sc400 slowly overheating

[spf_lexus]

So I finally got some time to test my sc400 out on a hot 95 degree day. The car warmed up to half way and sat there for about 20minutes, and then slowly climbed up to just before the red zone before I got home and shut it off.

Electric fan is on and pulling air (at all times, basic ignition tap).
No leaking coolant whatsoever.
Coolant level stay full, no burned cooland or white smoke.
No wetness areound the water pump/crank area.


I did see some scale inside the front bridge, maybe the thermostat is clugged up? Maybe a clogged radiator?

 

[cribbj]

Sean, here's what I would do, in order of increasing cost and difficulty. (My troubleshooting style is to try to eliminate the easiest & cheapest stuff first):

1) See if the water pump inlet hose is collapsing
2) Did you burp the system to get rid of trapped air?
3) Get one of those coolant check kits and test the coolant for exhaust gas to see if you've got a leaking head gasket. Cheap & easy to do before you spend lots of time & money doing other things.
4) Replace the 'stat
5) Replace the sender (engine may not actually be overheating - could be a bad sensor)
6) Run a flush through the system
7) Question whether the electric fan is really man enough? Most do not cool Supras sufficiently, which is why you find used setups F/S all the time on SF. Make sure you have as much of the radiator core covered as possible with the fan setup.
8) Have a look at the water pump - is it the original? Maybe time to replace

 

[Lextreme II]

Great advice John......

 

[stevechumo]

I'd like to chip in a little bit on John's advice, which is very great.

1) See if the water pump inlet hose is collapsing: Try to warm up the car to normal temperature and observe if the coolant level in the reservoir getting lower due to the water pump start circulating the coolant. You can also hold the upper coolant hose to feel any movement. Watch out because it'll get hot.

2) Did you burp the system to get rid of trapped air? Any bubbles?

3) Get one of those coolant check kits and test the coolant for exhaust gas to see if you've got a leaking head gasket. Cheap & easy to do before you spend lots of time & money doing other things. Leak down test.

4) Replace the 'stat: Remove the stat and drive the car without the stat to see if the car gets any cooler. If removing it doesn't help, then it's not the stat.

5) Replace the sender (engine may not actually be overheating - could be a bad sensor). After no. 4 above.

6) Run a flush through the system. Then fill 50/50 with drinking water and coolant.

7) Question whether the electric fan is really man enough? Most do not cool Supras sufficiently, which is why you find used setups F/S all the time on SF. Make sure you have as much of the radiator core covered as possible with the fan setup. N/A engine should have at least a dual 12" fans with 2500 cfm running all the time. This is my experience. A dual 14" fans will be much better. They should be pullers.

8) Have a look at the water pump - is it the original? Maybe time to replace. Check no. 1 above. If the coolant doesn't circulate and get lower when the engine runs, then the water pump isn't working.

 

[gloverman]

In this case I feel a simple flush of the radiator may not be enough. I would recommend removal of the radiator and have it professionally stripped and cleaned. After any overheating I always recommend replacement of the thermostat.

 

[spf_lexus]

Thanks for the quick replies fellas.


I just drove the car about 15miles on city streets on a cool morning and it came up to operating temps but the gauge never passed half way. I dont think the thermostat is bad, because after its brought up to operating temps it woud keep climbing no matter what. I have a weied half/half coolant mix of dex-cool and green coolant so who knows, maybe the mix is throwing off its heat transfer abilities? Its either that or the radiator is scaled inside, allowing flow.. but no heat transfer. Or like Cribbj said, a bad sending unit maybe a cause as well.

I just got a toyota genuine thermostat and gasket just so I know its new and working. I will be driving this car 1200 miles from CA to CO in the middle of August through the hottest parts of the country on the 20th so I need to know that this wont break down (I dont have A/C either!!).

I found a remanufactured radiator online for $150 shipped. Beats $600 for a brand new "OEM" unit.

 

[spf_lexus]

John, I popped the bleeder nut and coolant immidiatly rushed out so I dont think it has air pockets. I always fill my system up at this point. The hoses are brand new, (bought last week). The reservoir cap is very hot, indicating the coolant is flowing past the stat. When I did the t-belt, the water pump seemed to be replaced not too far back, pretty new looking. (OEM orange toyota RTV used). No leaks, or drips with no overall fluid level change.

 

[Zuffen]

Jsust one note on driving without the thermostat.

The thermmostat has the gasket that seals the water outlet to the waterbridge mounted around it.

You would need to work out some form of new gasket if you wanted to drive without the thermostat.

 

[spf_lexus]

The thermmostat has the gasket that seals the water outlet to the waterbridge mounted around it.

You would need to work out some form of new gasket if you wanted to drive without the thermostat

Thanks for the heads up. I just decided to buy a new OEM thermostat and gasket just so I dont have to worry about it freezing up when im 100 miles from civilization. Probably get a radiator just for the peace of mind even thought it may not be the issue. These 1200 mile to/from trips I take to school really get me nervous, you'll see signs that say "no gas/services for 120 miles" and I plan to do this at night because it will be over 110 during the day (and my english bulldog cant take the heat lol).

 

[spf_lexus]

7) Question whether the electric fan is really man enough? Most do not cool Supras sufficiently, which is why you find used setups F/S all the time on SF. Make sure you have as much of the radiator core covered as possible with the fan setup.

