Re: 86 cobra 4.3 20drg.s to hot
I figured that was a fresh water boat judging by how clean the motor looked. 180 is a bit too hot, my 88 usually runs between 160-170, it might come up to almost 180 after slowing down to idle after running on plane, but it will run cooler than that 99 % of the time. In addition checking the manifolds, check the water outlets in the thermo housing, sometimes they get necked down with rust. Also when you run it on the muffs, take off the cover for the impeller and look at the impeller housing, if there is water leaking (even just a little) out, then you might be pulling air IN when it's on plane and that will cause it to run hot. If you see water leaking try tightening the mounting bolts just enough to stop the water leak. The gasket for the impeller housing has to be in place properly for it to seal, I put OMC gasket sealer on them when installing. Did you use an OE or aftermarket impeller?
If it still runs hot try the water intake test with the boat in the water, you can remove the raw water intake hose at the thermo housing and have someone hold it up straight, and start the motor for just a few seconds, look at the stream of water coming out, it should be at least 2" tall, if not 2-4", that's the spec from OMC for the raw water pump output. If you don't get that, you could have a blockage in the PS cooler on the back of the engine, or leaks in the gaskets for the water tube that feeds the water from the lower unit to the upper unit.
You might want to remove and test the thermo in a pot of boiling water, I use a car radiator thermometer and the OE ones do open at just about 160 and are fully open at 175 or 180.
Mine had a running hot problem that new manifolds helped (see blockage in the pix) but did not cure. It was barnacles growing in the water intake holes (common in boats moored in salt water) and I had to break out the water screen to get all the barnacles out. After that it went back to running at 165-170 which is normal for a 160 stat.
Did it ever have a bad overheat in the past? IF so sometimes the hot exhaust water can melt the rubber gaskets for the water tube, or they could just be old and leaky. The raw water intake test (boat has to be in the water not on muffs) will tell you a lot.
You have the one piece units, how hot do they get? After a lot of use, they will show some clogging in one or more of the exhaust outlets. They are not hard to remove because OMC had the good sense to use studs and nuts instead of bolts. Due to the design, you must loosen and slide down the lower exhaust rubber boots on to the Y pipe to get enough clearance to pull off the manifolds. I spray some silicone spray on the Y pipe to make it easier to slide em down. Here are some pix, of my old ones vs new. The old ones were in salt water 5.5 seasons and never cleaned out. After rodding them out there were ok but the ones on your boat are original (can tell by the 4.3 stickers) and so I'd check em very carefully. Fresh or salt water? you are looking for rust inside the exhaust ports of the head and manifold.
ya I really do live in S-Cal. high desert. Plus my gauges are OE 26yr. old and always been close to right on even the speedo is not to far off the gps speed once above 10mph. Thanks again will update when I can test.
