Temperature "spiking" at start-up...
 

Yesterday morning I fired my boat up for the first time that day, and after idling it at around 1000 RPM's for a few minutes (to charge the batteries), the alarm buzzer went off. I immediately shut it down, and noted that the temp guage was up around 195*. It was definitely hot because it actually wanted to run-on a little bit when I shut it down. We figured it had sucked up an ice bag around the pickup, but nothing was seen when we raised the drive. After waiting a few minutes, I fired it up again, and the temp immediately dropped from around 190* down to the normal 140*. It ran fine the rest of the day with no temp issues.

I have noticed the temp fluctuating on a couple of other occasions upon initial startup. Both of these times, it never went above 170* before it dropped back down to normal. I have never noticed a temp issue with this engine at any other time - cruising, idling, WOT - it always stays between 140* - 150*. Most of the time, it does not do this.

Engine is a gen IV 454 with stock Merc exhaust, circ pump, 140* t-stat, etc. It does have the old-style T-stat housing, with the spring-loaded bypass valves. T-stat was replaced with a new Merc unit last year. Manifolds are stock late-model Merc - fresh water, low hours. Entire motor was rebuilt last year. Drive is a later-model Bravo1 with standard pickups. Seawater pump was rebuilt last year. I have probably put less than 100 hours on the rebuild.

I am suspicious that the T-stat bypass check-balls may be sticking from scale buildup. Could this cause the problem?

Any ideas would be appreciated!

Sounds to me like you may be getting some steam pockets in the engine causing the sensor to read high......either that or like you said there may be some scale buildup. You can try drilling two 1/8" holes in the t-stat to help get rid of air pockets. :)

BEEN THERE, DONE THAT!

Take the bypass hose between the intake and water pump (or however it is plumbed) off and check for a restriction. If the fitting has got clogged in the intake or the size reduced from sediment, it won't let water flow through the block before the thermostat opens (thats the bypass's job), trapping the water and letting it pre-heat (or overheat) before the t-stat opens... thus the spike in temp, and returning to normal after water starts circulating. If you do not have a bypass hose, there has to be a way for water to circulate through the block during warmup, and whatever you have... THAT will be your problem. Not only have I seen this in boats, but big block cars as well, and your description fits EXACTLY! It's a simple fix, and very common with BBC's... I would bet my ass on it!

If your seawater pump is going out --sometimes at idle it wont pump untill you rev it up and then the pump primes itself. REPLACE THE IMPELLER.

FindMe,

I really can't recall if my engine uses the bypass hose. I'll take a look at it. Hopefully, that will be the problem rather than the seawater pump. It is an absolute whore to get to.

Budman,
You better hope it is your seawater pump:eek: . I had a very similar problem in a Chevy Blazer. It would overheat on initial startup...puke anti freeze all over and then run fine all day:confused: .
Turns out Wally was right. I had a steam pocket form while the t-stat was close during the warm up period and it all formed around the t-stat so it wasn't in the water and didn't open till it was really hot. Once the engine was warm and the t-stat opened it was ok.
Drilling a couple of 1/8 holes in t-stat solved the effect but not the cause:mad: A couple of weeks later when things got worse I found out the real problem was a bad head gasket...that's were the "STEAM" was coming from:( .
The give away was when the radiator had oil floating on the surface of the antifreeze , but you won't be able to see this. I never had any sign of water in the oil , or any excessive steam out the tailpipe.( Did get some steam after a few more weeks)
So like I said..Hope it's your pump:rolleyes:
Good luck:cool:

One additional thought would be a loose clamp where the sea water intake hose hooks to the transom. Make sure this is double clamped. Also the gasket under the gooseneck where the intake hose hooks to the transom can fail or come loose. Either one of these can cause the sea pump not to prime. I would plan on replacing the impeller at first chance regardless of what you find.

BT :cool:

Mopower,

Shouldn't a blown head gasket show up with a compression check? I guess that will be part of my winterization plan.

Funny thing is, it doesn't do this every time. I have been out a few times where it ran perfectly, from start to finish. This might be a tough one to hunt down. I'm hoping to take it out this weekend (suppoesed to be near 80). We'll see what it does then.

Thanks for all the replies. This gives me some things to check.

Pull the t-stat assembly and clean it to solve the scale theory, I would change the t-stat to,, might be sticking.

Bud,
If the leak was really bad , yes compression would be down AND you would be getting water in a cylinder. This one was such a small weak , it went for months with holes drilled in the t-stat. As it got worse , it started to steam a little more than usual. Not till it didn't want to start (hydo locked) did I finally wake up:eek:
. Thats when I saw and oil sheen on the antifreeze and it smelled like gasoline:( . After pulling the plugs one was snow white clean:( and when pulling the heads that same cylinder was also clean. this is because it was being "steam cleaned" as it ran. All other had normal color and carbon deposits.