You will not "fry" the impeller in 10 seconds.
If you (or a previous owner) have ever had an impeller failure/disintegration, and you have not pulled your oil cooler to clear it of fragments, it would be a good idea to do so soon.
When I removed my oil cooler, I tried to back-flush it with no success. I finally had to use needle-nose pliers to pull the impeller fragments out of it to completely clear it. The 7.4L engine oil cooler water passages are small enough to prevent passage of impeller fragments or other large debris.
You certainly could. You would need 2 valves to prevent flow to the drive and one for the hose input (or use a cap, etc)
It would be unnecessary, and you run the risk of having the valves in the wrong position or the cap leaking or getting lost etc.
Just get a good flush adapter, make sure it doesn't slip off and make sure the hose is on before starting.
Cheers,
Rick
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Thanks Rick (and everyone else)! I've made some progress. Firstly, I went an bought the mercruiser muffs. Worst $40 I ever spent! I fitted them up and no matter how tight I squeezed them, I couldn't get them to seal well enough to push water up the leg and out the end of the hose that normally hooks to the impeller. Further experimentation is needed I think. But, I did put the end of the garden hose in the hose that normally goes to the output side of the impeller and to the engine. I got flow all the way through. Learned something interesting by doing that, the water was coming out of two holes up close to the transom as opposed to out of the prop hubs. What that tells me is that all this time I've been flushing expecting to see water exiting the props, I should have been looking at these other holes. (Which I didn't know existed). I've been having my helper watch the prop area, and alert me if nothing comes out. I realize this thread is making me seem pretty stupid, but I swear I'm a pretty decent mechanic, I've just never had a Mercruiser before.
To answer your other concern, there was an impeller failure, I believe caused by lack of flow when the seller fired the boat up during my inspection. I was inside the boat at the time, so didn't notice a lack of flow. The reason I believe this is that when I unhooked the hoses at the impeller, the system was full of anti-freeze, which would have been flushed out had the impeller been working when it was running on muffs. While replacing the impeller, I was getting decent flow when I back flushed, but I knew the amount of fragments weren't enough to comprise the entire impeller so I went exploring. I found a lot of my impeller jammed up in the oil cooler as you describe. Between what I found in there, what back flushed out and what was jammed in the passages of the impeller housing, I'm confident I got it all.
Tonight I pulled the drive off. I'm going to try to replace the shift cable bellows so I can sea test it next weekend. Good news is, drive bellows are good. No signs of water intrusion. Gimbal bearings look good and feel tight and smooth.
