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2004 Honda 225 hp sounds overheat alarm and throws it into rpm reduction mode

I went out on a sea trial the other day and the motor runs fine at idle and cruising until I hammer the throttle then the overheat(red light) and a solid horn goes off and the motor sounds rough like it goes into rpm reduction mode. It pisses water just fine the thermostats were just recently changed, could it be the pressure relief valve?
 
Could be any number of things - Intake screens clogged up, t-stat not opening all the way, bad impeller in the water pump, worn impeller housing, blocked water passage, or a faulty pressure relief valve. Do you have a Helm shop manual? If so, there is a test in there for the pressure relief valve. If that tests out, then you need to go on a bug hunt. Start with removing and checking the intake screens on the lower unit. Then move to the t-stats and test them by heating them up in hot water and measuring at what temperature they open and how far they open IAW the owner's manual. If all that tests out, then you will need to drop the lower unit and inspect/replace the impeller. Autually, it's probably better to replace the entire water pump, it's not that expensive and not much more than the impeller kit.
 
There is a tube at the bottom of the manifold that goes back into the case mount and I bet that corrosion has collapsed the hose restricting flow at higher speeds.
Joe
 
You didnt mention if this happens all the time or is persistent. Ive had this happen on rough sees when I wass trimmed to high, also while running over very thick sea weed, and once I forgot to screw back the flush port and that apperently caused enough of a pressure relief that the engine overheated.
 
thanks guys it does not happen all the time the motor will run fine for an hour sometimes and then it happens randomly, i will have to take the boat out for another sea trial and see if it happens when the motor is trimmed up and ill check the flush port and t stats but the tstats are brand new so i doubt that they are the problem ill check back in after a few tests again thanks for your help
 
Sometimes brand new t-stats do not work properly. But, since the problem is intermittent, that is probably not the issue. You have two overheat sensors and an engine coolant temperature (ECT) sensor. Any one of them can set off the overheat alarm. At this point, I would first locate the two overheat sensors and make sure the two-prong connectors are clean and well connceted, and the wires to the ECM are not cracked or chaffed. If you have a Helm shop manual, there is a detailed testing procedure in Chapter 5, statring at page 5-56. If that does not fix the problem, then you need to run an engine diagnostic and see if the EPROM has storred any fault codes. If the alarm is coming from the ECT, you will get a "6" code (six short blinks). If the alarm is coming from an overheat sensor you will get a 24 or 25 code - two long blinks followed by 4 or 5 short blinks. That process will pinpoint what is setting off the alarm. Alternatively, take the motor to a dealer and have them hook up the HDS.
 
Nope - according to the shop manual, a solid MIL light means you have a faulty ECM or faulty wiring to the ECM. More and more this is sounds like you may have a grounding problem. Check your ground wires, starting at the battery. Especially check the ground connection to the ECM.
 
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