safulop

OK, I'd like to replace the temp II sensor on my S4. I don't normally work on my car, but I have a few tools. I do have work-shop manuals. Is this easy or hard?

davek9

Easy.. unplug it and unscrew it, install is the reverse (no water will leak out).

safulop

Uh, it's the one on the water bridge toward the passenger side right?

MainePorsche

Make sure threads are clean for proper grounding.

MainePorsche

Yep, that's Temp II.

SMTCapeCod

Anyone happen to know diameter/thread pitch?

THinking of a thread chaser to ensure powder coating is outta there on mine.

SMTCapeCod

^^^
whups, inadvertent double post.


Thanks, Ed! Everytime I think I'm asking something so foolish/unlikely that it probably hasn't been addressed before, I'm reminded of all those that have gone before!
\/

Andre The Giant

What does the temp II sensor do, is it the one that tells the electric fan on MY'85 to start ? The other fan works with the engine.

Ed Scherer

An old post of mine might answer that question. I think it's M12×1.5.

bureau13

No, there's a temperature-controlled switch in the front (lower, driver's side) of the radiator that turns on the aux fan. The Temp II is the one the car uses to make it run properly...I'm guessing it is for fuel based on how warm the engine is? But I'm not sure exactly how it's used. There's a different one for the gauge and overheat lamp.

hacker-pschorr

Why are you changing it?

Andre The Giant

Thank you Jeff. I will look on the PET to see where that aux fan switch is, it's bugging me !

safulop

Car has had some issues over the summer, got a bunch of things taken apart and put back together in search of the answer. The only remaining problem is that the cold idle is too high, but settles slowly down to spec idle when engine is warm. There is an accompanying drop in gas mileage.

While my shop said the Temp sensor was operating correctly, they likely only tested it warm. The sensor has a whole range of resistances it is supposed to provide to the LH, and if it is reading too high at cold temps then the LH will send too much fuel which will increase the cold idle speed and reduce the mileage during short trips, which is mostly what I do driving 6 miles to work and back each day. So possibly changing to a new Temp sensor might fix this weird problem.

John Speake

The idle control loop is designed to give a higher idle speed when engine water temp is below normal running temp. It's a closed loop system, so fuel enrichment won't alter it.

Cold idle speed is typically 200 to 300rpm higher than the hot idle speed of 675rpm for an S4.

safulop

Yeah, mine is idling up to 1600 when cold-started. It slowly settles to 675 as it warms. The idle speed is definitely dependent on coolant temp, but it is just way out of the normal idle speed range. So one idea was that the temp sensor resistance could be out of whack at colder temps. But it's good to learn that the excess fuel part of the logic there is incorrect, thanks John.