Are you sure you got your sender back with the engine?

If I can help let me know. I am a Gaffrig dealer.

Kirk

the motor went out to the builder with my stock merc. sending unit, and it always worked with the gaffrig gauge. but when it came back from the builder the gauge ran hotter. so i replaced the sending unit with a gaffrig one. then replaced guage and so on. i am going to try to clean the thread sealent off and give it a shot. thanks alot for the ideas keep`em coming something is bound to work.

there are anfew different senders. it depends on what your guage's sweep is, 130 -230 for example. or maybe 100 - 190. and there are also a high line series of senders which match the guages that have the little red idiot light built in to them. they to have different range's.
more than likely your sending unit got swaped with someone elses at the shop during its stay there.

You can check the gauge and wire by grounding the wire to the sender with the ignition on. It should read the highest reading the gauge has no matter what it is.

Some senders are made to trigger two gauges at once, like with a flybridge boat with a lower helm. These will give a false reading to a single gauge.

Good luck with this one.

Be sure to use a marine thermostat gasket with the rivets in them and not an automotive style. I had this problem once and found the thermostat housing wasn't grounding. Just ground the housing with a temporary jumper wire to check it.

wow that sounds like it rite there. i was in a jam and needed gaskets and went to auto zone and got gaskets, def no metal in them. i have the merc gaskets in my tool box i will make the switch today.

You would think that the hold-down bolts would complete the ground, but it is worth a try.

I thought the same thing and most times I think they will. It took me a while to figure out what was going on as one engine would read OK but the other wouldn't, couldn't believe the housing wouldn't ground thru the bolts. Likewise I had changed thermostats in both engines and used automotive gaskets, one worked one didn't. The housings I was using at the time were the 454 Mag style with the separate plate and gasket on top which makes the housing itself somewhat insulated from the bolts unless they just happen to touch. This woudn't be a problem if you have one piece housings.