need help diagnosing engine possibly running hot

[gmspd]

this is my first post on the forum. i have been lurking for a couple years and i have always been able to find answers by searching. i have been chasing a possible overheat issue on my boat for about a month. i have a 1991 wellcraft eclipse 233 with a mercruiser 5.7 v8 and alpha one outdrive. back at the beginning of the boating season the boat developed a vibration at low speeds when turning. it ended up being a bad u joint on the drive. i replaced both u joints, the gimbal bearing, all 3 bellows and an impeller since i had everything apart. i only replaced the impeller vane and not the upper and lower housing. everything looked like new in the housing. fast forward 2 months later, i was out cruising along and i happened to glance down at the gauges and i saw the temp gauge at almost 200 degrees. at the time i was at 4200 rpms around 33 mph. i backed down to about 3000 rpms and the temp came back down to around 150. if i went over 3500 rpms it started to climb again. first i tried a new thermostat and it made no change. then after reading on various forums and asking a few local boat mechanics everyone suggested that i replace the impeller again along with the upper and lower housing. i did that last week. when running the boat above 4000 rpms now, the temp gauge slowly starts to climb and levels off around 170-175 degrees, but when i back it down around 3000 rpms the gauge goes back down to 140-150. today while i was running it i monitored the engine temp with an infared laser thermometer. nowhere on the engine did the temp get over 140 degrees while above 4000 rpms while the gauge was showing around 170, but as soon as i back the throttle down the gauge goes back to 140. having the thermometer directly on the water temp sensor it never showed above 140. everything is pointing to a bad gauge but it makes no sense that it is only inaccurate above 4000 rpms. the water temp sensor is brand new, i just put it in today.

thanks for any help at all. this boat is driving me nuts!!!

 

[wellcraft-classic210]

--Hmmm hard to say with all the conflicting findings

The gauge can be checked with various resistors for accuracy --- It needs a good 12 volt or 13.6 voltage supply. ( I seen the chart on other posts)

Its also possible that you have some sort of Flow restriction and the infared monitor is not sensing the hot spots.

The exhaust manifolds can rust & start to block the flow. A visual inspection after pulling he risers might be in order.

The experts here may have some more detailed input as well.

 

[gmspd]

i put the infared on both risers and manifolds and they both showed around 170 degrees. im wondering if it is a flow issue as well, especially since the issue only happens above 4000 rpms.

 

[alldodge]

Pull the exhaust elbows off and see if your exhaust flappers are intact. If they broke off they can cause a problem with water getting out. Also check your power steering cooler for a restriction

 

[gmspd]

Wouldn't you think that the engine would be showing hotter temps with the infrared if that was the case. It read 140 at the thermostat housing, intake manifold, water pump, and the front face of both heads while the gauge was showing 170.

 

[gmspd]

Wouldn't you think that the engine would be showing hotter temps with the infrared if that was the case. It read 140 at the thermostat housing, intake manifold, water pump, and the front face of both heads while the gauge was showing 170.

 

[Stamey]

The temp you read on the metal outside the water jacket tends to be lower than the actual water around the sender. The water temp can vary greatly as it moves around the engine and in my experience a thermometer on the outside can only be used for relative temps, such as one manifold or elbow being significantly lower than the other, which points to a restriction in the cooler manifold. I have never had a laser thermometer read exactly the same as the gauge.
Another possibility to look at, besides the manifolds and elbows, is the intake hose between the out drive and the bell housing. My 4.3 actually has that problem now. I know it is the intake hose because when I replaced it I twisted it and collapsed it as I had a devil of a time getting it on there. I will be using more lube when I put the next new one on.

Chris

 

[gmspd]

thanks for the tips chris. the intake hose between the bell housing and and the transom is also brand new. i replaced it when i did the bellows. yesterday i decided to compare the readings on my laser thermometer with my truck gauge and sensor. my truck was reading 210 degrees on the coolant temp gauge and i pointed the laser right on my coolant temp sensor on my left cylinder head. the thermometer read 205 degrees. that quick test right there tells me that the laser thermometer is pretty accurate in giving actual engine temps. my next step is to trace all the wiring from the sensor to the gauge and make sure all connections are good.

 

[gmspd]

thanks for the tips chris. the intake hose between the bell housing and and the transom is also brand new. i replaced it when i did the bellows. yesterday i decided to compare the readings on my laser thermometer with my truck gauge and sensor. my truck was reading 210 degrees on the coolant temp gauge and i pointed the laser right on my coolant temp sensor on my left cylinder head. the thermometer read 205 degrees. that quick test right there tells me that the laser thermometer is pretty accurate in giving actual engine temps. my next step is to trace all the wiring from the sensor to the gauge and make sure all connections are good.

 

[alldodge]

You said that as rpm's increase so does the temperature. This tells me that you have a restriction, with either water coming in or with being able to move hot water back out.

Good luck