rapidly fluctuating temp gauge - SBC
 

I'm hoping someone can point me in the right direction. Last Saturday had the boat out and everything was going great. While cruising home I looked down and the starboard motor was running VERY hot (220+) so I instantly shut it down. Luckily I had twins so getting back to the dock wasn't an issue. Occasionally I would start it up to try and circulate some freshwater through the engine but could never get the temp to drop.

The boat is new to me as of a month or so ago so I initially thought it was probably just the impeller in the drive (Alpha SS) so last night I replaced it. While tearing it down I found some fresh sea weed in the pickup ports which leads me to believe that was the initial culprit of the overheating. Went ahead and replaced the impeller anyways while I had it apart. Put everything back together fully expecting to have fixed the issue and I would be good to go. Started the boat up and now the temp gauge is fluctuating very rapidly. I took a video and posted it here to show what I am talking about. Sorry about the quality. It was getting dark and I only used my cell phone. when the gauge is straight up its roughly on the 200 mark.

https://www.youtube.com/watch?v=LuIuGCjzebI

I swapped temp sender with the port motor and got the same result. Also swapped gauges without a change as well. I then back flushed the oil cooler thinking it might be clogged restricting the water supply but everything was nice a clear.

Before this the motor was rock steady at 150 degrees. Has anyone had a similar issue? Any ideas before I just start throwing parts at this thing? I plan on doing a compression test later this afternoon after work and hoping for the best.

Is that in the water or running on a hose????

I've never seen a temp guage move faster than a tachometer. LOL.

I tried both. First go around was on the hose but then I filled a large tote with water past the pickups and just let the hose overflow the tote. no difference between the two.

Me either haha! I'm hoping its something relatively simple but I'm starting to think that might not be the case...

I have.

It was a puked head gasket on my 05 Duramax. Combustion gas in the coolant will make the needle whack. lol.

EDIT: It only did it at peak torque though....just enough cylinder pressure to push that much exhaust past the gaskets I guess.

Probably what it is with it seeing those temps.

I have seen many headgaskets push the temps up somewhat fast, just not that fast.

My EGTs were pretty high at the time...rolling into it pulling my boat with a tuner installed. lol. The first few times it happened, the needle moved so fast I thought for sure it was a wiring issue...."no way could the temp be raising that fast". Found out that wasn't exactly true. ha.

This was the answer that I was hoping not to hear but kind of presumed it is the culprit. I will be running a compression check later this evening and will report back with what I find. Just hoping that I caught it in time and didn't crack a head...

Debris can get hung up in the T stat and give pretty erratic readings too. Saw that with some broken impeller pieces years back.

Well the results are in..


http://i16.photobucket.com/albums/b2...pszzaaalrl.png

Looks like I will be pulling the heads. Now I just have to decide if it is worth it to pull the motor or just do the work in the boat. Neither really sounds all that fun.....

Sorry for your trouble. As I understand it, Merc specs compression to be a minimum of 100 PSI with a max difference of 70% between the lowest and highest readings. It looks like you're in their spec for what that's worth. It may still be worth digging into it, I just wanted to throw that out there.

How did the impeller look when you replaced it? Did you replace both impellers and how did they compare? If it's a new to you boat, it's probably worthwhile to change both impellers so you have a baseline. Basic, simple stuff but just wanted to ask.

Post back what you find out as the history of these threads provide great information for those of us looking into issues down the road.

I would do a leakdown before ripping anything out of the boat. A compression test will get you in the ballpark, but with a leakdown you can pinpoint the issue(s) a lot closer before digging into anything.

The sound of leaking air can tell you a lot of the story.

What he said ^^^. Still keep checking the easy stuff like thermostat, back flushing, etc. Pulling heads is not fun.

Well last night I went ahead and pulled the heads off the engine. Really wasn't to bad of a job. Just glad I have small blocks (never thought I would say that! haha) so there is a bit more room to work back there. I hear horror stories from people who own/work on the big block 272's. Start to finish was about 2.5 hours and that was not being in a huge hurry. I found exactly what I was expecting/hoping and that was obviously burned & leaking head gaskets between cylinder 3 & 5 as well as a burn mark on cylinder 6. I will try to get some pictures up of the gaskets. I'm dropping the heads off this morning to get checked out for cracks & warpage. Should have the heads back by Monday at the latest which means I should be back on the water next weekend if all goes well.

We had that same temp gauge issue with a 402 in one of our drag cars. It wasn't a head gasket, but actually a crack in the #2 cylinder water jacket. I have seen a head gasket react the same way.

It's been a few weeks since I updated this thread so I want to make sure and post the results in case someone else has as similar issue. The issue was a blown head gasket between cylinders 3 & 5.and possibly #6 as well ( Need to do another compression check to see if it was leaking as well but the gasket sure looked like it was). I had the heads checked for cracks and re-surfaced as a precaution and then re-assembled everything the following week and was back on the water that Saturday. Motor is running great again and the temperature is rock steady at 155 degrees without the crazy fluctuations I was seeing. Thanks everyone for the help!