Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM
OK, I know nothing about OMC. Having said that, I had a similar problem with my Merc 140 I/O. Early on, somebody told me to confirm that the entire flow path for water was clear of rubber impeller pieces, which get released into the stream of water when the impeller shreds. I am assuming here that the OMC has a rubber impeller similar to a Merc. The water path is from the pickups, through the pickup pump, up through a tube, into a "water pocket" (on some Merc drives), through an orifice, into a hose which goes to the transom, through a fitting, into the hose from the transom to the thermostat housing, through the head and block passages, into the manifold, up through the riser, out the elbow, into the exhaust passage in the driveshaft housing, down through the gear housing, and out the center of the prop. I did what you did, by pulling certain hoses and checking flow. The problem I ran into was that I always had great flow at idle on the muffs, but would overheat whenever I sped up on the lake. Well, one day on the lake it overheated and stayed that way, i.e. it would overheat even at idle. This turned out to be a good thing, because I could replicate the overheating with the muffs back at home. I was able to determine that I wasn't getting good flow from the sterndrive to the transom, even though the pickup pump and impeller were brand new. So, OK, I pulled the lower unit (gear housing). The tube looked fine. Then I pulled the plastic cover for the water pocket. I did this because some references mention this cover warping due to exhaust heat, and then at speed the cooling water can leak out around the warped cover. Nope, mine was perfectly flat, but I had a new one anyway. Then I reached way up inside the driveshaft housing, into the water pocket, and guess what I found--old pieces of rubber impeller shreds jammed into the orifice in the wall of the pocket. It took awhile, but I was able to get all of them out. I could tell that some were very old, and that I had pieces from at least two previous impellers. These pieces were lying in the water pocket, staying near the bottom at idle. But as the cooling flow increased with higher rpm, they would get caught in the flow and clog the orifice. Now I realize your OMC probably doesn't have a water pocket, but the point here is what some guys told me right off--if you think an impeller has ever been shredded in that sterndrive, you need to check the whole water path for impeller pieces. You can use the "divide and conquer" technique as you've been doing. I got lucky in that my problem got stuck so I could find it. My next step was going to be to put a clear hose in place of the transom-to-thermostat hose, because many people told me you'll see bubbles in there if the flow from the sterndrive isn't keeping up. And the clear hose can be observed at speed. Good luck, I hope you get lucky like I did (after I fought my overheating problem for about 4 years, 2 mechanics and 1 rebuilt head).<br /><br />M140P<br /><br />P.S. By the way, here are the little buggers that caused me so much grief...
pic1 pic2
