1991 Sport Nautique Running Hot

[bgjmn]

'91 Sport with 351 PCM. Had the boat on the water and it was running at least 200 degrees on the gauge, so we quickly got it back to shore. Did not go to limp mode. Only ran for a half hour or less. Replaced RWP impeller, even though existing impeller looked fine. Ran again, same result. Now have it on the trailer and have replaced the thermostat and circulator pump. Have run a number of times on land using both the Fake a Lake and the Bucket method. Have water exiting the exhaust as it should. Pulls 5 gallons from bucket in 35-40 seconds at idle rpms. Infrared Thermometer reads 20-30 degrees less than the gauge at the temp sender. Tops of exhaust risers are cool when the water is running through. The only place it reads hot on the thermometer is on the exhaust manifold down by the spark plugs. Are we maybe looking at electrical/voltage issue? The red warning lights for volts and temp at top of dash seem to be on immediately upon starting the engine even on cold start. That might be another clue. Any thoughts would be welcome.

[mpost]

Check out the owners manual and PCM manuals here for your 91.LINK
It mentions the trouble light in the dash that can be temp, oil pressure or charging issues. If this is on as soon as you start the engine, it would eliminate temp. However if you are really measueing 170 to 180 with the IR gun, that is stilll a little high. Mostly the carburated engins run a 160 degree thermostat. What does your oil pressure gauge read? Not sure on the 91 but my 98 has both a oil pressure sender (for the gauge) and a oil pressure switch (for the warning light.).
Where are you located in MN? PM me if you are near by.

[bgjmn]

Will be great to have the engine manual to refer to! Mine has three separate warning lights on upper part of dash, volts, temp, oil. It's the volts and temp that seem to be on all the time. Am not with the boat right now but recall the oil gauge always read high, yet that warning light never came on. As far as the IR gun the highest I got it to read at the temp sender was around 150, while the gauge on the dash read around 180.

[shag]

You could have a sticking thermostat... I often just removed my thermostat on older carbureted boats I had... Here in Fla, never had an issue and they always ran 145-165 once warm...

[bgjmn]

Interesting idea, but I just put in a brand new T-stat.

[ScrewPusher]

do you have a strainer that may have some debris slowing the water down? Also pull the hoses on the trans oil cooler (or all coolers) and make sure they are clear. Buddy was having similar issues and several impeller parts were stuck in his oil coolers. It was on a Malibu, so not sure if was engine or trans cooler.

[Jonny Quest]

Temp sending unit. In the last 8 years, I’ve had 2 PCM temp sending units go bad.

JQ

[bgjmn]

Awesome, I have a new Temp sender that I have not yet tried. Look forward to popping that in!

[Wkebrdr]

It sounds like you have an IR thermometer and a new sender you can troubleshoot with.
Remove the old sender from the engine, and then wire it back up ensuring you give it a ground. Use a heat gun or hair dryer to warm up the sender confirming the temp with your ir tool and compare to the gauge reading. Repeat the process with your new sender. This will show if the old sender is "bad" in comparison to a new one.
If the readings are the same for both senders then measure the resistance of the sender at a certain temperature and record it. Get a resistor(or potentiometer) as close to that recorded value as you can and attach it between the back of your temp gauge and ground. Does the gauge read the same as it did before? If yes, then you have a gauge issue. If no, then you have corrosion/short in a wiring connection between the gauge and engine. You can prove this by running a temporary wire from the gauge to the sender on the engine.

[Wkebrdr]

Also, I've seen old impeller blades lodged in the thermostat housing causing an engine to run hot at rpms, but ok at idle. Doubt this since you replaced thermostat.