1988 sunbird omc 4.3 impeller help

Ok I'm new here and have read every post I could find on my problem to no good end results. Like everyone else mine pumps fine with the muffs and everything is fine but in the lake it won't pump at idle. I have back flushed every hose, disconnected every hose in sections and flush. I have new impeller housing kit sealed and made sure it is sealed. Replaced engine water pump and removed t star just to see if I could keep it cool on the lake. Running at higher Rpm will get the pump pushing some water and going across the lake it will cool and I can watch the temp drop. I have checked for kinked hoses everywhere and replaced the one from the drive to the gimble. Even spoke with a local marine mechanic that didn't know what else to try but he doesn't work on I/O that much. The family is really ready to go boating and I'm three weeks into this and at a standstill and don't know where to look next. Any help or ideas would be greatly appreciated!

Re: 1988 sunbird omc 4.3 impeller help

Did you put the impeller in correctly or backwards?

Re: 1988 sunbird omc 4.3 impeller help

My experience has been that the symptoms you describe point to an air gap or small blockage somewhere in your raw water cooling system. It runs nice and cool on muffs because there is pressure behind the water that overcomes the air gap or partial blockage. When in the water, the lack of pressure behind the water means that the impeller cannot overcome the air gap/blockage , and your engine runs hot.

You need to test that when in the water, your impeller is doing its job properly. Find the hose coming from the outdrive, through the transom, that feeds the cooling system. It should connect to the 'stat housing (at least it does on my 5.0L GM). Disconnect the hose from the 'stat housing and connect a piece of clear hose to the end. When in the water, lower the outdrive to as near vertical as is possible. Have a crew member start the engine while you hold the hose so the water comes out straight up. If working properly, you should have a column of water coming out of the hose that is several inches high. If not, look through your clear hose - if you see bubbles along with the poor flow of water, you have an air gap. If you just see low flow from the hose, it could be a bad or poorly installed impeller, a physical blockage in the system, or an issue with the outdrive internals.

My issue ended when I had the lower unit resealed. Have there been any issues and/or service performed on the outdrive recently?

Good luck.

Re: 1988 sunbird omc 4.3 impeller help

I've been working here lately but I'm going to start tomorrow checking every hose from the foot to the engine.

Re: 1988 sunbird omc 4.3 impeller help

Ok so I have removed the lower unit checked the tube in the foot and reseated it. Removed the out drive and back flushed everything and re sealed it. Still same problem it won't pump water... Im list at this point!