7.5 Merc cooling issues/blockage in the powerhead...

[tcmsu]

Hi there,

I have a new to me 1981 7.5 Mercury that will not pump water. The guy I bought it from said it needed a new water pump so I started by installing a new impeller (the old one was trashed - pieces everywhere inside the pump). I cleaned things up, made sure the fins turned with the drive shaft and tried bucket testing it. No water came through the pee hole, just a little bit of exhaust. With the help of a mechanic friend (he is an evinrude guy) we have been able to determine through compressed air and water that the blockage is somewhere in the block itself. When we put air through the telltale output it comes out the exhaust ports fine, but barely trickles out the hole at the base of the powerhead where the copper tube from the water pump connects. At this point I would like to go in after the blockage so my question for you all is where would be the best place be to start (i.e., safest and/or easiest access to the block)? Just by looking at the motor, it appears that the easiest entry would be by removing the side plate (exhaust?) just below the thermostat on the port side of the motor, but I'm not sure how much access I would have. My other idea is the rear plate that holds the spark plugs. Any other thoughts or ideas? Where would be the most likely place in the powerhead where the obstruction is? We were both very suprised that we couldn't free the obstruction with compressed air or the water hose as it has worked in the past on other motors. I haven't seen any other situations like mine on these type of forums before. In case this helps, the motor starts up on the first pull and runs very well, just no water out the telltale.

Thanks for any suggestions,

pete

 

[emckelvy]

Re: 7.5 Merc cooling issues/blockage in the powerhead...

If you can't get air or water up the copper water tube, the most likely problem is a squeezed-shut water tube grommet on the upper end of the tube (or plugged shut by broken bits of impeller).

You'll have to pull the powerhead to get at the grommet and the washer that sits on top of the grommet. Replace both parts. You'll probably have to scrape out the hole in the exhaust adapter plate where the grommet goes, so it'll sit in there without squishing the water tube.

Before pulling the powerhead, on the off chance that there's silt or other debris built up in the block, remove the 'cylinder block cover' for inspection. That's what Merc calls the water jacket cover at the back of the block.

If there's any debris built up in there, it'll definitely impede circulation thru the block. Clean it all out then run air/water up the copper water supply tube and see if it flushes out into the block cavity. If not, the problem is most likely with the water tube grommet & time to pull the powerhead!

There's a ring of studs at the bottom of the motor. Remove the trim cover under the lower cowling to access the nutted fasteners. Disconnect any wires, hoses, linkages, etc. in way of powerhead removal.

Leave the nuts on the front set of powerhead studs, just loosen them all the way until the ends of the studs are flush with the nuts. Then you can use those nuts as prying points to break the powerhead loose.

If any bolts/nuts are reluctant to break free, don't force them, use a combination of soaking with PB Blaster or equivalent penetrant, and propane or MAPP torch to heat the fasteners up and bust 'em loose.

Here's a place to pull up diagrams of the block, part #'s, etc:

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

HTH & let us know what you find when you tear into it........ed

 

[tcmsu]

Re: 7.5 Merc cooling issues/blockage in the powerhead...

Thanks for the quick and thorough reply Ed! I already pulled the powerhead to get at the top of the water tube and grommet. There was some build-up there that I was able to scrape away, but when I put compressed air through the telltale it would come rushing out the exhaust ports and barely trickle out the grommet area. Because I figured that I would have to go in after the blockage I tried pushing air through the grommet and again, most of it came out the exhaust ports. I will now focus my attention on the cylinder block cover as you suggested. I also plan on getting a shop manual today. Thanks again for your help, I'll be sure to keep you updated on my progress.

pete

 

[tcmsu]

Re: 7.5 Merc cooling issues/blockage in the powerhead...

It ended up being a combination of a pinched shut grommet and a filthy mess behind the water jacket cover. Thanks so much for your help, I love these motors!

pete

 

[todd22]

Re: 7.5 Merc cooling issues/blockage in the powerhead...

i am having a similar problem with my 85 9.8 merc. i have water coming up the copper tube when i take the manifold cover off but when i put it on no water. any ideas ?

 

[emckelvy]

Re: 7.5 Merc cooling issues/blockage in the powerhead...

Did you renew the gaskets on the exhaust manifold cover and inner exhaust plate when you put it back together?

Have you tried taking the cover off the back of the block? See the earlier discussions in this thread for info on that. If you have lots of debris in there you're not gonna get water all the way thru the block.

What happens when you blow up the telltale with air, does it blow freely?

 

[todd22]

Re: 7.5 Merc cooling issues/blockage in the powerhead...

i didnt put new gaskets on the manifold cover. i blew air from the pee hole down and felt air. didn't try removing the back cover. i will try that next