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.

Mopower,

I just pulled the plugs last week, and none of them were as you described. They all looked good - slight chocolate brown color on the porcellain, maybe just a tad rich. First step will be to take a close look at all of the bypass hoses and passages.

Thanks for the help!

Bud,
Glad to here your plugs look good...but so did mine in the beginning:( Like I said , this saga went on for a couple of month and toward the end is when the plugs were white.At first , when the cyl. fired it blew a very little into the waterjacket and the water pressure wasn't enough to blow the other way so no excess steam or hydrolocking. I really hope you find something else stupid , but next July if your still fighting this problem , remember this. It drove me nutz for I don't know how long:rolleyes:

Tom,
If the leak is sealing itself up once it is warm your plugs will look fine after you have run it. If the head gasket has a slight leak it could seal up once warm. Best to get a compression gauge on there so you have the winter to pull the heads if needed. Better yet lets rig up a leakdown tester and check all cylinders. Mopower may be on to something, check it out while it is a slight hangover instead of a major headache.

cp,
I couldn't justify pulling my own heads , particularly after drilling a couple of 1/8 holes in the 'stat to let the steam go by and allow the 'stat to sit in the water and function properly. I was too stubborn and lazy to do what had to be done:(. Besides that , I still wasn't convinced it was the problem when it was steaming out the pipes just that 'little extra'. You know , blame it on the weather...slightly cooler ...more humid.. or "well the engines not up to complete operating temp". I mean face it pulling the heads is a pain in the a$$! Especially if you don't know for sure:confused: . Not till it hydrolocked did I finally admit to it and pull the heads and found an ever so small rust mark at #5 between the water jacket and the cylinder:(.

Well, I wanted to run it this past weekend...
 

when it was warm, but none of my fellow boaters were going out! I guess that's a lame excuse. Now I'll have to break out the winter coat to run it!

Tim - we need to hook up anyway and try that outdrive swap sometime.