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Tiara Open 7.4 Crusader Overheating
#1
Hello. I have a 2000 31 Open, 7.4L Crusader. Port engine is overheating and we cannot seem to find out why. New pumps, thermostat, flushed the system and still overheats. Overheats at idle. It will cool when I put a load on it. Any thoughts appreciated. A bit frustrating.
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#2
It may be poor raw water flow from a restriction that doesn’t present problems when pressure is high (higher rpm). if you haven’t done so clean and inspect the raw water side of the heat exchanger. Old impeller pieces and or barnacles can limit water flow and heat rejection. Also you want to check your risers. Rust and debris can get lodged in there and prevent raw water flow.

IR temp guns are cheap and can tell you a lot without disassembling half the engine. Check both motors at different places in the cooling loop and look for dissimilar readings or abnormally high temperatures.
[-] The following 1 user Likes jclark003's post:
Sans Peur (09-18-2021)
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#3
What temps are you seeing?  As jclark003 mentioned, get a IR gun...it will save you lots of time in diagnosing the issue.  Get the temps on the elbows/risers, heat exchanger, thermostat housing, portions of the engine where the temp sensors are, etc.

Did you change your circulation pump or just the raw water pump?  The circulation pumps can deteriorate as well.

Best course of action would be to get the temps of the elbows/risers to figure out if it is a raw water issue or a fresh water issue.

If temps indicate raw water issue, cheapest thing to do would be:
-check the heat exchanger for debris - take off the inspection caps and spray water through the tubes.  You may find some old pieces of the pencil zincs, impeller fins, shells, etc.  When was the last time the elbows/risers were changed?
-check the seacocks
-check the sea strainers for debris
-check the seal on the sea strainer and the hose connections for air leaks

What do you mean when you say the system was "flushed"?  Was the coolant flushed or did you flush the raw water system?  Was the raw water system back-flushed such that any debris would come out of the oil/trans cooler(s).  Did you run rydlyme through it to descale?

If a fresh water system problem:
-did you put the right thermostat in?  I'm not sure if the 7.4 is a 170 or a 180, but check the manual.  Is the thermostat correctly oriented?
-inspect the circulation pumps
-what is the condition of the coolant?  when was it last changed?

Had you not said that it idles hot then cools down with load, I would have asked whether you:
-burped the system after the coolant flush, 
-used the correct coolant (dexcool 50/50 mixtures)
-verified the seacocks were open and clear of debris
,but that likely wouldn't be the problem if it cools down with a load applied, at least to my knowledge.

Good luck!
-M1
[-] The following 1 user Likes Medicone's post:
Misdirection (09-19-2021)
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#4
Agree with above. The IR gun is a great way to track down a problem. You can use it to follow the flow and see where the temps start to rise. Also could be a bad thermostat that may be getting stuck closed at idle. I know it’s new but wouldn’t be the first time I’ve heard a new t-stat was bad. It should be 170deg.

I always start from the source. When was the last tome your pick up scuppers were inspected and cleaned. They can easily clog up with gunk and debris or bottom paint for that matter. Could be not enough flow at low rpm but as pump spins faster becomes enough.

Also on the fwc side, when was last time you flushed coolant? Could have a pin hole somewhere causing air to get trapped. You will need to burp the system to get all the air out.

Good luck
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#5
Have you checked the belt tightness to the water pump, as well as the one to the alternator? I had one overheat a while back when the when the alternator belt became worn and loose. Might seem weird, but the belts balance out tension in some manner around the central pully. Higher RPM would overcome some of that so idle would be hot and some cooling with higher pull at higher RMP.
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#6
When I purchased my 31 Open one engine ran hot.
I removed the bronze intake screens off and removed half a dozen dead crabs about the size of a quarter and a colony of barnacles.
I have no idea how in the world they got in there.
I also reamed the intake holes out with a 1/8" or 3/32" bit that were half plugged with bottom paint ,
No more heat issues since.
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#7
Talk with an expierenced mechanic,I had the same issue with anti freeze coming out of the overflow tank.

He replaced the thermostats

My mechcanic had to "burb" the motor to get rid of the air locks,and slowly fed the antifreeze back into the system.

No problems since
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#8
Have you run it without the thermostat? I just had an overheat problem. Waiting for new thermostat. But without it temp doesn’t go past 125. Damn $60 for a thermostat!

Pull raw water hoses from risers make sure they’re flowing strong.
Run with cap off stick your finger in make sure you have strong coolant flow.
1 other thing. I had a boat that was sucking a little air on the raw water intake hose. Drove me nuts.

Wouldn’t it be nice to be able to buy a new boat every 2-3 yrs ?
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