Leon Speed

A few days ago I happen to notice the air flaps are at 30% with the engine warm. I read about 83C on the dash temp indicator. Question was is the dash temp indicator accurate so I measure with an infrared gun. I measure in different areas, before and after the thermostat. That got me wondering about what temperature this engine is supposed to run and where to measure exactly. I still haven't found an answer, if there even is one.

Find out the flaps and fans are controlled by inputs from the temp sender on the left bottom of the radiator (plus intake and trans temp sender, A/C pressure). Then I think, why are there two different temp senders (excluding the intake and transmission) for engine temperature: the one on the radiator and Temp I for the dash indicator. Then there is a third one for the LH: the Temp II indicator.

Anyone willing to take a stab at it because I can't think of a good reason.

the flyin' scotsman

Interesting question; optimal engine operating temperature is dictated by the cooling system size (when the t.stat is fully open). It is also affected by the coolant type and age.

I don't know the absolute number but like you get various reading with my IR gun.

For the flap discussion I'd delete them.............the factory did

linderpat

subscribed

Leon Speed

Me too!

m750rider

On an S4, the standard thermostat begins to open at 83C and is fully open at about 98C. The temperature indicated on the gauge is measured at the water bridge with the temperature sending unit sensor on the drives side (LHD) so if you check with an IR gun, use it there. It is close to the thermostat position. It is a gauge signal only, and does no control.

The other sensor in the water bridge on the passenger side is the Temp II sensor for the computer.

The coolant temperature is controlled by the thermostat, water pump and radiator, plus the cooling fans. The temperature sending unit in the bottom corner of the radiator on the drives send sends a signal to the fan controller that adjusts fan speed, from off when the temperature is low to on at full speed when it is high. On early S4 cars, the controller also adjusts flap position. The WSM shows the temperature and output(speed) of the fan controller signal, but if I remember right they come on at about 75C. Note this is the lower corner radiator temperature (return water temp), not gauge temp. Gauge will be higher. They will speed up and slow back down to maintain the engine temperature. Check the WSM for the actual temperature setpoints.

The fan controller sends the signal to the fan final stage, located on the passenger side in front of the radiator which actually powers the fans. Both fans should run at the same speed if everything is working correctly.

The fan controller also gets signals from the transmission temp, intake air temperature, and AC system and will also adjust fan speed based on those inputs.

Bottom line is the coolant temperature will run about 85 - 90 C, right around the thermostat setting, when operating normally, and maybe a little higher in summer temperatures with the AC system on. Because sensors are not identical from car to car, same with thermostats, each car might operate at a slightly different indicated temperature.

Bertrand Daoust

Very good explanation.
Thank you.

dr bob

Adding some to m750rider's excellent description: The fan and flap controller does not know the engine temperature. It's job is to provide coolant to the engine at what Porsche decided is optimum for the thermostatically-controlled system. The controller wants to run the flaps at alosed position to reduce drag, run the fans at slowest position to reduce horsepower lost to fan operations, and therefore provides return coolant that "just cool enough" to allow the thermostat to manage actual block temperature.

Engine temp is allowed to vary between the thermostat set temperature and 98ºC or about 210ºF, based on the thermostat position. This is the normal, allowable operating range for the engine. The WSM describes the actual thermostat operation in this range. When the gauge reads significantly above 210ºF in normal driving, it's time to be concerned since the system is no longer able to manage the heat in the engine. At the same time, some folks get excited when the fans are not running at full-speed as the gauge creeps past 190ºF. If the coolant return temp going to the engine is in line with the system setpoints, the fans won't speed up; it's the thermostat that's controlling engine temps at that point.

There was a huge amount of hot-climate testing done on the cars during development. Quite a bit of extra capacity was included, sufficient to allow the upsizing of the engine all the way to 5.4 liters and more ponies without changing the radiator capacity. The only allowance made was moving the oil coolers out of the cold-side radiator tank on '90+ cars. Meanwhile, radiators and the system do deteriorate over time unless they are maintaine religiously during their full life. Corrosion and scale happen on undermaintained systems, chewing into the extra capacity designed into the system. At some point, all that 'headroom' is lost and we start to see higher engine temps under certain conditions. That's a sign that the radiator and maybe the coolant pump are not sufficient for the job anymore. Both really are known "consumable" parts of the system. Pumps get replaced when the impellors get so eroded that flow is reduced. Radiators can be inspected, but more often get replaced based on symptoms. Sometimes radiators can be chemically cleaned, but mineral scale is tough to remove from the aluminum tubes without sacrificing some aluminum in the process.

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I'be long been a believer that thermostats are not wear parts, but some recent experience has changed that belief. I was seeing higher gauge temps on extreme hot days (95ºF+) cruising at higher (75+) speeds, a condition that should be optimal as far as airflow and engine heat load. Changing the thermostat as part of a routine maintenance solved that, and in the process reminded me about how stable and overcapacity the rest of the system really is. The old thermostat was in the car for about 15years and probably 75k miles. During those years the front and rear seals have been replaced twice. The new thermostat causes the engine to warm up more quickly even with the flaps pinned open. The gauge is extremely stable after that warm-up, never varying by more than 10º (from 185º setpoint) even in extreme hot (over 100ºF) conditions with AC cranking. So my recommendations on thermostat replacement moves to some regular period short of 75k miles. I'm thinking that the timing belt interval period might be a good target schedule. It's handy since the tools are already out and the coolant is drained from the block anyway. If you are experiencing stuck-thermostat symptoms similar to mine, immediate replacement would certainly be a high-on-the-list option. Thermostats and the two seals are together pretty darn cheap from our vendors. Along with coolant reservoir caps, they easily qualify for the spares-on-hand list too.

RKD in OKC

Yep, thermostat is a bi-metal spring that reacts to the temp of the water to open and close. Over time the spring work hardens requiring more temp change to operate. You can test it in a pan of water on a stove and a thermometer.

My mechanic measures the operating temp with his ir gun on the metal where the top radiator (return) hose connects to the block. After the engine has warmed up enough to cycle the fans.

the flyin' scotsman

The '87 Service Info Technik states several temp trigger points (as measured at the rad temp sensor) for the fans and flap operation:

79c flaps 30% fans half speed (7.5v)
89c flaps 100% fans faster (8.5v)
95c flaps 100% fans max speed (9.5v)

Various other inputs are also considered with a/c and auto transmission sensors with coolant temp being seemingly primary input.

Although the '91 Service Info describes the deletetion of the flaps and relay theres no further mention of the other sensors although it seems reasonable with the other sensors remaining the controller was modified/changed.

Given the system is designed around the rad temp sensor it would seem that this is where we should read the coolant temp with our IR guns to gain sense as to checking correct function.

Leon Speed

Great points and noted that the thermostat has a certain temp range. A few years ago before I had an IR gun I put in a 75C thermostat. Now the engine runs at about 75 degrees normally and at 88 C on hot days.

I still have no explanation for why three different temp sensors, unless one sensor isn't enough for all functions needed.