Fluctuating engine temperature in my 2000 7.4 mpi Mercruiser Bravo 3
 

I I have a 2000 7.4 mpi Mercruiser Bravo 3. Out of about 7 thermostats that I have replaced in the past 15 years, only about 2 have maintained constant engine temperature -- approximately 170 degrrees. When I go above idle, the temperature may fluctuate suddenly between 100 and 170 degrees, and then return to a constant 170 degrees for a while. Then it becomes erratic again. I have replaced both the sender and the gauge, but the problem still persists. Any suggestion please ????


I also get a warning alarm which sometimes goes off after about 15 minutes when I speed up to 30 mph . However, all gauges indicate engine temperature, oil pressure and outdrive oil levels are normal. However, no alarm when I drive slow below 15 mph.

Do I have a faulty outdrive level alarm or some other problem ??

have you replaced the sea pump body&impellor lately?

There is a pressure transducer in the power steering cooler for water pressure. The other thing is corrosion over the years can crush the hose at the ends were the plastic insert is. This can cause a restriction or worse cavitation. Cavitation bubbles will erode the transom assembly just under the plastic housing on inside of the boat were the seawater hose connects. Just did one with a history of erratic temps and normal maintenance to the seawater pump. Then an erratic water pressure alarm. (Cavitation bubbles probably did in the sensor) The sensor fixed the alarm but mid season developed a water leak. Upon inspection we found the transom assembly destroyed by cavitation bubbles.

Mefi 3 does not have water pressure monitoring. nor any sensors to do so. That is only for PCM 555 and ECM 555 which started in 2001 on the 496 PCM 555 and small blocks and V 6 using ECM 555. Merc racing as well.


IMO you have a bad water circ pump

And guess who sells them all USA made

if yours is 10 year old or older you need one anyways. these are a maintenance item regardless. Often over looked.

160 - 170 dollars to your door. Cheap insurance and possible your problem. Clear hose testing replacing the areas of your regular cooling hoses can also confirm this and or other problem area(s). Very simple testing to pin point your problem. You are looking for air bubbles in the water flow - when found that is your problem area. Many times if your TEMPS fluctuate it can be traced to the water circ pump as the problem. Also will agree about the transom assembly water hose inlet restricted or collapsed but most of the time that causes hotter running temps and or overheat and a lot times within minutes depending on how bad the restriction is.

Next back side oil cooler honey comb plugged causing a waterflow restriction. Most likely hotter running temps and or over heat.

Exterior transom water hose that is cracked or dry rotted or allows air leaks can cause fluctuating temps also.

Again clear hose testing will find your problem area. Proven testing I have done at least a 1000 times.

I am not disagreeing with anything that was mentioned prior, All extremely good information. But when was the last time that you back flushed your cooling system?