I recently swapped out my thermostat and did a coolant flush on my 93 SC4 (complete block drain/refill with Toyota red coolant, Toyota thermostat w/ bleed hole placed @ 12:00, burped the system once) and have had probably 5k miles of trouble-free driving, alot of which was in 80-90* weather w/ the A/C on. Until yesterday.
I was sitting in a parking lot, A/C on full blast, probably ~90* ambient for maybe 15 minutes and I noticed the temp begin to creep up slightly from normal. It was about that time my fiancee came out of the store she'd been in, so I decided I'd see if it'd drop back down once we were out on the road again. Sure enough, back to normal the rest of the afternoon in mixed city/highway driving.
Today, I noticed as I parked at work that it was doing the same thing, only this time I'd been driving on the interstate for ~100 miles w/ the A/C on, ambient temperature maybe in the 70's, and had only driven a couple blocks in stop & go traffic. I'd kept a pretty close eye on the temp gauge, though, and never saw anything unusual until the very end of my commute. Anyway, I opened the hood to check things out and noticed something I thought was peculiar; in spite of the A/C being on, the cooling fan wasn't moving as quickly as I'd expect it to, considering the temperature gauge was ~ 1 tick mark higher than what it normally is.
Back in the day, my first car was a 92 ES300 which at one point blew a headgasket. Probably the very first indication that anything was wrong was that I heard the cooling fans ramp up one day as I drove along, only to look down and notice the engine was getting warm. My question is this; shouldn't the SC's fan behave in much the same way, i.e., if working properly, shouldn't the fan run at high speed until/unless the engine cools back down?
I went out on lunch and did some poking around; with the engine at normal temp., ambient temp. close to 90, the cooling fan reservoir is full and doesn't bubble when the fan is operating. I jumpered the diagnostic connector to ensure the fan would operate in fail-safe mode, which it did. And I also switched the A/C on/off as I was watching the fan and could see the fan kick up in speed w/ the A/C on. But I'm puzzled as to why it wouldn't do the same in the event of the engine getting warm, which is what happened this morning.
In the case of the ES, that fan was going so hard and so loud that I heard it inside the car with the windows up that time it got hot. I've never once seen/heard the cooling fan on the SC go fast enough to even be audible, although it is pushing a good bit of air. I'm just wondering if I've got a fan problem or if I need to burp the system again, etc.
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SO...update on my car. It's pretty consistently getting warm after a few minutes of city driving following long jaunts on the interstate without issue.
Yesterday afternoon, I was able to affirm that the fan does, in fact, increase with engine speed sometimes; the problem - in my opinion - is that it must not be all the time, hence the engine's getting hot in stop/go traffic.
At this point, my speculation is that maybe I just need to burp the system again. Since the temp. sensor for the gauge is at the top of the motor and the sensor for the fans is at the bottom of the radiator, it makes sense to me that if there were enough air in the system, it'd cause the temp. gauge to rise, the thermostat to not operate correctly, all the while the coolant in the bottom of the radiator is cooler than that in the block and the fans don't come on reliably. Plausible?
I don't suspect any other major issue, due to the fact that the thermostat and coolant are new and 100 miles of highway driving in 90+* weather, and even city driving, don't cause it to overheat. It's just when I get into stop and go traffic that it starts to struggle.
This was an issue with the early SC's in stop & go traffic in warmer climates (such as California) which is why in 97 they added the extra opening above the bumper to allow for more air flow thru the radiator. If you look around online there is a nice OEM fit 2 core radiator brass & copper available for 200.00$ that might fix your problem. I hate plastic & aluminum ones.
I've been reading up on the nature/operation of coolant temperature sensors in general and am feeling like perhaps I should focus my attention on replacing them, starting with the radiator-mounted sensor for the cooling fans. Through my research and observations, I've learned that the fan does appear to be working properly when it works (i.e., increasing with throttle/ A/C operation). As I'd said before, I'm just skeptical of whether or not it's reacting quick enough to prevent the engine from getting warm in the first place.
Still dealing with this issue, as I haven't had time to fly into it, plus it's fairly predictable and I'm trying not to succumb to the ole 'just throw some money at it' approach.
As I've said before, I believe the issue to be directly related to A/C operation, and I thought of something today that I wanted some input on. Two things I've done recently (prior to the cooling issue arising) were having the refrigerant switched to R134 (A/C works GREAT), and I swapped out my CC unit from a project SC300 I picked up recently; it's the same year as my SC4, but it had a CC unit w/ a PERFECT LCD, whereas mine had the typical bleeding issue, as well as a backlight that was burnt out. I've since learned that the fan control unit communicates with the CC unit to determine A/C operation as well as desired temperature. The CC unit works perfectly, in terms of whether or not it functions the way the original one did, and I even went so far as to plug the original unit into the SC3 to see if it'd work there, which it did. HOWEVER...(and this is where someone will undoubtably say "duh idiot"), I did notice the part numbers on the 2 units were one digit apart from each other. I'm thinking at this point that, although both units seem to function properly in both cars, the SC3 has a different cooling fan setup, which would lead me to believe that the CC unit might not send the signals the SC4's cooling system needs to ensure proper fan operation w/ the A/C on.