cylinder head temp sender / idle & start probs?

[Bigbartguy]

I've looked over a *bunch* of previous posts and am looking for some additional input.

1985 just stopped wanting to start. I'm able to get it start by *really* working the throttle (shouldnt need to with fuel inj), but it refuses to idle and will die. Once started, she purrs over 2k RPM.

I tested the Cylinder Head Temp sensor per Bentley's and received 1.09 ohms, just under specs if my memory serves. But tried starting it w/o the CHT connected and it fired right up and idled smooth......although richly based on the smell. Started it up several times and it definitely works.

Is my sensor bad ?

new in last 2k miles: DME relay, O2 sensor, dist cap, rotor

[teardrop]

I've just been through this (hopefully ok now) on my UK 89.

Has the Cyl Head Temp Sensor been upgraded?

Mine did not fail completlely it was very intermittant. It would sometimes test ok sometimes not.

I found that when the car wouldnt start I was able to use a 800k resistor instead and it would start. I immediately swapped the resistor out and but the sensor back and it started again!

The old style sensor earth through the cylinder while the new earths through a second wire. I think that the earth can go intermittently bad depending on age, engine temp, air temp. humidity etc... all of which made it very difficult to profile exactly what was wrong.

Since I replaced the sensor with the newer two wire verison things have been 99% perfect. One none start since and I have now reflowed the transistor joints in the DME box. So far ok.

I suggest replacing it. Its not too expensive and not that hard to do.

Tips: Buy a long reach socket and grind a slot in it. Then you dont need the porsche special tool and its easier to tighten the sensor up. Dont over tighten it.

Carefully use a small sharp knife to cut the old rubber grommet around the edges. Then you can reach in behind the engine and push it out.

When replacing the grommet (supplied with the sensor) lubricate it (I used KYJelly) and I got it in in about a minute.

Hope this helps

Andy

[Bigbartguy]

thanks Andy. I think I'm going to replace it anyway. I'm not sure if it's the old style or not, but it's worth a shot.

[Jorgeman]

BigBart--Andy has it right on. (Except he 'earths' and we 'ground'). A quick and dirty test is to attach a jumper clip to the body of the head sensor with the other end of the wire clipped to a good engine ground. The sensors usually don't fail internally but the outer shell is steel and it's screwed into a dissimilar metal which causes an electrical barrier to occur after awhile and the ground connection is compromised. That's why they came out with the two wire model--the second wire provides a ground to chassis.