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350 XLi temps

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Koko

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I have a 1998 FWC 350 XLi with 410 hours which is overheating. After running at 3400 rpm for 10 minutes, I used an IR temp meter to determine that the top of the risers are at about 135 deg. and the center of the manifolds, just below the gasket up into the risers are at 225 deg.. The temp gauge reads about 180 at this point, but I know that it will go up to 190 if I keep running causing the alarm to activate. There is plenty of flow coming out of the exhaust. I am thinking that I must have a problem with the FWC side of the system since the risers and all seawater hoses are fairly cool. Can a faulty thermostat cause this condition or could there be a blockage or air gap somewhere in the fresh water system? Maybe the heat exchanger is clogged on the FW side?
Also, does anyone know where the temp gauge sending/sensor unit and overheat alarm sensor is located on these engines? It seems like the manifolds are a lot hotter than what the temp gauge is indicating and the alarm siren won't shut off even after the engine cools.
Any input would be greatly appreciated!
Thanks
 
Last edited:
Koko,
welcome... while the answer is yes to all of your suppositions, my belief is that you are only going to cure this by getting into some of these components. Your heat exchanger may be an easy starting point as you can simply pull the end cap off and look inside to see if the passages within are clogged. You will spill about 1/2 a gallon of raw water but that's really not anything to keep you from looking. The temperature alarm and sending unit on my slightly older engines are located on the thermostat housing on the engine.
 
The manifolds should be hotter than the gauge as they are the last items to get cooled.

"There is plenty of flow coming out of the exhaust." - sorry, I'm having trouble you can make this assessment by looking. When was the impeller last changed and the raw water pump inspected?

As far as the sensors and switches go, yours should be a GEN VI so the "standard locations" apply (intake manifold and cylinder heads).

also, i doubt the inside of the HX is clogged on the closed side. as noted above, removing an end cap will provide an answer.
 
Thanks for the replies...
The raw water pump has not been inspected recently. I will definitely look into it. I figured that since the raw water hoses and risers are all remaining cool (135), the problem is probably in the FW side of the system. I will also pull the end cap on the heat exchange. These are a couple of fairly easy checks. Do you know what temperature readings are considered normal for the middle of the manifolds and the top of the risers at full operating temps on this motor?
 
You really need to measure the raw water flow...also, if the pumps have not been maintained regularly, you may also be experiencing the u-cooler issue with the zinc "parts'.

As far as specific temps, they will vary with the local environment. we've discussed this in many threads over the years so some searching will likely provide a better answer. based on memory, 120 deg F, at the elbows is a nominal number with the engine in the cruising rpm range.

Also of benefit would be more IR temp data points. The temp differences at the HX, both closed and raw sides, gives insight into how the rest of the system is working.
 
You said it's a 1998, have you ever changed the risers ? I think 13 years is pushing it , just a little. Does the engine still overheat if you run at lower speeds ? I used to be able to run my 318's all day at 2000 but at 2500 they would boil over in less than 10 minutes, turned out the new risers I had put on earlier had half of the outer cooling ring plugged with casting material, I changed the risers and no more problem.Sometimes the IR gun can be misleading if you're not familiar with what temps you should be looking for at what points, and just looking at the water flow comong out the exhaust won't really tell you much unless you have almost a complete blockage...Let us know what you find...Lee
 
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