Temp switch went off - overheating engine?

[Joshto]

Hey folks,

17 footer 1992 Cadorette bowrider
3.0 Merc 1998
Alpha one gen one.

Need your input here. I just replaced the blown head gasket of my 3.0 (reconditioned/resurfaced the head and had the block's flatness verified) and I also replace the ripped exhaust bellow and installed a brand new exhaust flapper as I noticed it was missing completely.

Once I put together the engine, I ran it on the mufflers fast idle for about 20 min to check for water or oil leaks. Everything perfect. The engine runs smoother and quieter than before. Happy with that.

Went out for a sea trial today. First I put the boat on the water and let it warm up in neutral...increased revs slowly to about 3000 for a few minutes. Everything perfect: temp and oil pressure. No leaks anywhere.

Went cruising for 10min at 3500-4000 rpms. All good. Stopped the engine to enjoy the lake views in a hot day. After 45 min with the engine off, I noticed the temp gauge wouldn't drop slowly as usual but stayed at 140f (60c) as if it the engine was actually running. When I turned the ignition switch to start the engine, the temp alarm went off right away. I turned off the engine and did some troubleshooting.
If I disconnect the wire (brown/Green) from the temp switch the alarm buzzer stops immediately. So that confirms the wire isn't grounded (short circuit) anywhere else and the switch is actually enabled (internally closed)

I then grounded the temp sender wire (brown) to make sure the temp gauge was accurate; it seems ok: needle drops to zero when I disconnect the terminal and goes all the way up when I ground it.

I wonder if the exhaust flapper has anything to do with it. I've read that the flapper makes the engine run hotter.

I didn't have more time to troubleshoot so headed back to the ramp slowly just in case the engine was truly overheating. When I came back with the trailer to pull out the boat (engine had been shut off for 10 min or so), the temp gauge had climbed to 150f (70c), as if heat raises inside when the engine is off.

I will check again in a few days when the engine cools down fully to see if the temp switch keeps triggering the alarm. If it does, then it's just the switch that is stuck inside.

What do you think or suggest to check?

Thanks for your help.

 

[alldodge]

Nothing is consistent, I would take a IR temp gun to the thermostat housing to see what temp is

Has the drive impeller been changed?

 

[Joshto]

Nothing is consistent, I would take a IR temp gun to the thermostat housing to see what temp is

Has the drive impeller been changed?

No. But unless it broke out of the blue, engine was fine at the ramp and performed well all the time. And if it broke, so did the temp gauge?

I'll bring a IR temp next time I get my hands on the boat. I want to think it's just the temp switch that went bad. Hopefully!

 

[achris]

... I ran it on the mufflers fast idle for about 20 min to check

How fast? Any more than about 1200-1500 and you may have cooked the impeller.

I wonder if the exhaust flapper has anything to do with it. I've read that the flapper makes the engine run hotter.

Poppycock!

I didn't have more time to troubleshoot so headed back to the ramp slowly just in case the engine was truly overheating. When I came back with the trailer to pull out the boat (engine had been shut off for 10 min or so), the temp gauge had climbed to 150f (70c), as if heat raises inside when the engine is off.

The gauges are 'unsprung' and will not return to zero on key OFF. Whatever the gauges read without power is completely invalid... Ignore them when the engine is off.

Chris........

 

[Joshto]

How fast? Any more than about 1200-1500 and you may have cooked the impeller.

Poppycock!

The gauges are 'unsprung' and will not return to zero on key OFF. Whatever the gauges read without power is completely invalid... Ignore them when the engine is off.

Chris........

Thanks, Chris. By fast idle I mean 1000 RPMS. At the ramp with the boat on the water and trim all down, I did bring it close to 3000.

Temp sender and gauge seems to work fine based on my initial testing. IR temp gun will tell the truth. It's so strange that the alarm went off when I turned the switch after almost an hour with the engine off...I'd say the temp switch is gone.

 

[Lou C]

Verify your engine temp readings with the IR temp gun, before changing parts. The temp sensor can be tested with an Ohmmeter against manufacturer's specs.

 

[Joshto]

Spent some time troubleshooting the boat's alarm issue. Now I'm pretty confident it's the temp switch.

To my surprise ( as I hoped it would), the alarm didn't go off when I turned the ignition. So I took the boat to the lake armed with a brand new IR thermometer. On the water, in neutral, I let the boat warm up slowly while monitoring the temp closely. Gauge and IR thermometer showed increments of exact same values, capping at ~145f. After 5 minutes sitting at that temp, no alarm. So I went out crusing to push the engine a bit further. 2 miles into the lake and boom, the alarm showed up (increased loudness progressively - as if the internal switch resistance decreased slowly). Stopped the boat and checked the engine's temp with the IR. I couldn't get any value above 165f.
Stat cover: 135
Stat housing (below and in between the sensors):145
Manifold: ~160
Valve cover: 150
Exhaust elbow: 135
Block (port-under manifold): 147

Interesting part: with the alarm active, I started the engine and pulled forward, shortly after the alarm's loudness reduce significantly (I could hardly hear it but it was there); it came back again to full volume a few seconds later. Stopped the boat and revved up in neutral, the alarm didn't go away. Repeated the same steps a few times; everytime it came back, I measured the temp on same spots over and over: temps barely changed. Pointed the IR to another spot, like the cool water intake hose, and it showed a dropped in temp immediately (so I'm also confident the IR thermometer was actually working)

Then I went around for another 5 min, the alarm disabled and did not show up again. I rattled the harness around the engine but nothing I couldn't get to reproduce it again.

I think the switch is a goner. Ordered a new one.

 

[achris]

I think the switch is a goner...

The switch is just that, a switch. If it's varying volume, then it's no longer a switch, it's a variable resistor, and that's something it shouldn't be....

 

[Joshto]

To wrap this up. I replaced the Sierra temp switch with Merc OEM and the issue is gone.

Tip: the Merc original Temp Switch retails for about 80 or more. Instead, get the entire Audio Warning Kit from eBay for 50 bucks or so. It includes not only OEM temperature and oil switches but the warning housing kit and all the wiring. I actually purchased a kit for USD 30. Great deal.

 

[achris]

To wrap this up. I replaced the Sierra temp switch with Merc OEM and the issue is gone.

Wow, a Sierra part that was faulty out of the box. Who'd have thought...

 

[Joshto]

Wow, a Sierra part that was faulty out of the box. Who'd have thought...

Oh no. The Sierra that failed and I replaced with the Merc, had been in the engine for 2 or 3 years.