Engine temp fluctuation

We have a 94 sport with a pro boss 351 engine. The first two years we owned the boat it held 160 pretty consistently. Last season we replaced the impeller and thermostat and since then the temp has been less consistent and when it does hold it's closer to 180. We've tried troubleshooting with a CC dealer and their suggestion was see what the motor does without a thermostat in and when we did that it held 125 consistently. We just put a brand new thermostat in and once again were back to a less consistent temp and hovering more around 180. Both new thermostats were labeled 160 degree thermostats.

Anyone have any suggestions as what could be causing this?

Needs to be a balanced thermostat.Check it in a pan on the stove to see what actual opening temp is, I use a meat thermometer.If you lost impellor pieces brfore change they maybe lodged in hose.

Bob

I had exactly the same thing two weeks ago in my 1990 SN. Every so often the boat would get too hot on starting up. I would shut off, let it cool a little, re-start and it would be fine. Clearly the thermostat was sticking.

So I wanted to change the impeller anyway so I ordered an impeller and thermostat.

Put them both in and the same as you, boat always used to sit just below 160, even a little lower. With the new thermostat in, my boat would run hotter, nearly 180, occasionally a little lower, but 170 at best. What I also noticed was when I gunned it back to get my fallen skier, shut off and started back up, the gauge would be high of course, but take forever to come back down, unlike my old stat that would drop real quick.

My new thermostat was rated at 140 degrees whereas the original I took out was 146, although the old one used to run colder with it in, strange I thought.

Okay, so what did I do. I cycled the old original thermostat three times in boiling water and it opened and closed perfectly.

I took the new thermostat out of the boat and what I noticed was the actually opening (orifice) in the thermostat once open at it's temperature was much much smaller on the new (Sierra) thermostat than the old one. So the new one, despite opening 6 degrees earlier, had maybe half the flowrate as the old one.

I stuck the old one back in, and my boat is perfect again, opening as it should, holding at between 150 and 160. It has not hung up again since, so probably some grit or rust had gotten in there which cleared out when I cycled it.

You should compare yourself, the opening difference was huge so very little water flow was getting through the new thermostat despite being fully open, and open even a tad earlier.

Good luck, but I think you will find that the problem, just hope you still have the old stat. If not, order a real PCM one and not an after market one as I did.

Tony

Definitely compare the opening when they are both boiled in a pot of boiling water, I am sure you will find that the problem. Even if you put a 143 degree opening stat in there, that just means it will open earlier, but if it is the same small opening once open, you won't get the water flow needed to keep the engine cool.

Frankly I don't think you have any blockages, if you did, by taking the stat out and running it without, you wouldn't have been nice and cool at 125 degrees as you were.

I have a friend with a Mastercraft whose impeller came apart and caused some blockages. I can tell you from working on that with him that free flow of cold water is crucial to keeping them cool. Even with his stat out, it would get too hot due to the blockages.

Take a look at this skidim.com link where you can see the PCM stat, when it is open, the entire three triangular areas are fully open allowing lots of water flow. On the Sierra one I tried, the opening was just a small ring open once the disk had opened up at temperature, it looked much like a car stat. I would say the flow rate was a third of the old stat. Even skidim make the comment that only the OEM stats work in these engines.

http://www.skidim.com/prodinfo.asp?number=RP026002

Good luck.....Tony

Hey,

I purchased what was supposed to be a 143 degree thermostat from Ski Dim. I replaced the old thermostat and when I put my boat in the water it went over 200 and I shut her down. Waited a little bit started up and did the same thing. Took the new Ski Dim thermostat out and put old one in and I was back to around 160 and when I run hard it rises to 180 or so.

The Ski Dim thermostat was a pcm and only had a catalog number on the wrapper. I boiled it on the stove and it started opening up above 160 and at 200 plus finally opend up all the way. How fast is it supposed to open up?

Could the new thermostat have been stuck?

Tom

BRI892001 was absolutely correct!! We finally got around to ordering and installing the 143 degree thermostat and the engine now holds 160 like it used to. So I guess the pro boss EFI engines are the one the one exception to the all EFI engines use the 160 thermo. Unfortunately we weren't able to find the original thermo we took out that would have been the best way to figure out which one we needed.

Thanks for the help guys!