Normal Engine Temperature For 1999 Ls400

[TheeSlowPoke]

I have read through many posts - you guys are FULL of information, wow! My wife says I spend way too much time reading through the information.

I have read that the thermostat in these cars is something that goes bad often. In one or more of the posts I read that an indication of a bad T-stat is less than 1/2 way up on the temp gauge. My LS 400 runs dead on the second tick of the temperature gauge. That is below 1/2 way up, does that mean my T-stat is bad?

The gas mileage is around 23 mpg when averaging 45 mph. Most of my driving is highway, but the part that is city driving is plagued with stop signs and stop lights.

Thanks for your insights,

TheeSlowPoke

[kc92hatch]

Hi SlowPoke,

I wouldn't worry about it - sounds like you are fine. My needle (97 LS400) runs a little bit below the half-way line also. Half-city / half-highway driving I'm averaging 22 mpg, so we sound about the same.

The thermostat functions like a little valve to either put the radiator into the cooling system (essentially adding cooling capacity) or remove the radiator from the cooling system (reducing cooling capacity.)

I'm not sure about these cars, but the thermostat can fail open or closed.

If it fails closed, then the radiator is out of the cooling system and your engine coolant will just keep getting hotter and hotter from the engine and won't cool off because it can't move through the radiator.

If it fails open, then the radiator is always in the cooling system and your car will take a long time to warm up. But will not overheat.

My car seems to reach operating temp pretty quick (within maybe 5 minutes of so) and then stabilize a little bit below the 1/2 way mark.

If your car is performing similar, then I think you are good.

Stop looking for problems ;-)

Good luck!

[blake918]

My temp needle in my '99 sits on the third line sloping down a little....

It's hard to tell if your mileage is on par or not since mixed city/interstate is tough to judge. Strictly on the interstate, I see 28-28.5 mpg in my '99 (cruise set at 75-80mph with the a/c).

Edited October 8, 2008 by blake918

[SW03ES]

If you guage looks like blake, you're fine.

Also FYI, T-stats can fail open or closed, but they are all engineered to fail open now...part of the reason why cars don't overheat as much as they used to.

[oarnura]

My temp needle in my '99 sits on the third line sloping down a little....

It's hard to tell if your mileage is on par or not since mixed city/interstate is tough to judge. Strictly on the interstate, I see 28-28.5 mpg in my '99 (cruise set at 75-80mph with the a/c).

Is the mpg rating the Current reading or Tank Average? My 98 LS' needle is usually a little higher than that and I am averaging 20.7 right now highway city mixes. I wasn't light on the throttle though, I just got the car and friends have been trying it out.

[blake918]

Is the mpg rating the Current reading or Tank Average? My 98 LS' needle is usually a little higher than that and I am averaging 20.7 right now highway city mixes. I wasn't light on the throttle though, I just got the car and friends have been trying it out.

That's the tank average. I only flip to "current" mpg for amusement since it's all over the place especially with all of the hills we have here. Check for leaves/pine needles/etc. in between your radiator and ac condenser. They really like to hide there in my car. Once you get your car out on the interstate, I can't imagine you won't see great mileage like I'm getting as long as it doesn't need any maintinance. When I say 28+mpg, I'm not doing any stop-and-go driving at all. I fill the car with gas 15seconds from the interstate, set the cruise, and watch the tank average climb.

[oarnura]

Is the mpg rating the Current reading or Tank Average? My 98 LS' needle is usually a little higher than that and I am averaging 20.7 right now highway city mixes. I wasn't light on the throttle though, I just got the car and friends have been trying it out.

That's the tank average. I only flip to "current" mpg for amusement since it's all over the place especially with all of the hills we have here. Check for leaves/pine needles/etc. in between your radiator and ac condenser. They really like to hide there in my car. Once you get your car out on the interstate, I can't imagine you won't see great mileage like I'm getting as long as it doesn't need any maintinance. When I say 28+mpg, I'm not doing any stop-and-go driving at all. I fill the car with gas 15seconds from the interstate, set the cruise, and watch the tank average climb.

I have been plagued by stop and go traffic. I did hit 26 mpg when I got on the highway 2-5 minutes after exiting a gas station.

[SKperformance]

I would replace the temp sender on the head as it may be giving bad readings ,

The gauge on the car is not meant to be read but a basic indicator.

It will give you an idea if it is warmer or colder but it is not in any degrees.

Basically the center line is about a 60-80 degree variance in a mm while the whole gauge may be 0-200 below it and 300-400 above it.

It is not showing a true indication just that it is working.

Start by replacing the sender as it is cheap , it is works you can buy me one too as they all need to be changed every 5-8 years or so .

[chilkoot]

the t-stat on my '93 failed open. when mine was bad the temp was pegged at the 1/4 mark, without any fluctuation once it was up to heat. Now it sits about 1/8inch above Blake's, right where my '90 did.

edit: i'm not so sure I believe that the temp gauge is so inaccurate as to be only a good-or-bad indicator. the temp rises at a steady rate as the car warms up and when my car was being overcooled by the failed thermostat it pegged at the 1/4 mark, it seems to be fairly accurate to me.

[SKperformance]

The gauge is in indicator only . It is not that it is in accurate in any way , as it will show fluctuations .

The problem being is that the change of the center of the gauge can vary wildly but still show the same reading as it is within normal operation.

It is similar to a gas gauge , they show an avg amount by what it says is half a tank is usually 1/3 which is why it always on all cars drop so much faster once that point is reached.

I won't disagree the thermostat is bad but i would not determine that solely on the gauges position. If you have no heat after 20 minutes of driving then it would start to point more to that.

[SW03ES]

This is how most stock temp gauges are...

[TheeSlowPoke]

Thanks for the responses!

kc92hatch, you are right - I am constantly looking for problems. It's my nature!

As for the T-stat, I think it is OK. I will be able to tell better this winter. If the T-stat is stuck open, then it won't warm up in the Minnesota cold.

Besides the vibration I am chasing in another post, the decision whether, or not, to go synthetic oil and tranny oil is next. Again, I will review what is written about the oils before posting.

My last tank of gas resulted in 24 mpg. I can live with that. :)

Thanks again,

TheeSlowPoke

[oarnura]

Thanks for the responses!

kc92hatch, you are right - I am constantly looking for problems. It's my nature!

As for the T-stat, I think it is OK. I will be able to tell better this winter. If the T-stat is stuck open, then it won't warm up in the Minnesota cold.

Besides the vibration I am chasing in another post, the decision whether, or not, to go synthetic oil and tranny oil is next. Again, I will review what is written about the oils before posting.

My last tank of gas resulted in 24 mpg. I can live with that. :)

Thanks again,

TheeSlowPoke

I just got my car two weeks ago and am going through the same thing. :)

I do feel vibrations on the shifter and steering at some RPMs. I was looking at the same things on all the forums.

I realized that when I get my car in for my 90K service I'll have all the mounts and suspension components checked. Till then just drive it and enjoy it.

[TheeSlowPoke]

My temp needle in my '99 sits on the third line sloping down a little....

It's hard to tell if your mileage is on par or not since mixed city/interstate is tough to judge. Strictly on the interstate, I see 28-28.5 mpg in my '99 (cruise set at 75-80mph with the a/c).

My needle runs exactly where yours is shown. Wow, 28 mpg while cruising at 75 to 80. Are there any hills down there?