Annoying overheat problem at start-up, 1995 290 with twin 4.3L

[GrandMaster]

First, the specifics:
Twin 4.3L Mercruiser I/O's
Alpha One Gen II's
Raw water cooled...100% Salt water
Carbureted engines...no fuel injection or ECM
Dry-stacked for the last several years

So here's my issue, starting about 8 months ago whenever I have the boat put in the water, first start my engines & let them idle, the stbd comes up to temperature (about 160-170) normally. However, when I start the port engine the temp will just keep climbing to the point where the alarm goes off...and beyond I assume although I've never let it get any hotter. The water flow seems about the same coming out of both engines.

Now, if I engage the throttle-only button when the port engine starts to go above 180 and give it about 1 second of 2,500-3,300 RPM, the temp will stop rising and drop/stay in the normal range all day long no matter what I'm doing or how I'm running. Get back to the dock, shut down, and have the guys pull her out to flush and the same thing will happen all over again even while on the muffs...and I can easily stand there to see the water coming out of both engines.

In the past few months in an attemp to correct this, I've replaced the thermostats and had the drives serviced with new water pump impellers, but neither have made a difference.

My only other thought would be something with the engine-mounted circulating pump but I cant see how that would play into this specific scenario since it should start turning as soon as the engine is started.

Because I know what to do to avoid the overheat when I start the engine, its not that big of a deal and doesnt really affect my boat usage, but things just annoy me when I know they are not working as they should so any suggestions would be appreciated.

 

[floriduramax1]

I had this happen to a 7.4L in a Rinker once. I fought it for a while and eventually found an in-line cooler was completely plugged! I removed it, soaked it in CLR, and then poured Hydrochloric acid through it and it was perfect for the next few years. It had what looked like crushed shale and salt build up in it. The previous owner never flushed after salt water runs!

One thing to remember...T-stats can only open if they are sitting in water. So either you're not getting water to it at idle or water from it is blocked. Have you tried an infra red temp gun to measure the t-stat housing?

 

[Lazy Daze]

On your t-stat housing, do you have what looks like an extra-set of hoses attached to a "T" fitting that comes off the top, middle portion of the housing? If so, these two hoses should be "higher" or "above" the others.

 

[GrandMaster]

On your t-stat housing, do you have what looks like an extra-set of hoses attached to a "T" fitting that comes off the top, middle portion of the housing? If so, these two hoses should be "higher" or "above" the others.

I have some engine pictures on my laptop at work so I will look through those tomorrow to see if I have those hoses and how they are configured.

If I have them, what are they for and how might they be causing a problem?

 

[GrandMaster]

I had this happen to a 7.4L in a Rinker once. I fought it for a while and eventually found an in-line cooler was completely plugged! I removed it, soaked it in CLR, and then poured Hydrochloric acid through it and it was perfect for the next few years. It had what looked like crushed shale and salt build up in it. The previous owner never flushed after salt water runs!

One thing to remember...T-stats can only open if they are sitting in water. So either you're not getting water to it at idle or water from it is blocked. Have you tried an infra red temp gun to measure the t-stat housing?

I have not tried the temp gun because I haven't actually had an overheat recently since I just bump the throttles when I start every time now to avoid the problem. I thought about some kind of blockage too I but had the cooling system acid-flushed when I had the thermostat replaced. Also, I'm not sure why a momentary RPM increase is solving the problem even if I run the engine at idle all day long it wont overheat The problem only comes back if the engine is shut down & restarted...even if it restarts after being off for a minute.

 

[Nehalennia]

How old are your manifolds?

 

[GrandMaster]

How old are your manifolds?

About 60 hours