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Intermittant Overheating Problem (only over 90 degrees outside) with 2001 Lexus ES 300

I have a 2001 Lexus ES 300- 120,000 miles. For over a year, I have been having problems with it over heating sporadically. It only happens when it is 90+ degrees outside-I live in Texas, so this happens alot. It always happens the same way. The air conditioning in on and the air starts getting warm. This goes on until I am idling at a light or in traffic. I then hear a loud clunking noise–gauge is still showing my engine is not hot. The clunking stops. Then all of a sudden my gauge jumps to extremely hot–goes as high as it can. I turn the air off and try and get out of my idling situation. Steam starts coming out of my engine off to the right. I pull over as soon as a can. I let the car cool and then I can drive it again with no problem. This is very intermittent. My auto shop has done the following and has now thrown in the towel and wants me to go to the dealer. They cannot make it happen for them. Since it has gotten cooler after Sept, it has not happened again. We are now in January and nothing has happened for several months. Can you give me a clue on what this can be? Summer will be here before we know it and I will be afraid to drive again. I have tried everything. My dad thinks there is something in the air conditioning that is the cause it because it only happens when the air is on.


Some of this was Maintenance Items.

9/12/2015 Replace Radiator Cap, Flushed Coolant, Replaced Lower Radiator Bushings
11/21/2015 Replace Water Pump Timing Kit, Replaced Timing Belt, New Coolant
11/21/2015 Replaced Valve Cover Gasket
11/21 15 Replaced Spark Plugs
11/21/15 Replaced Fuel Press Regulator
5/17/16 Replace accessory relay 1 ea wells
7/7/16 Replace Radiator Fan Assembly
7/7/16 Replace Fan Clutch
9/6/2016 Replaced Thermostat

It has overheated since replacing the thermostat.

The problem might be with a partially restricted radiator.

When the AC is turned on, the condenser in front of the radiator releases heat.

If the radiator is partially restricted, and the condenser is releasing heat, and the ambient temperature is high, the restricted radiator just can’t keep up and the engine overheats.

To check for a restricted radiator, you get the engine up to operating temperature. Then take an infra-red thermal gun and point it at different radiator cores. If some of the radiator cores are cooler than others, it indicates a restricted radiator.



Do most shops have this capability or do I have to take it to a specialist?

Another question, if this is the case, how does it get fixed? What is done?

Most shops have an infra-red thermal gun.


The radiator is replaced.


Yes, but if the engine has been overheating–for over a year–to the point where the OP sees steam coming out from under the hood, don’t you think there is a strong probability of the need for additional repairs?
Under the circumstances that the OP describes, I would be very surprised if the cylinder heads weren’t warped by this time.

If they were warped, wouldn’t I be having more consistent over heating? I have not had an issue in several months since the weather cooled down.

Tester’s suggestion is IMHO easily the most likely possibility. And his description is excellent.

Any parts store, Harbor Freight, or even big-box hardware store will have an IR thermostat for a very affordable price, typically under $50, and it’s a good investment anyway. I use mine often, including to scan the house for areas that need to be better insulated.

The OP made the comment that the overheating occurred after the thermostat was replaced. I once had a new thermostat installed in a 1955 Pontiac I owned. The gauge would unpredictably and intermittently head for the top of the scale. I would shut the car off and let it cool down. I would start off again and it would be fine for a while. I replaced the thermostat again and had no more problems. While it is unusual, a new thermostat can be defective.

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“defective thermostat” theory is kinda begs to be entertained more :slight_smile:

another thing which did not yet get much attention here is what that sudden “clunking noise” is and why temperature reading is surging right after clunking noise?

probably it would help to understand if temperature probe is in “short loop” (where coolant circles before opening the thermostat) or in “long loop” (where coolant goes to radiator)

another good question to ask would be: is temperature sensor is located at the top (where air pocket would insulate it) or at the bottom (where it would not be the case)

my “imagination running wild” might imagine next 2 theories (or their combination):

1: it is indeed a bad thermostat, and/or possibly even some dirt stuck in it => it takes a lot of effort to dislodge it, thus loud clunking as it is stuck and then opened, temperature reading surges from coolant previously stuck in short loop, pressure drops, steam is generated and thrown via safety valve

2: it is not enough coolant in the system and/or air pocket was not properly bled during coolant replacement, it happens to be containing temperature sensor => reading is low, then pocket is collapsed by the rising pressure and reading surges… hard to explain clunking noise in this theory

Good ideas above. I think the key to this is the clunk. Once you figure out what’s causing that, you’ll likely know what the problem is. I’d had two cooling system related clunking sounds before. One was a sticking thermostat in a Ford Galaxy. It would stick then quickly unstick. When it decided to unstick all of a sudden it made a very noticeable clunking sound, followed by the sound of water running inside the engine. Do you hear any water-running-like sounds coincident with the clunk?

