Overheating Mercruiser 5.7

[StuartAdair]

Hi,
Sorry for resurrecting an old post but its very relevant to an issue I have on a 5.7 Mercrusier, Alpha 1 outdrive. Ran it all last year and the temperature gauge rarely moved over 100°, so much so that I always thought that it was broken. Had the boat out this winter and put in a new water pump and water pipe leading to the transom as a service item. With the boat back in the water now its running at 140° at 1400 rpm but as soon as I drop to idle the temperature goes way up. I initially thought I'd made a mess of the water pump change but there seems to be good flow of water. Please check the video which is the engine at 700 rpm idle speed. https://www.youtube.com/watch?v=MkDbXCcvL98

I then checked the thermostat which looks like the attached pic and has seen better days.

I've ordered a new stat and circulation pump as I suspect both are original 30 year old items.

What do you think about the water flow video? Does it look about right for the engine at idle ?

Many thanks

Stu

 

[jbcurt00]

Hi,
Sorry for resurrecting an old post but its very relevant to an issue I have on a 5.7 Mercrusier, Alpha 1 outdrive...

Please dont post to old inactive topics.

As your motor is a 5.7 and you posted to a 7.4 motor topic, it isnt really relevant to that topic which is considered a hijack, which is so discouraged.

Started a topic @ your motor and transferred ypur post and a reply here.

Discuss your motor here.

Is this the same boat/motor you posted about back in Oct?
https://forums.iboats.com/forum/eng...engines-outdrives/10807710-its-all-gone-wrong

 

[Rick Stephens]

Not where you check flow. Should pull the 1 inch line off the thermostat housing that comes up the port side from the transom and through the oil cooler. That's the input line from the impeller in the drive.

 

[StuartAdair]

Thank you all for your advice and apologies for my error with the forum rules. Rick, the reason I pulled that particular pipe off was to prove that there was a flow of water from the impeller which seemed to be the case. I'll put the new stat in and replace the circulation pump at the weekend and report back.
Many thanks
Stu

ps: Yes, its the same engine I posted about previously.

 

[Rick Stephens]

Thank you all for your advice and apologies for my error with the forum rules. Rick, the reason I pulled that particular pipe off was to prove that there was a flow of water from the impeller which seemed to be the case. I'll put the new stat in and replace the circulation pump at the weekend and report back.
Many thanks
Stu

ps: Yes, its the same engine I posted about previously.

I knew why you pulled it. That isn't a good place to check impeller flow. The in line to the thermostat housing is.

 

[StuartAdair]

Which one should I pull Rick Stephens

 

[Rick Stephens]

Trace F. It goes down by the port motor mount and connects to the oil cooler. From there it goes to the top of the transom , right below the steering arm, which is directly from the impeller.

 

[StuartAdair]

Will do Rick, Many thanks.

 

[StuartAdair]

Hi folks. A further update. I've replaced the thermostat and just for good measure I also changed the circulation water pump. Both were 30 years old I suspect so needed changing either way. I ran the boat and within a few minutes the temperature gauge started to rise, ending up in the red fairly quickly. The strange thing was that the engine didn't actually feel that hot. No signs of steam blowing out. the water pipes were still soft and I could comfortably put my hand on the risers.
I pulled off a water pipes A and B (See pic above) and water poured out everywhere at a rapid rate.

I then remembered that I had a temperature probe onboard so started to probe the engine in various locations. I had it running perfectly for an hour in the marina between idle and 1500 rpm and the probe never gave me more than 150°. Most of the time it was in the perfect 140° - 145° range. I probed the actual temperature gauge sender unit at 145° whilst at the same time the temperature gauge was reading 240°. So I think it reasonable to assume its either the sender unit, gauge or connecting wire. Indeed, the wire is brown at the sender unit end and pink at the gauge so there must be a connector somewhere which is another possible failure point. I'm just going to replace the whole lot.

Interestingly when I removed the old gauge to measure it the needle sprung straight across to the right at 240°. I would have expected a zero reading with no power connected ?
Anyway, lets see what happens tomorrow when Mr Amazon brings the new gauge (Sender is coming mid-week)


Stu