The other little thing I've seen cause false readings is having too much teflon tape on the sending unit threads.
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Do you have a pressure relief valve comming off the pump ? To me it sounds like when at WOT the jet is pumping way to much water in and out of the block to quickly to absorb alot of the heat. With out a pressure relief valve your pump can and will put WAY to much pressure into the engine. Pumps can build 100 + pounds of pressure leading to blown hoses, head gaskets and even blow out the soft plugs. Been there done all the above.
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I had the same problem with a bypass thermostat kit, temperature stayed at 160* on hose, but as soon as I ran the boat on the lake the temp would cycle between 160 - 200+*, tried just about everything you did.
I decided to have a look at the head gaskets and found that it blew between a water port and cylinder. No water in the oil.
I came to the conclusion that the thermostat could not cycle fast enough to keep up with the super heated water coming from the cylinder and that made the temp fluctuate.
I had the block decked and the heads resurfaced, fitted everything new, and the problem was solved.
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Just a though for your saftey, considering some of the "heating" issues your dealing with , highly suggest you peak in at the exaust hoses (inside) to check for "bubbling and delaminating" , exaust hoses can look perfect externally and be failing on the inside from heat Tom
Dang, talk about the responses today I've gotten on this, much appreciated and I will try to answer all that I can!!
For Outlaw, yes both do have bypass holes in them, they are small but for 6 years or so that I've ran the thermostat it's usually made for a boring gauge that was pretty stationary at 160 degrees most of the time!! Might try to enlarge them a fraction bigger, just gotta figure out the temp fluctuation problem first before I do something else to disguise the problem at all!! Great idea on making them bigger though!
I'm almost pretty sure that by taking out the thermostat all together is gonna make the gauge barely register like it use to pre-thermostat and I probably won't see anything out of the ordinary in the gauge due to pretty much no restriction for the water flow!! Flow in and out fast, but we shall see!
For CAS, I agree with your thought on no thermostat will equal little to no fluctuation in the temp due to the lack of restriction for the water to flow!! When I first got the boat, it didn't have a thermostat and alot of times you wouldn't barely see the temp register on the gauge, it was around 100 degrees and wouldn't warm up completely!! For the teflon tape idea, the last time I took it out with the new sending unit I actually didn't put teflon tape on it and it still fluctuated like the old one did!! Since that last run, I put teflon on it and installed it like it is now!! Never heard about the teflon idea, I love learning new things and ideas!!
For Michigan Daaaaaa, I don't have a pressure relief valve anywhere in the system but am going to install a gauge here in the next day or so to see what it has for pressure at idle and WOT!! What I'm not sure of is why all these years, since new in '75 till today it's never had a problem with pressure causing any problems until now. I know stuff can happen and I can accept that, but I know the complete history of this boat and motor and it's never had a problem with pressure messing something up!! I will be running that gauge and will take it from there, if I do have way too much, I'll be picking peoples brains on the best and right way to reduce the pressure without losing the flow to the motor!! Just trying to diagnose what's causing my temp flux problem right now, fix it, identify what's caused it, and fix that!! It never ends for me lately I guess, always something wrong with something!! hahahaha
For Jet Mad, before this problem, my boat would run in the driveway and in the river at 160 pretty much all the time except for a long WOT then it would creep up a bit but then would always drop back down to 160. Also would obviously heat up with I would suck something into the jet like I did this time, but what's different about this time was that I kicked out whatever was in the jet but my heating up problem never went with it!! This is new to me, I've had this setup for some time now and this is out of the ordinary!! So now I'm looking at possibly having a problem with a head or both, and thinking I may be having the same problem you described!! Question I have for you is, did that gasket show up in the compression check? Also, after the motor work, I assume you continued to run the thermostat setup and did the fluctuation continue after that or does it do something different now like how I described mine ran like previous to my problem? Is it steady now around 160?
Sorry for that long response, your heads probably hurt now from reading it as mine does from typing it!! lol
Thank you all for your words, keep the thoughts and ideas coming folks, they are much appreciated!!
Sooo many posts so I might be repeating this but did you pull the frost plugs and check for sand?
I have seen water come out of the water jacket areas but there was still sand built up in the block when the frost plugs were pulled out.
I replaced mine with rubber fixit ones that were only a couple bucks and always kept a few extra in the boat in case one would come out but never had that happen, and when you want to flush out the block just loosen the nut and pop out your plugs and get all that sand out and pop them back in...just a thought.