I researched the previous posts on the subject and i cant figure it out. Car idles a hair hot. I would say close to the middle of the 2nd and last white lines on the temp guage. Drive on the highway about 45 min and the guage approaches redline and stays there but never actually hits it. Fans go on and stay on until after the car shuts off. Coolant level is good. open the hood and the heat is unbearable. From this description, is it possible that the thermostat is not good? Wouldnt the car overheat in a matter of minutes? please help.
Thanks,
James

 

James,

I'm not a guru, but if the fans stay on (I'm guessing '87+ here), and the flaps are open (if your car has them) you have a few possibilities. Get a cheap IR thermometer and see how hot it is, and where.

Thermostat and/or rear seal are bad: pretty easy fix and a bad seal would let it run hot.

Bad coolant tank cap (not holding pressure), or a cracked tank(mine was cracked on the bottom : pretty easy fix.

External crud blocking radiator: easy to clean out

Internally blocked radiator: Not as easy to clean, but can be done or improved.

Loose water pump impeller: longshot, and unfortunately only one way to check.

Hope that helps, that would probably be my troubleshooting order.

Jim

 

great info. radiator should be fairly clean. car is 93 gts.
Thanks,
James

 

""" radiator should be fairly clean. car is 93 gts."""................Sorry to say that a 1993 car is now 14 years old and the radiator could very well be plugged up by now.Jim r's info is spot on for checking cooling system issues except that he forgot the last thing that I would check for and that is combustion gases getting into cooling system.

 

radiator is from an 87 s4. it was removed, an end tank was replaced, it was tested and installed. how does one check for cumbustion gases?

 

James,
Could also be the oil thermostat. Located above the oil pressure sender. Do a search for this, focus on a post referencing this about 3-4 years ago by Fred C of Denver (cfc928). Dave Roberts sells a kit of the thermostat and two new relief pressure springs for ~$35.
Gary Knox

 

holy crap!
Thanks.

 

first get an IR reader see what the real temps are on the top hose,you might be chasing your tail till you really know, if its running hotter than 205 then you have a cooling problem if on the other hand your seeing temps around 185 to 200 then this is OK for the the cooling system, they have a paste that you put on the inside of the cooling cap to test for combustion gasses, this would indicate a bad head gasket or cracked head. dont panic do the simple stuff first, it might just be a bad cooling sender or gauge or connection

 

if the fans are coming on thats a good sign, it means the thermostat is working. When i overheat, it is on the day's first start up and the t-stat doesnt open. car over heats, becuase the t-stat doesnt open and the radiator is cool (relatively) to the touch. fans dont come on , because that temp sensor is at the bottom of the radiator. could be not opening wide enough. easy enough to check.

radiator clogging will be the best guess, or are your levels low. filll the water above the middle of the seam on the overfill bottle. Just learned that trick. below it, causes air in the system, or poor circulation. If the radiator is clogged, it will be warm to the touch, not hot. you want that radiator to be real hot, that means its disapating heat and doing its job!
MK

 

I cant touch the radiator because the fans are on one side and the condensor is on the other side. when i first started the car and let it run i noticed that the lower hose was cool and the upper hose was hot. then after more time both hoses were extremely hot. this leads me to beleive there is a poor circulation problem. the fluid level is to the center of the expansion tank.

 

When was the water pump last replaced?

 

Check your powered air inlet louvers (gills).

 

the water pump was recently replaced. that doesnt mean that it cant fail.

 

Replace the radiator with a NEW one.

When you get the old one out and apart, you'll find a hard crust of calcium precipitate in the lower tubes, and also partially clogging the other tubes as you move higher in the radiator. Unless you are religious about coolant changes and just as religious about using distilled water, your 20+ YO radiator will be as I describe. Take a look into the bottom hose nozzle with a light and a mirror and you'll see what I mean.

It is possible to have the core cleaned after the tanks are removed. For the most part, the chems used for cleaning are also tough on the metal parts of the radiator. A good radiator shop will tell you what your options are as far as cleaning vs. remaining life vs a new radiator.

 

The T-stat should have been done when the rad was done. If not, it's a $10 - thirty minute job on a cool engine. Next, when hot, check the temp of the lower hose, it should be significantly cooler than the upper hose with the engine running. At the temp you describe, I'm betting that both hoses are very hot, and very firm to squeeze. This indicates that the rad is not dissipating the load. Bad mojo. If the lower hose is cooler than the upper hose, your other possibility is the impeller on the WP has come loose and is not circulating. The only test for this is to open the cooling system and supply water to the engine while watching the return line. Very difficult on the 928. It requires a clear 1-5/8" hose adapter from the T-stat housing to view the circulation. Worth it to check before cracking into the TB/WP.

I also suspect the rad, because it sounds like that's been fiddled with. Ocham's razor and all. I changed the rad on my Mondial last month, and it was like a new car! No overheating even in town on +90F days.