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accurate cooling temp - thermo housing?

B

bobct

Advanced Contributor
When measuring with an IR gun, is the thermostat housing the most accurate place on the engine?

I ran my starboard engine again yesterday and it's displaying 178 at idle, port engine was 180. Both new gauges and temp senders which I tested off the boat. They were both within a couple of degrees.

When I bought the boat, one gauge was off 20 degrees so I just changed them both along with matching sending units from Teleflex. In the survey, the #'s for each engine were 160/161 at idle and 181/176 at full. Not sure where the surveyor took those measurements though.

At the housing (on the "178" engine) I was getting 156 degrees at idle. So, no significant change after basically changing or cleaning the whole cooling system (u-coolers aside). Am I chasing a problem that doesn't really exist? After all the work I've done, I wanted to have gauges that didn't require a conversion factor to get the real temp.

I'm still connected to a "hose" and the strainer is staying full. I noticed that the belt tension spec in the Crusader manual is an 1/8" inch deflection which seems awefully tight to me. That's hardly any deflection at all. My belts aren't that tight, probabaly more like a 1/4" - maybe a hair more.

What do you guys think?


Bob
 
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you said you did everything except the u-coolers ,the first time my engines started getting hot,traced it right to the u-coolers,I put muratic acid thru them and it cleared it right up
 
That belt tension does seem very tight to me. Tight enough not to slip when you gas it is my standard. (Any more than that wears out bearings, fast!)

On where to zap for accurate temp readings: I use the body of the temp sensor itself. Hell, that's where IT'S reading, so....And there's no paint film there to screw up (usually).

Jeff
 
So, I did some double checking today, ran both engines. One showed around 175 and the other 180. If I hit the sending unit with the I/R gun, I show around 150 on both (within a few degrees). On the thermostat housing, more like 158 degrees. Those seem too low.

The sending units don't have any paint on them (brand new) and I aimed at the vent fitting on the housing which is pretty clean. I'm probably overthinking this but with brand new/matched and tested gauges and sending units, I expected to be spot on... and maybe I am with 175/180?? Maybe the surveyor did the same thing I did and got erroneous low readings initially.

The elbow/risers on both engines were just a little over 100 (105ish) which seemed good to me.
 
IF this is a raw water cooled motor, then you're spot on. (Mine run exactly that temp.) Strange how the gages are off so much.

Jeff
 
Jeff - no, I have FWC 454's. I'm going to give Teleflex a call today and see if they have any ideas.

thanks
 
Bob:

teleflex's troubleshooting info states a reading of +/- 15 deg, at 180 deg (all F) is normal.

Read that as the tolerance they allow for manufacturing parts.

Your data infers the sensors may be a little on the low side for the resistance they provide. Your wiring should be fine if you can verify 12+ VDC at the gauge with the key on. You can always measure the sending units' resistance (to ground without the wire on the terminal) to see if it is the root cause. I'd discourage any shunting of the sender.
 
Stewart Warner didn't do any better with my senders and gauges. Mine read 15 deg low, so I shunted about 300 ohms at the guage. Since yours seem to be reading high, that would call for a series resistor. It's a good thing these folks are not trying to measure anything critical! Like anything in the medical or process industry. Quite dissapointing overall. I must note, per MM, that these resistor "tricks" don't really solve the overall problem; you might get a single point to be correct, but you will loose some dynamic range. For me though, I wanted to see the actual temp at cruise +/- 5 degrees.
 
I had an interesting conversation with a guy at "Vee Three" (now Teleflex) who manufactures the gauge. I told him I was running a Sierra equivalent sending unit but he suggested I get an actual Teleflex sending unit which I ordered today.

He thinks that'll get it closer, we'll see. He also said not to use any thread sealant when installing, definately no teflon tape. I asked him about "where" on the engine the most accurate temp reading can be taken with an I/R gun. He said the "block" once fully warmed up and thermal transfer has occurred

I agree about the 15 degree delta, that seems crazy on what could be the most important gauge - at least in the top two.

Once I get the new senders, I'll scrape a little paint and see what it comes up with and compare it to the readings I have at the sender and thermostat housing already. I just want to establish a baseline and if I need to "convert" at the helm, I'll live with it.

I'm fairly confident I'm not actually running "hot" at idle now. Interested to see what the cruising temp is once the oil coolers are in play...
 
I had a problem with one of my temp gauges recently. I replaced the sender, then the guage and it still read hot (actually pegged after only a few minutes). IR gun said 160. Turned out to be the wiring connection between the engine wiring harness and the gauges (where you would unplug the engine wiring harness if you were going to pull the engine). I believe there are 8 or 10 wires in the bundle. I'm not sure which wire was the problem but I guess it was a voltage problem somewhere.
 
I agree that the engine/hull harness connections may have something to do with this... not exclusively, but MAY.
However, since you have twins, why not swap temp senders, and see what happens?
 
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Bob:

Those senders have thermisters inside of them. I'd have the IR gun pointed at the component the sender screws into if you are checking for accuracy. You can get 'calibrated' sensors but yu'll pay for them and they will make the marine label cheap. This is the economics of mass production; if the buyer (teleflex) is happy with +/- 15 deg of accuracy, that's all the mfg will deliver.
 
Gee, most people have a problem with motors running too hot!

Ignoring the gage screw up for a moment, are you planning on changing the thermostats to get the temp up? That will improve gas mileage since the motors are running 20 to 30 degrees too cool.

Jeff
 
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