Randy V

With the warm weather approaching, my car is now running very close to the third mark on the temp guage with the AC on. It has had excursions approaching the red zone in hot weather if I do not turn the AC off.

I have the standard 83C thermostat currently installed.

If I replace it with the 75C thermostat will I see a lower temperature reading on the guage?

I realize that I may sacrifice some fuel efficiency, but I'm thinking it may be worth it to reduce the possibility of overheating.

Thoughts?

Thanks.

Mongo

I have a 75C in my car and the temp remains around 1/4 when driving on the freeway. If I'm in stop and go traffic, it will rise almost to 3/4 but the fans turn on and drop it to half. Mostly, I see the needle oscillate between 1/4 & 1/2 when driving. I'm an advocate for a low temp thermostat.

Speedtoys

50 Kirborts, easy.

You will see a lower guage reading, of course.

BUT...overheating isnt a thermostat issues, its a 'cooling system not shedding heat as fast as you pump it in' issue....doesnt matter what your at-rest cooling system temperature is.

Mongo

BTW, Behr and Weyler make 75C thermostats.

Also Randy, do your fans come on when your car exceeds 1/2 temp?

Randy V

Yes. All cooling systems appear to be operating correctly.

David L. Lutz

When the cooling system is not maxed out, your temp will be lower with the 75. When the system is maxed out (depending on system condition, outside temp, etc.) the 75 will not help lower the running temp.

However, when the system is not maxed, the engine runs 8 degrees cooler because you have the 75.

If your cooling system is all the sudden put under extreme load from stop and go traffic (for example) it has a little head start because it was cooler to begin with if you have the 75.

I run the 75 for this reason, because in the hot southern climate with the AC on my system gets maxed quick in stop and go traffic with the SC. (I am running a new Behr, new thermostat)

Speedtoys

"it has a little head start"

Like what..one thermostat cycle? 15-20 seconds?

jpitman2

All gains will be transitory, and at the expense of the internals not running at optimum temps. You have a fundamental cooling issue which should be sorted asap. As Wally keeps saying, if it overheats idling you have an airflow problem, if it happens while moving (30mph+), you have a water flow issue. At least I hope thats the right way around?
jp 83 Euro S AT 53k

dr bob

No.

What the other guys said, plus....

Is it running hotter on the gauge in slow traffic, but cools off when you get going at speed on the freeway? If so, the thermostat won't make any difference. You have airflow or a combination of airflow and waterflow problems. I'll bet airflow, like partially blocked raditor fins, junk in them, dirty, whatever.

Is it running cool enough in town but gets hotter when you are at freeway speeds? Water side problems, partially plugged water passages, leaking rear thermostat seal, possible water pump impellor not keepi up with the shaft.

----

I spent a couple hours pulling the radiator, cleaning all the accumulated bugs, junk, leaves, trash, rocks, sand and gravel out of the fins. Mine was a little warmer in city driving than I'm used to, but definitely hotter at freeway speeds in 90º+ ambients. So the airflow diagnosis shared above. The insides looked great using a mirror to look through the bottom row inside the discharge nozzle. The thin film of residue came out easily with a very diliute (10%) CLR flood, 5-minute soak, then rinse 10x with hose water. Results are good, with highway temps back where I expect them, less than half-way between the second (170º) and third (probably 190º) marks at 90º ambient and 70 MPH cruise.

I pulled the flap fuse (not an issuee for Randy's GTS...) for the first time this year, and on downhill runs with throttle closed, the needle quickly drops below the 170º mark on the dash gauge. Prior, it was a tug-of-war between the flaps and the fans as load changed; flaps would stay almost closed to keep the temps up, then the would open, then shortly the fans would run for 30 seconds or less. Fans would stop, flaps would close, repeat. This was just sitting at idle. I figured I could unload the fans a bit in normal highway driving, and it does. But the thermostat doesn't appear to be full open until the full 85ºC shows on the gauge. Meanwhile, the fans and flaps are responding to changes in radiator outlet temp, letting the thermostat do th precise temp management in the block. Putting the colder (75ºC) thermostat in S4+ cars will confuse the controls even more, as the brains believe the engine isn't warmed up enough, and add extra fuel and maybe some timing in response.

tveltman

I was going to say that you should check your rad passages to see if they are plugged, but it looks like Dr. Bob has already worked it out. Also, many owners simply bypass the fan control logic system, either by wiring the fans to run full time or by (ghetto-***) unplugging the sender at the base of the radiator. This forces the fan control to run the fans, but IIRC they will never run at 100% power unless you wire them straight to switched power.

Check your front cooling flaps also, as they may be stuck in some partiallly closed position which would cause problems under high load. I have had the flaps all the way cold, and the temperature will only climb when you lay into it, and otherwise you might not suspect the car is running hot.

hacker-pschorr

It's best to combine the lower thermostat with a lower temp fan switch so the fans kick on sooner to help maintain the new lower desired temperature.

There are about a dozen combinations of switches to pick from, it's a generic thread used by most VW / Audi's.

blown 87

Seems to work well with the stock stuff.

ROG100

Stay with stock 83C.

Tails

If you install a 75degree C thermostat you are only putting a "band-aide" on the problem.

Clean air and water passages and check fans operating correctly, to fix possible air flow, water flow and heat transferance problems.

Tails 1990 928S4 Auto

dr bob

S4+ cars have a temp sensor so the VW/Audi switches are not an option. You can dink with resistors in the controller but in the end that's a band-aid. Clean the radiator.