1990 S4 Auto.
Looks like the PO 'disabled' the cat temp sensors. One has a wire coming from the sensor but it goes nowhere. The other has no wire from it at all but there is a wire from above the TT that is taped off with some insulation tape (poorly).
I am aware that there were several cat fires when the engine management system shut down ignition on one half of the engine due to some issue and that porsche installed the temp sensors to help stop this.
Questions:
Are the sensors & wires still available?
How easy is it to run the wires to wherever they connect to the wiring harness?
Do the temp sensors have any other function eg feedback to fuel computer etc?
Are they really necessary?

Your thoughts please.

Myles Garmonsway

 

was your car originally in Japan ??

 

Jim,
yes, the car is ex Japan.

 

Are the sensors in the exhaust toward the rear of the cats ?? if so that was the Japanese required "warning" that was added to the car...

 

Jim, correct - the sensors are towards the rear of the cats. The car has a CAT light on the top right of the instrument panel and has occaisionally come up with a 'CAT TAMPERATURE' warning on the digital console (probably when the cut wires contacted the car body.

Regards,

Myles Garmonsway

 

Your 1990 should also have the factory temp sensors mounted to the cylinder head near # 3 and #7 as well as the clear plastic relay mounted to the brain box. That is the system which when it senses an imbalance of temperature shuts off 4 of the injectors under the assumption that 1/2 of the ignition system 2 cylinders on each bank has failed and the car is about to catch on fire ....

 

Jim,
does that mean that I don't need the temp sensors for the cats?

Regards,

Myles Garmonsway

 

Cat fires are fairly common in early S4's in "limp home mode" where the engine is running on only 4 cylinders....but gas is still squirting into the other 4 and being pumped into the cat..... later models, starting in 1989 or 90 had a sensor to disable the fuel injectors on the non running side during limp home mode to eliminate it

 

Please do not call it "limp home mode" call it my car only has 1/2 the power is running rough and the cats are starting to smell very HOT but I am too stupid to park the car and call a tow truck ...Got to make it home ,got to make it home ,got to make it home. The only "limp home" may be the default program when the mass air sensor takes a dump.....BUT running on 4 cylinders is NOT LIMP HOME !! it is a very good way to burn up the car.

 

FYI-- When chasing a 4-cyl mode problem early in my stewardship period, it was often difficult to identify that there was in fact a problem. The performance in normal driving was not so different, except that it took a little more throttle. My '89 has the telltale lights on the relay so was a bit easier to diagnose. Look to see if an LED is lit, tough stuff. So while it was only a 4cyl, it was hardly 'limping'. It's easy to see why the factory decided to add the EZK/ignition monitoring relay, since so many doctors' and dentists' wives might not notice on the way to the club or the salon.

There are also some horror stories. An east-coast lister spent the best part of a weekend on his earlier S4 TB/WP project. Got it together and it started right up, but it was a little rough. He left it idling in the driveway for a bit, until a neighbor let him know that the car was on fire. Primary damage was 'limited' to the floor pan area right above the cats, but that includes all the electrics and the console above. Cause was later determined to be a coil wire left loose on one side. Under load, with more fuel passing to the cats raw, the fires are bigger and happen a lot faster.

I grit my teeth when I read questions about how to disable or bypass the protection system. I think I'd be spending time building a retrofit protection system for the earlier cars if I owned one. That's how serious the problem is.

 

Back to the question..no you do not need the added behind the cat sensors.

 

Many thanks Jim & all. I will try to trace the wires & make sure that they are corectly insulated to avoid other potential problems.

Cheers,
Myles Garmonsway

 

Jim: Now I'm confused. Are you saying there is no such thing as "limp home", which shuts down 4 cyl, as in a feature designed by Porsche? I'm trying to know if there is a 4-cyl mode I don't have to worry about and one that I should not be running the engine. If there is one of each, how do I know which I'm experiencing?

 

"Limp home mode" does not run on 4 cylinders, just on a set of parameters set in the LH for that purpose. There is a NOTICEABLE loss in power in the 4 cylinder mode, IMO.

 

If the car has the ignition monitoring system 1989> The car if it sees different temperatures on the two head sensors it does cut off the fuel injection to the 4 cylinders two on each bank that it THINKS may have an ignition problem so it cuts off the injection to keep from filling the exhaust/cats with unburned fuel and fresh air . That prevents the cat fires which ignites the under coating burns up the fire wall and all the rubber plastic on top of the engine and melts the hood. So any car after 1989 (unless someone has bypassed the relay) you have no risk of FIRE when the engine cuts to 4 cylinders however Porsche maintains that driving on 4 cylinders the lack of smoothness MAY cause the driveshaft to break inside the torque tube. But more to the point if the car has lost 50% of its power it really should not be driven because you do not KNOW what is wrong it might be a timing belt getting ready to let go !! When your car is talking to you , LISTEN I was driving a 914 when it started running on 3 cylinders when I pulled to a stop fuel was pouring off the engine one of the injector hoses had burst !!! why it did not ignite I will never know.