The other clunking sound that was coolant related was on my truck. It turned out to be calcified debris had broken loose, probably from the radiator, and every once in a while would hit up against the water pump impeller.

Here’s my best guess. Either some kind of yet undiagnosed AC system problem, or the problem was the thermostat, and in replacing it the shop forgot to water bath test it, and you had the bad luck to get a faulty replacement. Or there’s air remaining the cooling system. Either would result in occasional overheating incidents but no clunks.

Unlikely, but possible and is AC related. The clunk is the radiator fan is hitting something and stalling out, causing overheating. Check for play in the radiator fan bearings.

One more thing to look at. Inspect the radiator from the rear and see if the thin fins between the tubes are corroded away. On a car, 5% gone is bad. Pickup radiators are bigger but you would still change it. If you see green and white coloring, it’s a bad sign. Your car is old enough to need a new radiator. You may have to pull the shroud back to fully inspect but if any fins are gone you can bet that more are gone with a full inspection.

Ok everyone, I wanted to get to back to you since everyone was so nice with their recommendations. I took all of your suggestions and went to have a heart to heart with the mechanic. It was finally discovered that I need new head gaskets. The block is fine. The hoses have expanded and are hard as a rock. They also said one of the baffles(?) on the radiator appeared to be sticking but thought it was because of the lack of pressure due to the head gaskets. After much deliberation, I have decided to replace them. The shop I go to is a chain and I think I need a more smaller experienced shop. My normal shop wants to charge me $2500 to replace the gaskets, hoses and for a radiator flush. Sounds expensive to me. Any recommendations on what to do at the same time since things will be pulled apart or what to look for?

Once heads will be pulled off, it is a good idea to make sure valves are properly seated and adjust lifters if they need it.

While at this, replacing valve seals would be a very good idea, since they get hard with age and you see oil to get past them: cheap parts, very expensive to replace unless your heads are already removed.

Replace spark plugs as you would have intake out of the way anyways?

It will be very visible if you have cylinders/pistons/rings wear, although if you maintained your Lexus well, I doubt anything would require much attention there yet.

Since $2,500 is the approximate book value of your car, I don’t see the sense of spending more than a few hundred $$ on repairing this 16 year old vehicle. What if you spend big bucks on repairing the engine, and then next month the transmission goes south?

I think that you would be better-off taking that money (plus a bit more, if it is available), and starting fresh with another car. Just be sure to have a well-reputed independent mechanic (NOT a chain-run shop) inspect that vehicle before you buy it.

Others may differ with me…

Let me shed some light on this. I have 14 yr old twins who will be 15 next summer. I will be needing a car for them in the next 1/1/2 years. Plus my dad is 90 and has a newer, fewer miles car I will get. I have already put alot of money into my car but it is a known entity for my kids. I have been trying to buy time until all of this happens. I hear what you are saying about getting a new car. Do not want to trade in one set of problems for another. I am the type to hang on to cars and do not like switching so I know that is kicking in.

This car has a straight 6 cylinder engine, I think. One head gasket, one head. IS300 Lexus are cars that young guys want, because they are fast and rear wheel drive, but still OK as transportation, and generally these are tough machines, not prone to engine failure. You need another opinion. The facts you list don’t scream head gasket to me. A new radiator is a fairly simple job. Getting all the air out of the cooling system should already have been done, so if you paid someone to do the work you have listed, go back and complain.

If any mechanic tells you that the work will be very expensive and offers to buy the car from you, tell him thanks and leave - and don’t go back.

Nope . . . you may have misread :fearful:

OP said he has a 2001 ES300 . . . that’s got a 1MZ-FE V6, and it’s FWD, not RWD

The IS300 wasn’t yet being sold, at that time, AFAIK

It is an ES 300 2001, not IS 300.