Fluxuating Engine Temperature Under Full Load

[obsessive]

Hi, all. Searched for this exact issue and could not find anything much on the web that seemed to mimic the issue. Unfortunately the threads I found didn't have a resolution reported.

My TBI Mercruiser 5.7 showed an odd symptom related to fluctuating temperature yesterday. It was (unfortunately) the FIRST time this year we could go out on the boat. Up until then, I would stop down once a week and start it and let it get to operating temperature along with trimming the drive up and down and taking it in and out of gear just to keep things moving.

Everything was just fine under idle. The boat ran an indicated 170'ish (I say that because it's the factory gauge) degrees as it has for the last 8 years. Under idle/under light cruise (1200RPM)/under plane (3200RPM) it stayed normal. After cruising around for an hour or so, I decided to give it a full throttle run. This is when I noticed the issue.

The boat has always gone a bit over 170 at full throttle, returning to normal when backing off. This time it was different. The boat slowly rose to 195'ish, stopped there for 10 seconds or so, then slowly went back (took about 10 seconds) to 170. It would then repeat the cycle. I ran it for about a mile like this, then backed off plane. It returned to normal. Took it back on plane (not full throttle) and it was again normal.

We decided to cruise around a bit longer, then anchor out for an hour or so. We wanted to get back to the marina to meet someone for dinner, so we did a full throttle run for 5 miles or so and it acted COMPLETLEY NORMAL. Went from 170-180 as it always has.

What gives??? Any ideas? Thermostat? Just needed to blow crap out of the system that had grown there? The raw water pump is new, but the thermostat and circ. pump are original from what I can tell (bolts still have all the factory paint on them). I'm a bit confused.

E

 

[GrandMaster]

If its raw water cooled, @ 15 years the thermostat & circulating pump have probably seen better days. Thermostat should be relatively easy to remove & look at.

Have to ask though, how old are the exhaust manifolds?

 

[obsessive]

If its raw water cooled, @ 15 years the thermostat & circulating pump have probably seen better days. Thermostat should be relatively easy to remove & look at.

Have to ask though, how old are the exhaust manifolds?

Same age.

I considered those, but didn't think the issue would just pop up, then go away. I was assuming that it would be more progressive. Not sure.

 

[Lazy Daze]

Could of just been crap in there, like you thought. But at this point, considering the t-stat is 16 years old, it's about time to change it. Whether it was/is the issue, it's long overdue.

 

[obsessive]

Could of just been crap in there, like you thought. But at this point, considering the t-stat is 16 years old, it's about time to change it. Whether it was/is the issue, it's long overdue.

Agreed - it's one of those things you don't think of until you have issues.

Would you suggest to do the circulation pump, too?

E

 

[Lazy Daze]

Yeah, probably. I'm less worried about that, though. But then again, it's certainly within the "timeframe" that no one would think twice or second guess you.

 

[obsessive]

Yeah, probably. I'm less worried about that, though. But then again, it's certainly within the "timeframe" that no one would think twice or second guess you.

Ordered both. OEM parts.

I'll install them and follow up in this thread.

Thanks.

E

 

[obsessive]

Well - finally got around to replacing the thermostat and circulation pump. No luck - the engine will still move slowly between 170 (gauge indicated, so obviously not exact) and 195'ish when under a full load. Yes, the thermostat was installed correctly and the sending unit resistance checked. No leaks. The temperature still stays steady under slow cruise/off plane.

There are a few things to note. If I run the engine in neutral at 2000RPM for a few minutes, the temperature will start moving from 170-175/180. This has never been characteristic of the boat and goes along with the issue above. Also, the Starboard side manifold and riser is noticeably hotter (I can only keep my hand on it for 5 seconds or so) than the Port side (I can keep my hand on it as long as I want).

All of this mysteriously started this year and they did (as mentioned earlier) replace the raw water pump. Could this be it???

I'm kind of lost at this point...

 

[Lazy Daze]

Do you know if the old impeller was pulled out intact - or where there any missing vanes? When you have the t-stat housing off, did you inspect the interior passages well for parts of impeller vanes? You could check the power steering cooler and fuel cooler for parts, too.

 

[obsessive]

Do you know if the old impeller was pulled out intact - or where there any missing vanes? When you have the t-stat housing off, did you inspect the interior passages well for parts of impeller vanes? You could check the power steering cooler and fuel cooler for parts, too.

Unknown if it was intact.

The T-stat housing was clean. The T-stat had a little corrosion on it, but not bad. It was indeed original.

Since the boat was sitting so long in the water, possibly something (we have zebra mussels here) plugged the lower intake (???) or like you said, maybe pieces of the old impellor got in the feed line.

E

 

[CliffA]

just to throw out another idea, could be one of the exhaust flappers has broken loose from it's mount or a large piece has broken off and partially blocking the water flow out of that side exhaust....this is another one of those things that should be replaced after the boat is 10 - 15 years old or older.....if one of the exhaust pipes were partially clogged with a piece of flapper it could cause engine temp fluctuations as you are describing.....sometimes the piece of flapper could shift and not block the water flow (normal engine temp) and then sometimes it could shift to where it blocked or partially blocked the water flow (high engine temp)....

i replaced my exhaust flappers a couple years ago....not too bad of a job if you have access to the exhaust pipes.....my flappers were still in tact and still in place but they had lost some of their 'spring' and did not completely seal off the exhaust pipe when the engine was not running.....

and i would not worry too much about the starboard side exhaust elbow running hotter than the port side....that is normal for our engines....i researched this in pretty good detail a couple years ago since my engine shows the same thing.....it has something to do with the way the Merc Engineers designed the engine from what i can tell....not sure why exactly they wanted one side of the exhaust to run hotter....one 'guess' that i have is that on my engine the Thunderbolt V ignition module was mounted on the port side exhaust elbow....maybe the Engineers wanted this elbow to run cooler to avoid heating up the ignition module...again this is just a 'guess'.....

cliff

 

[Chief Alen]

Sounds like the elbows, spacer, or flapper fell down.

Time to remove the elbows, check to see if they are clogged, if you have spacers check them, and while off make sure the flappers / shutters are still there and didn't fall down.

You have heat exchangers, those also need to be checked.

 

[Surftodd]

I had a fluxuating temp for the second half of last summer. Only at cruising speed though......It turned out to be some mono-valve slipper snail growth on the hose intake right after the outdrive hose connection! I couldn't believe it! If anyone has this issue and has an outdrive, pull the drives before replacing manifolds and rises!

 

[ttmott]

Check wiring to sensor. And,might want to consider your charging system and the grounds on the engine. Variations can indicate differently on gauges. A DVM on the engine ground and alt B+ when running full throttle will show or eliminate the electrical.