head temp sensor saga

[911FL]

Interesting story. Last week I had my valves adjusted by the local porsche mechanic. There was also an oil leak problem which he traced to the oil sender and that was addressed as well with a new sender. While he was at it I asked him to replace the head temp sensor which I happened to have still in the sealed porsche wrapper from when I bought the car (perhaps purchased by the PO). No idea how old it was but obvious it was new. From a previous post, I was a bit dismayed to find that the tech had degreased and sprayed a protectant all over the engine compartment. I cleaned it up as best I could and all was well until I tried starting the car on Saturday. The engine turned over fine but did not start. An hour or so later, the car started but ran poorly the first few minutes. Started fine until Monday when I had to flatbed the car back to the shop. I called and suggested the HTS was possibly the culprit but I was informed that it was the reference sensor and they get old and as soon as you start messing with them they can quit. Okay, so $300.00 parts and labor and they said come get the car it starts great. Had one problem starting Wednesdsay night but started after cranking a second time. This morning nothing again. Cranks but not even a hint of starting. The flatbed is on the way as I write this but after a discussion with the mech he suggested it might be the HTS...

I will let you know what the verdict is this time, but honestly doubt the mere coincidences of sensors going bad all at once.

Is it possible that an HTS sitting in a sealed bag from the factory for > 3 years (unknown when it was bought) could go bad?

[scottb]

Was the "new" head temp sensor the two wire version or the older one wire type. If the old one wire version, it may have been bad right out of the package. If they're installing a new one, be sure the shop installs the two wire version.

[ZOA NOM]

Pick another shop, or DIY.

[911FL]

It was the new two wire sensor. It seems odd that the original would last 23 years exposed to the environment, and one in a plastic bag would go bad...

Couldn't the mechanic test the sensor to verify it was functioning before replacing the other sensors?

[yelcab1]

It is easy to mess up the wires, or the connector. Although, after your first complaint, it should have been fixed.

[jbrinkley]

yup, DIY

and now you know why lots of people do DIY

[911FL]

They have now replaced both sensors plus the HTS. We'll see if this fixes it. I have learned a very valuable ($$$) lesson here. I usually do all of my own work but figured another valve adjust would take way too much time and this became a while they're doing that let them take care of other issues...

[CruiseControl]

To check the head temperature sensor measure the resistance of the sensor(s) at a range of temperatures. Refer to Bentley manual section 240-18 for corresponding resisrttance to temperature. e.g. at 0degC 4.4 to 6.8kOhm, 40degC 1.1 to 1.3kOhm, 100degC 160 to 210degC.
When the cold engine engine did not start, you could have substituted a potentiometer to confirm the sensor being the fault cause.
To bench test the engine temp sensors, place the sensor with ohm meter connected in a utensil containing water that you can change the water temperature and compare the ohm reading (resistance) against a range of temperatures. For the early one wire sensors connect the ohm meter second lead to the sensor frame.
AS can been seen from the range of resistances to temperature these sensors are not exacty a precision sensor, however are adequate to do the intended task of cold engine - fuel enrichment.

Paul

[DRACO A5OG]

911FL,

Have you metered the CHTS yourself? Check the tech's work, raise the car pull of the driver side rear wheel and see if the wire is twisted.

Also how old is the DME relay? No Start and infrequent starts sound like the DME relay.

Did they finally fix it?