Erratic temp. readings...

[DoctorJD]

Thus far this season I've been dealing with a temperature guage issue on my 91' Conbrio (351 PCM). I started with no reading at all, and now I'm getting readings all over the board. When I started trouble-shooting this thing, I started out with a new guage and now I've replaced the sending unit as well as the wire from the sending unit to the guage. Yesterday, the guage gave me a normal reading for the first time (170-180) and then it started jumping up by 20-30 deg. and then settling back into the aformentioned range. Then it started pegging out, at which point I got nervous and headed back to the dock.

The motor is exibiting no signs of running hot. Even after I got that pegged-out reading, you could rest your hand on the risers comfortably for several seconds. No vapor locking, nothing. Also note that this engine is a salt water version and has a captive coolant system (which is full) as well as a raw water system (still trying to figure this setup out).

Could the wire I ran be the culprit? I just ran a 16-ga. standard stranded wire like you get at the auto shop. Should it have had some kind of high-temp sheathing on it?

Lets play devil's advocate and say that it could possibly be a true reading. The one thing that scares me is that when I changed the impeller out this year, the old one was shreaded. I found some of the pieces, but certainly not all of them. I know the pump was re-installed properly, and I pulled the discharge hose off of the pump long enough to see that water was indeed pumping through it. I tried to see if water was coming out of the exhaust but there's simply too much turbulence back there for me to tell.

However, I keep going back to the erratic nature of the readings. How can it spike 20-30 degrees and then settle back into normal range. This happened reapeatedly until I got that pegged-out reading. I'm at a loss. Any experience with this or ideas at to what the problem could be?

JD

[csuggs]

Might it be a grounding issue? Does the sending unit ground through the block? And maybe the gauge grounds through a common wire in the gauge cluster? That's where I would start.

[DoctorJD]

Might it be a grounding issue? Does the sending unit ground through the block? And maybe the gauge grounds through a common wire in the gauge cluster? That's where I would start.

The sending unit simply grounds to the engine block. I'll start looking at an issue with the grounding of the guage cluster. The only reason I originally doubted this theory is that the temperature guage is the second in line of three guages that daisy-chain a common ground. My fuel guage and oil pressure guages are working.

That being said, I have had some grounding issues on some other accessories on the boat as of late.

[wotan2525]

Sounds like a really weird issue.... if it were me I'd pull out the thermostat and run it... see if that gauge moves at all. It shouldn't go up past 100degrees. This will tell you if the engine is actually heating up too much or if the gauge is failing.