99 Ls400 Overheats With A/c On

vevro · September 11, 2007

I drive a 99 LS400 with 160k miles on it. Minimal problems to date, but lately the car has been overheating with A/C on. There is plenty of coolant in the reservoir and it only overheats whenever the A/C is on. Yesterday i wasn't paying attention to the gauge and it actually reached the red line. I turned the A/C off and the gauge began to drop rapidly, until it reached the normal middle level. Any suggestions?

johnhoward · September 11, 2007

I drive a 99 LS400 with 160k miles on it. Minimal problems to date, but lately the car has been overheating with A/C on. There is plenty of coolant in the reservoir and it only overheats whenever the A/C is on. Yesterday i wasn't paying attention to the gauge and it actually reached the red line. I turned the A/C off and the gauge began to drop rapidly, until it reached the normal middle level. Any suggestions?

Vevro,

I just had a new thermostat put in my 99. My car was overheating a bit (to the 3/4 mark on the temp gauge) when parked in traffic with the A/C on. If I turned the A/C off, I got some relief, but not much... I suspected that the thermostat was stuck open just a bit, only to let a little of the cooled coolant into the engine block. A cheap fix, but may not be your issue.

Anyway,

When your car is cooled off - spray a hose to clean off your radiator in the front of the car. Bugs/dirt/dust get in there and can block flow of air. Cleaning this won't hurt the situation and see if this helps any. Provided you've got adequate coolant levels and a good working radiator, I'd suspect that your A/C condensor fan isn't working properly. When running your A/C it causes extra heat in the engine bay, the compressor fan removes the excess heat and helps to cool the engine. This would explain why your A/C being turned off helps the car come back to normal temp. When your A/C is on the extra heat has nowhere to go if that fan isn't working. A thermostat or clog in your radiator could also help this problem along.

Take it to a trusted service tech and have them test this among all other cooling system components; and your drive belt/fan clutch. If you get a high estimate take it to two or three other places. Tell each before they look at it, that you already have an estimate elsewhere. Shouldn't be expensive - DON'T WAIT! Good luck -

vevro · September 12, 2007

Johnhoward,

Many thanks for the suggestions. Spraying a hose on radiator this morning didn't improve the situation. Same overheating issue with A/C on. At least there was little need for A/C during commute, so the temp gauge stayed within normal range.

Tought part will be finding a decent mechanic. LS spoiled me over the years as i never had to keep good relations with any mechanics. I'll keep you posted on the findings and fixes.

Again thanks for your help.

Jerry Bransford · September 13, 2007

That type of overheating is usually less likely to be a defective thermostat and more likely your radiator is getting clogged with hard water deposits caused by someone having used tap water to mix with the coolant. Only distilled or deionized (the same thing) water should ever be used in a radiator to prevent hard water deposits from clogging the water channels.

The good news is that any radiator shop can get rid of the hard water deposits by mechanical process combined with, usually, an internal acid wash. They'll rebuild your radiator to make it perform like new which should cure your problem.

Look for a small independent radiator shop that does nothing but radiators, they're around and they generally will do the best job. A general garage will just send a radiator out to them and charge you more. A radiator-only shop will do everything from removing to reinstalling it and flushing the rest of your cooling system out. You'll end up with a like-new radiator for much less than the Lexus dealer would charge to do the same thing.

cardona6569 · September 17, 2007

I would change the Thermostat, I had a similar problem with a Supra and it turned out to be the thermostat. Second the AC/fan relay needs to be checked and the secondary fan also, and I agree with you in a radiator cleaning. The action of cleaning the outside radiator did not yield any result it means it is either thermostat, electric/secondary fan problem or in the failing of the later radiator clogged. Last I would check the bottom hose because sometimes the wire it has colapses and does not let the water circulate in a proper manner. Reminder the coolant should be replaced every 30,000 as per dealer/manufacturer with the OEM fluid which is best for this type of car.

CPR

That type of overheating is usually less likely to be a defective thermostat and more likely your radiator is getting clogged with hard water deposits caused by someone having used tap water to mix with the coolant. Only distilled or deionized (the same thing) water should ever be used in a radiator to prevent hard water deposits from clogging the water channels.
The good news is that any radiator shop can get rid of the hard water deposits by mechanical process combined with, usually, an internal acid wash. They'll rebuild your radiator to make it perform like new which should cure your problem.

Look for a small independent radiator shop that does nothing but radiators, they're around and they generally will do the best job. A general garage will just send a radiator out to them and charge you more. A radiator-only shop will do everything from removing to reinstalling it and flushing the rest of your cooling system out. You'll end up with a like-new radiator for much less than the Lexus dealer would charge to do the same thing.