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Backflush 7.4L Merc?

J

johnmsch

New member
I usually replace the seawater pump impeller on my '94 7.4L Mercruiser every 2-3 years. Its now been 4 years and I kept thinking all spring/summer "Probably should do the impeller thing soon." Well, you know what happened. Cranked it up today and within a few minutes I could hear the alarm. Checked all the gauges and sure enough, the water temp was rising way beyond the normal range. Always keep a spare kit, so I'm ready to do the repair.

Did a search just to refresh my memory. I noticed in one thread a mention of doing a backflush, especially if any of the vanes are broken. What is the procedure to do the backflush? I'm assuming I take off the hose from the output side of the seawater pump. But where downstream from there do I use a garden hose to do the backflush?
 
Follow the hose from the "out" side of the seawater pump to the first obstruction (probably cooler for either transmission oil or engine oil). Disconnect hose from opposite side of cooler and put garden hose in that end which will push any pieces of the impeller back toward the disconnection at the seawater pump. If you suspect more debris, do the same for the next obstruction until you get to the heat exchanger for the motor if you have a closed cooling system. If not closed cooling, then to the exhaust manifolds.

Gene
 
Yep, I would say this impeller should go in the "Shredded Wheat" hall of fame:

View attachment 9562

Rebuilt the seawater pump, then did the backflush. The hose from the output of the pump goes to a oil/power steering cooler under the port side of the engine. From there, it goes up to the thermostat housing:

View attachment 9563

I disconnected the hose from the inbound side of the cooler, then disconnected the hose from the inbound side of the thermostat housing. Used a garden hose to flush from there. Did see a few pieces of impeller bits come out of the cooler. Then, one by one, I disconnected all the other hoses and flushed them.

After putting everything back in place, put the muffs on, fired it up and let it run at idle. I'm fortunate to live in an area where I have exceptionally good water pressure and have been using these Mercruiser muffs for the 20 years I've had this boat with no issues. Once the temp hit the normal reading (about 140 degrees), I increased the engine speed to fast idle (in neutral of course), around 1000 RPM. I let it run for 20-25 minutes, watching the temp gauge and lightly squeezing all the hoses to feel that water was circulating through all of them. During this time, the temp gauge kept rising, very slowly. Once it hit just over about 165, I shut it down. In all the years I've had this boat, I've never seen it get that hot in normal use. Pulled all the hoses off the thermostat housing, removed the two bolts and took if off the block. Checked all the passages and couldn't find anything blocking the flow.

So now the question is, why did the temp keep rising? Could it be the thermostat? (Going to replace it anyway now that the housing is off the block) Keep in mind that when I first had this problem, I didn't notice the alarm until the engine temp had hit about 210 - 220 degrees. Would that have been too hot and damaged the thermostat? Any other ideas what to check?
 
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One other question please. I'm going to take the circulating pump off the front of the engine. I'd like to do a backflush of the block just to be sure none of the shrapnel from the impeller vanes got in there. What is the best way to do this? Since I already have the thermostat housing off, should I run water through that hole in the top-front of the intake manifold? The only other way I can think of would be to run it through one of the two holes on the front of the block where the water from the circulating pump comes & goes. The question there would be how does the water flow through there? Does that pump push the water through the port side of the block and it comes out the starboard side, or is it the other way around?
 
While it was running, did you happen to check the temperature of both exhaust elbows? One of the smaller passages in the system is the raw water dump to the exhaust.
 
sandkicker said:
While it was running, did you happen to check the temperature of both exhaust elbows? One of the smaller passages in the system is the raw water dump to the exhaust.
Click to expand...

No, I didn't even think of that. Should I pick up a temperature sensor, like this one? http://www.amazon.com/HDE-Temperature-Infrared-Thermometer-Laser/dp/B002YE3FS4

Any idea what a normal range for the risers would be? I suppose that one riser reading a lot hotter than the other one would indicate a blockage, but I'm curious as to what the temp should be.
 
On a raw water cooled engine, i.e., no heat exchanger, the "normal" T'stat is in the 130-140 range. The water exiting the elbow, and hence the elbow itself should be in that temp range. If you can't keep your hand on it with only minor discomfort, its too hot. On some MERC engines, one elbow sometimes runs a little hotter than the other normally.
The "chicken" test is to spit on the elbow... if it evaporates almost immediately, its too hot.
 
Thanks again for all the help/tips. After pulling the recirculating pump and backflushing the block and everything else I could, I put it all back together with the new thermostat kit and its running great. On the lake all weekend and the temp stayed right at 140 the whole time, even with some good runs at WOT.

BTW, it passed the chicken test. :D
 
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