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1979 Mercury 7.5 Cooling Issue

M

Moving Violation

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I recently got my 7.5 running after about 5 years. I knew I should have replaced the impeller before I started it, but I didn't...It immediately started flowing water pretty good and after a couple of minutes of checking other things, I noticed it wasn't flowing any water. I shut it off immediately. Sure enough the impeller was shot and in a lot of pieces. I fished out all I could find, blew the tubes out with air, removed the thermostat and put air down those passages, etc. I thought I had everything clear. I installed a new impeller and when I ran it again, there was very little water coming out the pee hole; as it got hotter, it began to turn to steam. Back apart and looking for more old impeller pieces (couldn't find any), blew out again with air...same thing. I read on a previous forum about checking a plastic washer at the top of the water tube, and I'll check that. Are there any other tricks or places to look for possible impeller pieces or anything else? Thanks in advance for any info...
 
This kicked my butt over the weekend...I've made sure there are no obstructions in the lower unit or in the main copper tube. I can also circulate water easily through the flush out hole while running and get a normal stream of water through the tell tale hole. When I remove the hose from the flush out it basically stops flowing and I get steam. This is telling me I have an obstruction somewhere just past where the copper tube enters the bottom of the motor. I couldn't find a plastic washer up there. Again, any advice would be very much appreciated! Thanks!
 
So; you are saying that you have already removed the powerhead and checked the rubber grommet where the copper tube connects?

One other thing....have you checked the little tube that directs water up into the pump? Sometimes that gets clogged and it's easy to overlook....
 
You might still have impeller blade debris stuck in the tube to the PH or in the PH itself. Remove the LU and back flush the PH with water and compressed air (via the Tell Tale opening).

Jeff
 
Thanks Jeff. No I have not pulled the power head. Yes, I've checked the little tube to the water pump...it's clear. I'm sure everything in the lower unit and tube going to the power head are clear. I have removed the thermostat cover and thermostat (I removed the thermostat completely while checking all of this and testing). I put air from the tell tale opening and made sure the opening and the hose going to it are clear. With the lower unit removed, but with the long tube installed, I've put air up through the tube and there seems to be some resistance (with the thermostat cover removed). I've put air and water through the flush out hole (with the thermostat cover installed) and both flow freely out the tell tale opening. I'm thinking I have impeller pieces just past where the long tube enters the power head. I like the idea of removing the lower unit and long tube and putting some pressurized water through the thermostat housing, or as you suggest the tell tale opening, and see if I can back flush it that way. I've tried air but I agree, water would be better. Thanks a bunch for the advice Jeff!
Mike
 
I have not removed the powerhead...I've never done it and the shop manual isn't very helpful. If I don't get anything (old impeller pieces) out by backflushing, I'm figuring that's the next step. Thanks....
 
Removing the powerhead isn't as bad a job as you might imagine. Just six bolts accessed from below.....one screw below the spark plugs.....assuming the corrosion isn't too bad it will be easy.

Just as well too...because you can clean the carbon out from the exhaust port and have a look-see at the seal on the crank.....

Over all....it's good that you're doing all this....it will pay in the long run.
 
Seakaye12 and racerone...you were right. It was the plastic washer under the powerhead...completly melted/clogged. Yes, getting the powerhead off was pretty simple. Have parts coming...thanks for your help!
 
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