Where can I get a new head temp sensor that screws in at cylinder # 3? I have one but should I be concerned that it is 40 years old?

Thanks

David

Autohausaz.com $11.49

All of your favorite 914/Porsche (and even VW) retailers.

--DD

Click-n-Buy here.

I shouldn't think age would be a problem. But hook it up and put it in boiling water. You should see 100 degrees.

WHAT?

The Cap'n

Why not check out the ohm values then check what the output of yours is. Not sure if it is a thermistor or thermocouple but both of those do have limited number of cycles.

One of the fringe benefits of laptop tunable injection-- on my recent road trip up to Portland I was having some stumbling issues at cruise that appeared out of the blue. Opened up the laptop and watched the sensor readings as I sputtered along. My CHT sensor had taken a dump and was reading erratically, causing the ECU to go into warmup mode erratically and flooding the engine. I was able to just turn off warm up enrichment.

The point is that YES they do die with age and can be very hard to track down when troubleshooting. Had a similar problem in a 912E (same engine) where the CHT checked out fine every time I put a meter on it (while parked) but while driving it would jump all over the place and cause problems.

(This is the long version of what the Cap'n said)

Actually, I was referring to the silly idea of putting the CHT sensor in hot water and "seeing 100 degrees". Aside from that foolishness, sitting on the shelf for a year, a decade, or even a century shouldn't have ANY affect on the functionality of a CHT sensor.

The Cap'n

I believe he was referring to Celsius.

Ahh, written communication. How I love you.

Thanks everyone

Doesn't make any difference. I know of no way to directly read temperature from the CHT sensor, and no chart delineating that process. The CHT sensor is a variable ground for the ECU, and the resistance provides a signal that is used by the ECU to adjust the mixture for the warmup period. The ECU doesn't see temperature, only resistance. Failure of the sensor is often, but not always, is in the increased resistance mode. Some failures result in an "open" ground path, and the engine won't start at all. Not because it's cold or warm, but because it has no ground. A failure which decreases the resistance results in a severe lean condition. A failure with increased resistance results in a very rich condition.

The Cap'n

The CHT sensors are thermistors (negative coefficient)
You can certainly put the CHT in both ice water 0degC and boiling water 100degC and measure it with a resistance meter.

I did exactly that to determine that the CHTs I bought are thermistor type 2252.

That particular thermistor will read 7373 Ohms at 0C, 2252 Ohms at 25C, and 154 Ohms at 100C.
They will read an almost dead short (close to 0 Ohms) at 190C (300F).
That's why I determined that they are essentailly useless to determine if your engine is too hot (above ~290F).

However there is more than one type, so I dont know the values for any other than the one I bought.

The "chart" can be found here http://www.veris.com/docs/support/faq/RTD-..._Z202030-0N.pdf

Or serach for "Thermistor tables" "type 2252"

Stu

Are you saying that with out an ECU the Cylinder head sensor will not work. I have switched over to carbs

David

I was wondering why you were asking about the CHT. It will have no effect on carbs. It is part of the fuel injection system. Did you get the wire for the fuel pump relay ground?
Tom

Correctomundo! Don't need it w/ carbs.

EDIT: Where you been hiding, Tom?

Without an ECU, there is nothing for the stock CHT sensor to talk to. Because it is only there to provide temperature information to the fuel injection.

There is a different kind of CHT sensor you can get which goes under a spark plug. That one can be hooked up to a gauge and tell you the temperature difference between the sender and the ends of the wires they connect to. (Usually in the engine bay, and usually pretty warm themselves.)

You can hook up the fuel injection's CHT sensor to a gauge that reads resistance. It will move the needle as the temperature changes, depending on what resistance the gauge expects to see. But any correlation between the gauge reading and actual temperature is pretty close to random, and as Stu mentioned there is basically zero resolution in the reading when you get near the engine's operating range.

--DD

Dave - So basically, on my carbed car, I can delete the stock CHT? (I run a gauge with the sensor on No 3 spark plug)

The stock CHT sensor tells the stock ECU when the engine is warmed up.

A CHT gauge is a whole different thing and shouldn't be used with the stock CHT sensor.