aptoslexus

I've got a strange problem I haven't been able to solve. The temperature gauge has been reading hot all of a sudden. Before, it would takes miles of driving to move up to normal temperature. Now, as soon as I start the car, the needle starts moving rapidly up. It's at the halfway point a block from my house. Then it continues up until it reaches just short of the last white line before the red H mark. Highway or idle makes no difference. Fan runs. This is a 98 by the way. It doesn't move any higher. It parks there like it used to park at the halfway point before all this started. It hasn't boiled over or shown any other signs of overheating. I thought it was the temperature sender gone bad, so I replaced it. No change. I swapped out the thermostat. No change. The reservoir was full. No signs of coolant leaking anywhere. I put a new water pump in it when I did the timing belt a few years ago. It might have about 25k on it. Any ideas? Bad gauge? Bad radiator?

spuds

Take your thermal thermometer and read the radiator temp.Then you will know what is needed to be troubleshot.

aptoslexus

I'll have to pick one of those up. Don't own one. My torque app says the coolant is 185-190 degrees F when the needle is near the top. That's about normal. Might be a problem with the gauge.

Banshee365

I’m no expert with UCF21’s, but on the 95-97 LS 400’s there are 3 coolant temp sensors. One for the ECM, one for the gauge and one for the fans. If you’re reading your coolant temp with a scan tool and it’s in a normal range you’re probably looking at an issue with your gauge sensor circuit. I would look at the sensor and wiring before the gauge itself.

The needle going to hot right when the start the car is the clear sign that it’s an indication problem. It would take a couple or minutes to overheat from cold.

ChaseFRA

My 2000LS had a similar (almost identical issue) and it turned out to be a blown head gasket. Had no idea that this car would do that with only 116,000 miles on it but after sending it to the dealership for a diagnostic and checking with another certified mechanic, I've resigned to the fact that this is what it is.

Stroock639

that's because an odometer reading tells you literally absolutely nothing about the condition of the engine or how those miles were obtained (doing continual 0-100-0 runs for 10 miles or setting the cruise control at 50 for 10 miles will both just read as 10 miles)

aptoslexus

Blown head gasket? Yikes. That would definitely ruin my day, but the car isn't losing a drop of coolant or running weird. It's been treated with kid gloves. If it was a blown gasket, I'd be seriously rethinking my opinion on Lexus reliability.

sdls

If it isn't losing coolant or running weird I would say that it's something gauge related. Especially since your app says that it's running at a good, normal temperature.
Gauges can and do fail on the 2nd gen LS, I went through a few clusters in the time that I had it.