Hello, my 2001 LS430 has been running hot the past few days and I'm wondering what the most likely culprit is. I think I'm missing coolant but I can't see any leaks, no puddles under the car, there's no white smoke coming out of the tailpipe when I start it and there's always coolant under the radiator cap. Both radiator fans are working. The coolant was flushed and timing belt changed at 95k miles, which was about 3 months and 11k miles ago. I've already ordered a new OEM thermostat (I should replace it regardless if it's related to this issue) but I'm worried about the missing coolant.

Here's a description of the problem (when I refer to heat % I'm going off the engine heat gauge, where 0% is the C line, 100% is the red H line and each of the lines between them are 20%, 40%, 60% etc)

• I first noticed the car was running hot 3 days ago. It was 97 degrees F outside and I was going 75mph on the highway for about an hour. At the time the temperature gauge climbed up to 65 or 70% (where normally it's 50% - smack in between C and H)
• The day afterward the temperature climbed up to 80%; I turned the A/C off and added some coolant to the reservoir after the engine had cooled a bit. I'm not sure how much coolant, I just remember that the reservoir was somewhat low.
• Yesterday while the car was cool I saw the reservoir was about 60% or 70% and there was coolant under the radiator cap
• I woke up this morning and checked the coolant while the car was cold; there was coolant under the radiator cap but the reservoir was dry. I filled it to about 80% of the fill line.
• I did my usual 15-minute residential morning drive. A/C and heat were off. The climate setting allowed outside air in; this air was warm even though the temperature outside was 65 degrees F.
• After driving then parking and idling without a/c, it took:
  • 5 minutes to go from 40% to 45%
  • 1 minute to go from 45% to 55%
  • 3 minutes to go from 55% to 60%
  • It sat at 60% for 10 minutes, then I shut off the car. With the engine hot, the reservoir is now at the full line.

 

It could be a faulty Engine Coolant Temperature Sensor. Doesn’t hurt to replace it together with a thermostats, as both parts are fairly cheap.

 

i would get an OBD2 gauge so you can see what the exact temperature is. But if you are missing coolant, it has to be either leaked or burned, neither of which are good. your symptoms sound like a headgasket failure, but don't jump to conclusions just yet. There are tests you can do to see if you have a headgasket failure.

 

What do you mean by under the radiator cap? Inside the radiator, or do you mean like coolant residue OUTSIDE the radiator under the cap? If yes then you need to replace the radiator cap, which is pretty cheap. When is the last time the radiator cap was replaced?

And agree with getting OBD2 gauge to see the exact engine temp.

 

Inside the radiator, not outside. I'm not sure when the cap was replaced last but it looks fine and there's no sign of leakage around the cap. I'll order a new one just in case.

What will knowing the engine temperature tell me exactly?

 

To me, that little gauge that shows that engine temp is not accurate. imho first thing first, are you sure the engine temp is going up, or is the gauge having problem and that has nothing to do with the engine temp. If the temp goes up, how high does it gets. If the temp goes too high, that is not good for the engine, so you definitely want to know when to stop driving and turn off the engine, especially when 97F outside.

Just for comparison, in my Land Cruiser, I have ScanGauge mounted permanently to monitor engine temp and transmission temp. The little gauge for engine temp never budge significantly, but looking at ScanGauge, the engine temp goes up and down a bit. I mostly monitor the temp closely when we pull a trailer, or climb up the mountains.

 

In addition to leaking and thermostat failure. a failed radiator fan may cause overheating. While these 3 issues may be most common, there are others which may cause overheating. How many miles on your car? Please be careful. Aluminum engine blocks may not fare well to overheating....

 

There are 2 sensors for engine temp. The main one for the ecu and a 2nd for the cluster guage.