water temperature gauge problem

[kopy kat]

I have a '99 5.7 EFI Mercruiser with a 160 degree thermostat. The guage will not register most of the time. I have tested the gauge to ground, and it pegs all the way over. If I shut the motor down after warm up, wait a few minutes, and turn the ignition on, the gauge will register 160 until I fire the motor, then it drops back down to 0. If I make a high speed run, then slow down suddenly, the guage will register 160 for a second, then drop to zero. Is this a thermostat problem?

 

[crazy charlie]

Re: water temperature gauge problem

Sounds like a thermostat problem or lack of thermostat or upside down thermostat.You can also check the sending unit.Has the stat been changed recently????Charlie

 

[Bondo]

Re: water temperature gauge problem

Actually it Sounds like a Bad Ground at the Temp Sending Unit...........

 

[kopy kat]

Re: water temperature gauge problem

crazy charlie--installed different 160 thermostat, replaced 140 thermostat--same problem with both--I think there might be a missing plastic deflector that either sits above the th. or under it.<br /><br />Bondo--where would I check for a ground problem?<br /><br />thanks for the response--hope for more---Larry (kopy kat)

 

[Bondo]

Re: water temperature gauge problem

At the Temp Sender Body.......<br />The temp sender Has to be Able to go to Ground to make the Gauge work........

 

[cobra 3.0]

Re: water temperature gauge problem

...Is there any teflon tape in the threads of the sending unit? Try without it.

 

[kopy kat]

Re: water temperature gauge problem

Bondo-ground appears ok<br /><br />Cobra-no teflon, just the original factory red stuff, but the tapered threads cut through that anyway.<br /><br />Is it really possible to put stat in upside down? and if so-would it really affect the water flow-once opened?

 

[Bondo]

Re: water temperature gauge problem

Did you have a Good Clean fit on the little Brass Rivit in the T-stat Gasket ???<br />Try Another Sending Unit.......<br />Or a Mechanical Gauge........

 

[kopy kat]

Re: water temperature gauge problem

Bondo--made sure of rivet connection--I guess I'll try another new stat and gasket---the prior stat before this one, was hot water boiled, and opened up at about 180--it was a 160 degree stat as well.<br /><br />this is slowly driving me nuts!

 

[Bondo]

Re: water temperature gauge problem

"Try Another Sending Unit......."<br /><br />There Appears to be Nothing Wrong with the T-stat.........

 

[kopy kat]

Re: water temperature gauge problem

ok--installed new thermostat and gaskets (with rivets making good contact)--installed new water temp sending unit (correct mercruiser oem part)--checked all wiring for loose connections--gauge still will not register temp while engine is running all the time--when trolling, it will sometimes move up to the 130 degree mark, then go back down to o--If I shut the motor down, wait a few minutes and turn the ignition key on, it will register 160-170 until I fire the motor and the water starts to move through it--then back down to 0!!! What am I mising??

 

[kcole683]

Re: water temperature gauge problem

What type of gauge is it?<br />If it is typical teleflex gauge (I think the others are same spec),<br /><br />room temperature is 550 to 750 ohms on the sending unit, 212 degrees is 55 ohms.<br /><br />Put the sending unit in a pan of water and when it starts to boil check it with an ohm meter.<br /><br />I found the mercruiser sending unit on my engine read way low and would only move to normal temp after a hard WOT run.<br />I bought one at NAPA and it is closer but it read a little high when I checked it with an acurate thermometer.

 

[kopy kat]

Re: water temperature gauge problem

Spark-Chaser---I've had the same problem with the original sending unit, and now the new sending unit----that's why I'm asking for help--I'm lost! thanks for your reply---got any other ideas?

 

[f_inscreenname]

Re: water temperature gauge problem

Just a dumb question. Are the gages and sending units matching? They don't mix and match. Made that mistake years ago. Also make sure the thermostat is installed exactly like the factory did. Mine if not perfectly set it will not work right. That is a little tough to do now that it is 30 years old and a little corroded.

 

[kopy kat]

Re: water temperature gauge problem

no dumb questions--just a dumb feeling because I can't figure this out! everything is matched-all mercruiser oem parts. Is there a power supply to the gauge that might be affected some how?

 

[Fishermark]

Re: water temperature gauge problem

Sounds like a bad connection somewhere to me. I doubt that you have a solid wire from the sender to the gauge - somewhere there is a pulg connection - like the main engine harness. I would pull all the connections and clean up any corrosion I found there. I am confident that must be your problem since you double checked the connection at the sender. (Even the plug that goes onto the sender can develop unseen corrosion. You might examine it more closely and wiggle it at the terminal connection and see if your gauge jumps).

 

[Boatin Bob]

Re: water temperature gauge problem

I'm with Fishermark...you grounded the gauge at the gauge and it pegged, now take the wire off the sender and ground it to a good ground on the engine and see if it pegs, could be corroded connection somewhere along the line.

 

[kopy kat]

Re: water temperature gauge problem

actually, when i tested the gauge, I did it by grounding the sender wire at the engine---should I also ground the connection at the back of the gauge itself?

 

[Boatin Bob]

Re: water temperature gauge problem

No....I thought from your first post you had grounded it at the gauge, doing at the sender is best because it checks the wire all the way up to the gauge, could still be a flaky connection, try unplugging and replugging the main cable harness connection.

 

[csanna]

Re: water temperature gauge problem

I think this is an easy fix.<br />mostBoat owners & mechanics (on carberated engines) never put the thermostats back in after engine repair. My guess is this boat doesnt have one wich the temp gauge wont ever red in the red untill its too late. I have a 71 Searay with a 3.0 140 in it, the temp gauge always read low or nothing until I turned the motor off (wich all motors heat up a bit after shut down, hence coolant flow stops) Until my motor overheated one day! To my suprise the temp gauge was in the red.<br /><br />most gauges work (especialy temp gauges) your boat is just running on the cool side<br />this is good for the enigine bad for gas mileage.