Confirming temp gauge

[psubill78]

I've got a '98 5.7L Mercruiser I/O w/ Bravo IIIs on my '98 Sundancer 310....

The starboard temp gauge appears a bit wonky, I see it bumping between 170 and 220 rapidly.

Is there any way to confirm the temperature on the engine? I do have an IR thermometer.

How can I tell if it's just a bad gauge?

Thank you

 

[rcknecht]

Lift up your hatch and shoot the IR temp gauge at your Heat Exchanger...

 

[psubill78]

Thanks for the quick reply. I'm trying to find the engine manuals online to figure out where all these things are. Seems that MerCruiser only sells the books! PFFT!!

Thanks again.

 

[psubill78]

So will the temp on the heat exchanger closely match the gauges?

Bringing her home this weekend, hopefully, and I want to as prepared as I can.

The marina is replacing impellers - so I'm hoping it will be a nonissue come Sat....

 

[mwph]

Is it only acting up once you reach 170? If it is not bouncing around while the engine is warming up, might that suggest that the problem begins when the thermostat opens? Or when the sender unit gets into that range? Just think aloud.

Anyhow, If the impeller replacement doesn't cure it, you can swap gauges from port to starboard as another way to verify gauge accuracy. Swapping temp sending units could eliminate that from the mix as well.

 

[psubill78]

Is it only acting up once you reach 170? If it is not bouncing around while the engine is warming up, might that suggest that the problem begins when the thermostat opens? Or when the sender unit gets into that range? Just think aloud.

Anyhow, If the impeller replacement doesn't cure it, you can swap gauges from port to starboard as another way to verify gauge accuracy. Swapping temp sending units could eliminate that from the mix as well.

Since the only time I noticed it was on our sea trial, and I haven't been on the boat since, I can't say for sure. However, I do not recall seeing the gauge moving oddly when the boat was warming up.

I'm new to all this, VERY new. I appreciate the advice, and will read up on how do the tests above you suggest. Swapping the gauges on the dash seems simple enough to rule out the actual gauge. I would assume that I can get some type of meter to test the signals coming to the various gauges as well, right?

The surveyor and broker didn't seem concerned. They both said marine gauges are notoriously wonky, and that if the engines were really heating up - that an alarm would sound? Their recommendation on the replacement of the impellers was due to not knowing when the previous owner did, and suggested it was a good maintenance idea.

 

[mwph]

The surveyor and broker didn't seem concerned. They both said marine gauges are notoriously wonky

Hang on just a cotton pickin minute!
I don't care if Moses said it. Hopefullly your offer was based on an acceptable sea trial. In my opinion this is unacceptable. This could be a minor problem that could be fixed for a hundred dollars, or it could be a warning of a problem. Either way, that kind of response is unacceptable.

 

[psubill78]

Thanks for the info.... Now you've got me worried though.

I suppose we will find out more this weekend when we try to bring her home. I've got my IR thermo ready to test... and now - praying that the impellers were just not great during our sea trial..

 

[mwph]

Not trying to worry you. Just pisses me off to hear comments like that from a surveyor & a broker who are trying to sell a boat. How long is the trip home? I don't know where you plan to shoot the gun, but I'll suggest that you shoot it at the exact same places on both engines/components & document any differences. Probably wouldn't hurt to have a thermostat & some basic tools on board for the trip. Will you have help with you?

 

[psubill78]

Yup - a good friend who's far more seasoned will be with. And yes, already have the took kit ready. Trip is about 65nm

We won't leave until we are confident she'll make it. We have a river we'll be able to tool around before we hit up Lake Michigan.

 

[Bucit]

I would warm up the engines at the marina and do the testing there before I would head out. Once you have a starting point and feel confrotable that it's something small (gauge) only then would I head out on a 65nm trip.

 

[aqua*holic]

I've got a '98 5.7L Mercruiser I/O w/ Bravo IIIs on my '98 Sundancer 310....

The starboard temp gauge appears a bit wonky, I see it bumping between 170 and 220 rapidly.

Is there any way to confirm the temperature on the engine? I do have an IR thermometer.

How can I tell if it's just a bad gauge?

Thank you

If by rapidly you mean it "jumps" almost instantly between 170 and 220 than this is most certainly a gauge or sending unit problem. It is highly unlikely the actual temperature of the block and/or manifolds can change that quickly. If it is a more gradual increase and decrease of temperature readings than this would indicate a cooling problem. The temperature you see with the IR gun on the heat exchanger will not be the same as the gauge, but it should be constant. I would shoot the manifolds for a better indication than the heat exchanger.

 

[psubill78]

Bucit - yes, this is the plan. I'd expect an issue with cooling would be apparent at idle, or in testing by the marina.

aqua*holic - this was our thought at well. By jumping, I'm talking a few second (3 or so) rise and decrease between them, with no change to throttle. I was thinking that as well, jumping that high of a swap. I plan to shoot several area's on the engines, and compare them. I would assume they would be close.

After reading lots of threads on here, it seems the 'normal' temps are all over. Merc Manuals states 160, however, I see higher listed around here as the 'norms'...

 

[rondds]

The sender for your gauge is actually in the thermostat housing - that's generally what I shoot with the IR temp gun. You have to give the engine enough time to warm up (thermostat wont open til 140 or so degrees). I'd also shoot readings under load at different rpms - this means run the boat with open hatches and a buddy to take readings while you drive (or vice versa). Many problems associated with running hot are caused by corroded/constricted exhaust elbows. You should always be able to touch them without getting scalded. They should run at around 90 degees even at hi rpm.

 

[aqua*holic]

Normal just depends on what thermostat you have. If you have a 160, then the motor should run there or slightly above.

3 seconds to rise from 170 to 220 still seems fairly fast for an actual rise, but it is not out of the question. If you raise the engine RPM without the boat in gear, say to 1200 rpm and run it there for a minute or two, does the temperature gauge act different or does it do the same thing?

 

[psubill78]

The sender for your gauge is actually in the thermostat housing - that's generally what I shoot with the IR temp gun. You have to give the engine enough time to warm up (thermostat wont open til 140 or so degrees). I'd also shoot readings under load at different rpms - this means run the boat with open hatches and a buddy to take readings while you drive (or vice versa). Many problems associated with running hot are caused by corroded/constricted exhaust elbows. You should always be able to touch them without getting scalded. They should run at around 90 degees even at hi rpm.

I read about the exhaust bellows on *******. Where exactly are you referring to, and how can I 'check' for corrosion? Isn't the exhaust below on the exterior of the boat? I know on the starboard engine (where we saw the supposed overheating) they replaced the bellow with a cuff....

 

[rondds]

Not bellows - elbows.
Elbow is #12 below...

The elbow is what that large exhaust tube is exiting from. There's one on each side...

 

[psubill78]

LOL. Elbow/bellow. My brain is just swapping out words now. Sorry!

I'll do a little digging on how to inspect that piece... if necessary.

I'm trying to be as prepared as possible when I got down there to ensure we can get her home safely.

 

[rondds]

No prob! My advice is to take a boatride with a buddy, armed with your IR gun. Get up to speed and shoot readings and write them down. Do this while at cruise speed, not after you slow down. The engine temps spike briefly when you slow down. Also, while cruising, put your hand on the elbows and get a sense of the temp.

 

[aqua*holic]

I had another thought on this. A temperature swing from 170 to 220 is a lot of heat. You should be able to feel that with your hand on the thermostat housing where your sending unit is.