil and PS coolers removed I had a close look at them and found the oil cooler so crammed with previous impeller failures that I had to pull several pieces out with "Needle-Nose" pliers before I could flush out the rest of them!!

If you have a failure (or any previous failures) You may need to remove the the oil/PS cooler(s) completely to clear them! YOu might even find pieces in the connecting hoses too....


Cheers,


Rick

 

[Fishermark]

Re: seawater pump failure on merc I/O 7.4 - more damage?

Here's a few pics of what happened to mine and where the pieces wound up when the raw water pump impeller decided it didn't like life anymore - after only two years of use.

 

[oldbuck]

Re: seawater pump failure on merc I/O 7.4 - more damage?

I ran some compression tests this afternoon - I think the 7.4L is supposed to be around 150 +/- 15% (from highest reading to lowest)?

#8 - 110 #7 - 125
#6 - 170 #5 - 130
#4 - 100 #3 - 130
#2 - 165 #1 - 170

Not looking so good...


Fishermark - its amazing where those pieces all ended up - you'd think there would be a design that would contain the failure - at least the big chunks...

-d

 

[Fishermark]

Re: seawater pump failure on merc I/O 7.4 - more damage?

you'd think there would be a design that would contain the failure - at least the big chunks...

-d

There is.......

It's called a thermostat housing.

 

[HT32BSX115]

Re: seawater pump failure on merc I/O 7.4 - more damage?

There is.......

It's called a thermostat housing.

Except on a 454 Bravo. On them (at least on mine), the pump discharge goes directly to the oil/PS coolers and they won't pass the fragments....

#8 - 110 #7 - 125
#6 - 170 #5 - 130
#4 - 100 #3 - 130
#2 - 165 #1 - 170

Those are pretty wimpy comps.

You might take it out and run it hard for a while and do it again and see how it does.....

 

[OldNBold51]

Re: seawater pump failure on merc I/O 7.4 - more damage?

I ran some compression tests this afternoon - I think the 7.4L is supposed to be around 150 +/- 15% (from highest reading to lowest)?

#8 - 110 #7 - 125
#6 - 170 #5 - 130
#4 - 100 #3 - 130
#2 - 165 #1 - 170

Not looking so good...


Fishermark - its amazing where those pieces all ended up - you'd think there would be a design that would contain the failure - at least the big chunks...

-d

my low hrs '95 7.4L showed all cylinders near 125psi with the exception of #6 at 110#'s. This is a low compression engine. I have no idea how you're getting 170#'s.

 

[oldbuck]

Re: seawater pump failure on merc I/O 7.4 - more damage?

my low hrs '95 7.4L showed all cylinders near 125psi with the exception of #6 at 110#'s. This is a low compression engine. I have no idea how you're getting 170#'s.

This is a '96 motor (with 300 hours on it), a Gen V I believe - could that be the difference with your '95?

I haven't been able to find the spec for the what the compression should be, but what concerns me more than the actual numbers are the differences between cyclinders - a 30% delta can't be a good sign.

 

[OldNBold51]

Re: seawater pump failure on merc I/O 7.4 - more damage?

This is a '96 motor (with 300 hours on it), a Gen V I believe - could that be the difference with your '95?

I haven't been able to find the spec for the what the compression should be, but what concerns me more than the actual numbers are the differences between cyclinders - a 30% delta can't be a good sign.

Mine's a gen V also. I agree with you on the 30% delta...kinda ominous. I'm questioning the 170 psi's. Run it and check comp. again and see if those numbers improve.

 

[oldbuck]

Re: seawater pump failure on merc I/O 7.4 - more damage?

Run it and check comp. again and see if those numbers improve.

Well I'd like to blame it on my 30 y/o compression guage that hasn't been used since the last time I rebuilt a motor 20+ years ago... but it's probably just me :redface:

As you suspected those 170's I read earlier are gone -

#8 - 110 #7 - 120
#6 - 130 #5 - 122
#4 - 105 #3 - 125
#2 - 125 #1 - 130

 

[oldbuck]

Re: seawater pump failure on merc I/O 7.4 - more damage?

with the encouraging compression #'s I decided maybe it's time to start the motor - but 1st I decided I'd change the oil...

1st thing I noticed was the oil level, while looking clean, was abnormally high on the dip stick. When I pumped it out I got about a half gallon of water followed by the oil

Is there any way water can get in the crankcase on this motor and it not be a really bad sign?

 

[HT32BSX115]

Re: seawater pump failure on merc I/O 7.4 - more damage?

1st thing I noticed was the oil level, while looking clean, was abnormally high on the dip stick. When I pumped it out I got about a half gallon of water followed by the oil

Is there any way water can get in the crankcase on this motor and it not be a really bad sign?

That is not good.

Where do you live? Did you "winterize" last fall and did you use a piece of wire to clear rust & debris from the drain holes etc?

That much water could be from cracks. the BBC likes to crack "in the valley" at the cyl area under the push-rods.


You might consider doing a block pressure check.

 

[oldbuck]

Re: seawater pump failure on merc I/O 7.4 - more damage?

I'm in N.C. - the boat has been winterized each season in case we get a cold snap, although it seems more people don't winterize than do around here.

The boat has been running well all season up until the impeller failure.

Can I do a block pressure test or is that something a dealer has to do?

 

[HT32BSX115]

Re: seawater pump failure on merc I/O 7.4 - more damage?

It's not that hard to do....a compressor, some "home Depot" plumbing fittings and some hose.....


Go up to the "Adults Only" sticky message. There's instructions on how to do it.