I have a 01 500efi that is overheating and I cannont find the issue! fresh water boat. Here is a run down of the issue and what I have done to fix it.

Last weekend it start to run warm.. 170ish... Then when I went wot it would get over 180 until i backed out. Then I could run 3500 rpm all day long and it would get over 160.

I then replaced the impellar and housing and the thermostat. Went out and ran it this weekend. Now I cant even run it at 3500 without it going over 180 degrees. I can Idle all day long though and it stays at 155.

I took the boat back home pulled the oil cooler and found it to be severily clogged with debris. I ran a coat hanger through all the holes and cleaned them out and repeated the same process with the powersteering cooler. Ran it again with no change in temp. It was still overheating. I am getting great flow out the exhaust.

I also back flushed through the transom assembly and water pours out so no blockage there.

Would a circulationg pump cause this? I am all out of ideas. I would really appriciate any help! Thank you!

Do you have a water pressure gauge? It may not be a bad idea to install one, at least temporary. Is it a salt water boat? Corrosion can cause the gimble to corrode right where the hose comes through into the engine compartment. There is a plastic insert that expands the hose to hold it in place. That plastic piece will crack, fold over and block off a large part of the water hose.
I have also seen a circ pump cause what you are seeing. I removed and inspected it and couldn't find anything wrong with it. I decided to replace it since it was off and was quite a few years old. Problem solved. Did the exact same thing you are experiencing. Ran fine at idle and low speeds. Brought the rpms up a little and it would run hot. Seemed to have a suffecient amount of water, but it just wasn't enough.
If you want to check the water hose, then just bust off the plastic fitting on the inner transom assembly. It comes off on about a 45* angle and the main water hose attaches to it. Take the water hose off, then remove the fitting (2 bolts) and have a look at the inside of the hole with a mirror.
Hope this helps.
Eddie

Thanks for the help! I don't have pressure gauges and the boat is fresh water.

Do you know what the gm part number is for the circ pump so I don't have to pay big money for the merc one?

You need to use a marine pump with a SS impleller and SS backing plate. Don't use an automotive pump.
Eddie

I didn't know there was a difference with them. Thanks for the heads up.

Does this boat run a fresh water cooling system or does it have a closed cooling system (i.e. with glycol). A buddy of mine has a 425Mag HO with the closed cooling system and it was doing exactly what you say yours is doing. Also the later it got in the summer as the lake temps went up the hotter it would run. We ended up pulling the heat exchanger from the front of the motor, pulled the end caps off of it and found the tubes that circulate the fresh water through were about 1/2 clogged (some with lake debris and some with chunks of previous impeller failure debris). At this point we flushed it all out using a long piece of wire and high pressure washer. We went ahead and replaced the impeller even though it looked fine and the temp has been stellar ever since.

If you have debris in the cooler you may also have some that has lodged in the exhaust manifolds/risers, also your manifolds/risers may also getting plugged from rust how old are they?

It has headers...

I inspected the plastic ring that goes through the transom assembly and it looks great. There is no corrosion at all. The only thing left would be the circulation pump. If that doesnt fix it I suppose a loose drain plug will!

Well Now i have replaced the thermostat, sea water pump and housing, circ pump, cleaned the oil cooler and powersteering cooler, back flushed the system, inspected the platstic screw in inserts on the transom assembly on both ends so the entire cooling system has been gone through. It is still overheating! this is getting very annoying!

Sometimes the rubber hose that connects the drive pickup water flow to the inner transome will get sucked closed as RPM increases due to age. Try changing / checking the hose for softness. I have had this happen on another boat.

The header water inlets can also get clogged with debris. Check them too. Look for the header to discolor in some area's. If it has, its getting hot from little water flow.