Verify my CHT sensor test results

[autojack]

My 3.2 has always had a kind of rough idle when it is first started. It starts out strong, but as it begins to settle down towards the 880 rpm speed it starts to hesitate a bit. It isn't hunting around wildly and hasn't fully stalled, but sometimes seems it is about to before suddenly correcting. Once it has been running for 5-10 minutes the problem goes away.

The distributor cap, rotor, plugs, and wires were replaced 18k miles/7 years ago. I just cleaned the IAC valve, it was pretty clean to start with and didn't change anything. So next I measured the CHT impedance. I got the following results:

4.3K ohms when engine was dead cold, 21C outdoor temperature.
160 ohms after the engine had run for 5 minutes or so.

Based on the chart in the Bentley manual, it looks like the cold reading is too high (indicating colder than it should) but the warm reading is in spec. Seems like this would explain why it starts off rough and then gets better. I have a replacement sensor, and it measures 2.18K in ambient air.

I was hoping someone with experience can check my work and confirm before I go to the effort of replacing the sensor, because I know from experience doing the speed and reference sensors that it's going to be a pain in the neck.

[proporsche]

here is a little help for you

[spuggy]

Bosch NTC sensor is nominally 2.5 KOhms @ 20C, +/- 5%.

4.3 KOhms @ 21C is a very long way from 5%.

Bosch datasheet says:

Characteristic Application
T [°C]	R [Ohm]
-40	45,313
-30	26,114
-20	15,462
-10	9,397
0	5,896
10	3,792
20	2,500
30	1,707
40	1,175
50	834
60	596
70	436
80	323
90	243
100	187
110	144
120	113
130	89

Here: Temperature Sensor NTCÂ M12

[autojack]

Oh, the data sheet info is great, I didn't even think to look for one! Thanks. I'll definitely be replacing it. I hope it helps with the cold idle issue.

[autojack]

Just to follow up on this: I did replace the CHT sensor and it fixed the idle issue. No hesitation anymore when it's cold, just nice steady running.

[Wasatch 911S]

That is good to hear. Where did you buy your CHT sensor from? Right now the only one I can find is available from Porsche, which to say the least is a lot more than I want to pay. I Assume you replaced it with the updated "2-wire" version? Thanks.

[spuggy]

Originally Posted by Wasatch 911S

That is good to hear. Where did you buy your CHT sensor from? Right now the only one I can find is available from Porsche, which to say the least is a lot more than I want to pay. I Assume you replaced it with the updated "2-wire" version? Thanks.

The VW/914 sensor fits the 3.2 head (same reach/thread) and available all day long for $20. It's only a single-wire - and you'd need to extend it.

[mysocal911]

Problems associated with a temp sensor are simple:

1. Temp sensor resistance indicating greater than 5,000 ohms when warm, results in black smoke when the engine is warm.
Use a paperclip to bypass it for testing.
2. Temp sensor resistance that's too low when cold, results in poor starting/running when cold.

[autojack]

Originally Posted by Wasatch 911S

That is good to hear. Where did you buy your CHT sensor from? Right now the only one I can find is available from Porsche, which to say the least is a lot more than I want to pay. I Assume you replaced it with the updated "2-wire" version? Thanks.

I bought the Bosch version from Pelican. $97.
https://www.pelicanparts.com/catalog/SuperCat/819/POR_819_FULEMI_pg1.htm?highlight_part=0-280-130-070-INT

[Wasatch 911S]

Originally Posted by autojack

I bought the Bosch version from Pelican. $97.
https://www.pelicanparts.com/catalog/SuperCat/819/POR_819_FULEMI_pg1.htm?highlight_part=0-280-130-070-INT

I didn't realize they were back in stock, thanks. Good to know. URO was suppository working on a replacement parts also.

[UROParts]

Looks like the Bosch is currently out of stock at Pelican, if you try to add it to the cart it redirects to Genuine. Strange that Pelican has the Bosch listed for such a low price, it's $299 at the big jungle retailer. Maybe it was on clearance?

In any case, our Cylinder Head Temp Sensor 930 606 915 00 is now available, and we've sold about 130 so far: https://www.pelicanparts.com/More_Info/93060691500.htm?pn=930-606-915-00-M253&SVSVSI=0819&DID=13864