Need your ideas. While driving my 98 LS400, I have noticed the needle on my temperature gauge just drop all the way to the bottom and then it will come back up to normal.

 

It might be a loose connection to the temperature sensor I would start there.

 

My guess is one of 2 things. First, when was the Tstat changed last? Secondly and probably old is the water temp sensor at the top of the motor. Cost about $30 and will also increase gas mileage.

 

I would suspect either the wiring between the temperature gauge sensor and the gauge/instrument display, or the sensor itself. If those check out, then it might be a problem with the gauge itself, but I think that's the least likely scenario. Btw, I'm pretty sure the temperature gauge sensor is different than the coolant temp sensor used by the ecu for fuel management. If it isn't an obvious problem, the trick is going to be getting the problem to occur while you are doing your investigating with your multi-meter.

 

I would suggest to check the connection... apply switch lubrication ... this happen to my Supra MA70 too

 

More information. I just bought this car and not sure when thermostat was replaced?. I have been watching and it seems to only happen the first 2 miles when the car has set and is cold. After about a mile, it begins to rise and then drops (back and forth) for about 2 or 3 miles and then it goes to normal position and stays.

 

May sound crazy but, is your coolant full? Maybe the gauge isn't getting a good reading until the pressure is built up.

 

Just had timing belt changed, with thermostat, and new coolant. still there???
I have had some sticking of the tech. and speed om. I just tap the dash a little and they work. The dealer seems to think its related and the board needs repalcing.
I'm confused on which temp sensor you are talking about. There is suppose to be one over the engine and I heard it was hard to get to.

 

I think it's time for a new instrument cluster...

It happened in mine...

 

Ya, mine does the same thing. it'll drop and then go back up.
But once I changed my cluster, it fixed the problem.

 

Don't go changing the cluster. I just read this again and it sounds like there's air in the system. Gauge going up and down would point to that. Bleed the system (search) and that should fix your temp gauge problem. The tach and speedo sticking is a common problem, and in my opinion, doesn't warrant spending sooo much money just to fix that. There is a fix for that (thanks to Och) where he drilled a tiny hole in there and inserted a pin so the needles don't drop all the way to 0.

 

My vote is for a problem with the thermostat...