I have a single 16" electric fan w/o a shroud. I dont know what brand it is but it has straight curved blades w/o any logo. I got it from Lex a while back. I have it pulling air thru the rad like it should. Gotta remember that this is a bone stock motor. I wonder if the exractors + de-cat will allow the motor to expel the hot exhuast "quicker" and possibly eliminate some heat soak that occurs around the log manifold/cat area?

 

[stevechumo]

You'll need the shroud for sure. The fan is used to collect the heat and direct toward the fan and sucked out. Try to make a shroud for it, and I could guarantee you'll see big improvement.

 

[spf_lexus]

You'll need the shroud for sure. The fan is used to collect the heat and direct toward the fan and sucked out. Try to make a shroud for it, and I could guarantee you'll see big improvement

I think your right Steve, I wonder if I can modify my LS400 fan shroud to have my 16" fit in the middle? I drove it for an hour last night and it never got above 1/2 way but that was at 75degrees, not 110. I am going to try today when its really hot. BTW, I know this is sorta off topic but I finally got my SC on the freeway last night and wondered if cruising at 75MPH yields 3000rpm or so. Does this seem normal for an SC400? My LS used to be at 2400-2500 but I know the final drive is different.

After John mentioned wether my fan is "man enough" I started thinking that maybe its not. I might just yank my dual 14" fans from my LS and use those instead. At least I know they are Hayden name brand fans... mine is no name, no specifications. Plus I know the 14"s pull A LOT of air. I have had my LS out on 110 degree days, climbing mountains under full boost and it never once raised above 1/2 way.

 

[stevechumo]

I'll have to take out my SC on the freeway to know the RPM. I'll let you know. But I think 3000 RPM at 75 mph is too high.

 

[spf_lexus]

Actually its more like 80mph at 3000. It migh be my tire size, but I doubt it. I have a weird setup right now as I am waiting to swap my 19's but right now the sizes are a little off. 225/60/16's on one side and 225/50/17's on the other side.

 

[Thermactor]

My LS400 has a really pathetic $18 eBay fan on one side, and the stock A/C pusher fan on the condenser on the other side, and the stock radiator which is about .4" thick. It does not overheat and doesn't need the fans to run unless it's in lousy traffic. Something isn't right

 

[spf_lexus]

My LS400 has a really pathetic $18 eBay fan on one side, and the stock A/C pusher fan on the condenser on the other side, and the stock radiator which is about .4" thick. It does not overheat and doesn't need the fans to run unless it's in lousy traffic. Something isn't right

I deleted my A/C so I dont have the condensor fans "pulling" to feed my radiator fan.. the 16" is on its own . I drove her around city streets for a while and my gauge never moved until I started driving the car hard light to light, and even then the needle jumped up a mm or so until I layed off the gas and cruised to the next light and it jumped back down. Im thinking I just need to swap dual fans and flush my incorrect coolant mix of dex cool/green coolant mix (I have a feeling its contributing to the bad heat transfer). I drained a couple wuarts worth of this mixture and replaced w/ distilled and I think it helped some.

 

[stevechumo]

I took my SC on the freeway and it's about 2600 RPM at 75 mph. I have a Supra auto TT LSD, so that may change the gear ratio. I keep forgetting if the Supra LSD is low or high ratio comparing to the SC. But I think it's low, because the Supra can go up to 180 mph, while the SC can go to 160 mph. Just a guess.

 

[spf_lexus]

Your right about your diff. Your TT diff is around 3:74 or so and the stock 1992 diff is 3:92 so this would explain your 300rpm difference. I guess the N/A supra's got 4:10 gears. I spend an hour or so searching other forums regarding this and I found a few threads with 75-80mph at 3000rpm as the norm. It seems a bit high but its MUCH easier to pass someone than in an LS400. As a matter of fact your cruising speed is the same as my LS.

Thanks for the comparison

 

[spf_lexus]

Well I completely flushed my dex-cool/green ethylene glycol mix until it was pouring out clear water and then I added fresh green coolant and even then it started to overheat on the freeway. I can floor it up to 100mph getting on the freeway and cruise for a few miles and everything is good but then it starts to **slowly** climb. Once off the freeway the temp will slowly keep rising or if its slightly cooler outside the temp will drop back down.

I have a high stall torque convertor and I know it is adding to the heat that the radiator must deal with but I never had this issue on my supercharged LS400.

Coolant lines new
16" electric fan is new
coolant is fresh/not leaking

My radiator's fins are clear of any debris but internally I have no idea. When I pop my bleeder screw open I can see coolant flowing and if I rev the motor I get a little bit rushing out so I know the water pump is working.

 

[cribbj]

Sean if you're cruising at 60 or above (not accelerating, just cruising) and the heat is creeping up, you can just about eliminate the fan as the problem. The fans only make a difference below 50mph; once above that, there's plenty of air passing through the radiator core.

So in my mind, either:

1) The water pump isn't circulating the water well enough to carry the engine heat out to the radiator, or

2) The heat isn't being transferred out of the radiator (core crudded up)

I'd see if you could borrow a radiator from somebody and see what happens, and if you still have the same problem at 60mph out on the highway, then I'd change the water pump.