Thermo (Temperature) Switch Issue?
 

I believe I am having an issue due to a bad Temperature Switch (or Thermo Switch I think it is also called). This is the one on the right chain cover that tells the system to go into closed loop mode after 15 degrees C.

With a cold engine, the switch is supposed to be closed. Mine is open cold. Therefore, I believe the system is going straight to closed loop mode before the engine is sufficiently warmed up.

My symptoms are as follows: Upon a cold start the car idles at about 1,250 RPM for two or three minutes and then the idle begins to drop as it should, I guess after the WUR starts to raise the fuel pressure as it warms and the AAR closes. I believe all this is normal thus far.

Then the idle drops to around 400 - 600 RPM and seems to struggle to stay alive. Usually it will not die and will just continue to struggle at this RPM range.

Once the car is warmed up a bit more the idle will be begin to climb back up to 950 RPM and will stay there steadily.

Does this sound like it could be my problem?

I don't believe I have any air leaks, as I've replaced most of the rubber components of the CIS system during my recent engine drop and since then my idle has been rock solid (barring this one issue) where it used to be all over the place.

My fuel pressures are all within spec.

It's an expensive switch ($ 150), so I don't want to replace it, if that is not the problem.

You are describing the thermo time valve but are pointing at the thermotime switch.

The thermotime switch manages the CSV , the thermotime valve holds vacuum away from the WUR.

The switch works until about 15° and about 8 seconds.
The valve opens at about 40° and usually takes 50 seconds to reach that temp.

Pictures might help

I'm talking about the temperature switch on the right side that controls the lambda system.

With a cold engine, the switch is supposed to be closed. Mine is open cold. Therefore, I believe the system is going straight to closed loop mode before the engine is sufficiently warmed up.


Matt,

No, it won't go closed loop until there is also an acceptable signal from the O2 sensor, but the default duty cycle will be different. If the switch is bad, replace it. There are alternative temp switches available if your garaged car never gets below 59F. My advice would be to hook up a dwell meter and find out what the system is actually doing. Have you done this ? You may be only getting a 50 % duty cycle open loop instead of 65% because the system thinks it's already warmed up.

No. I haven't tried that. I'll hook up the meter, start it up cold, and report back on what kind of duty cycle I'm getting.

Matt,

Testing is good. You can also simulate a cold start by grounding the switch lead with a jumper wire, it won't hurt anything. If your problem was the cold acceleration system, it would have no effect on your cold idle speed, more like stumbling and stalling when you tried to drive off, depending on where you fixed mixture adjustment was set. I like the optional 78 F switch better than the 59F switch.

Here are my results from my testing tonight:

I hooked up the duty cycle meter and started the car with a cold engine. The idle immediately was low and stumbling. The duty cycle was dithering between 50-52%

I then grounded the 15 deg. temperature switch lead and the idle steadied itself. The duty cycle was around 65%.

Paul...it sounds like you are correct. It is getting a lower duty cycle because it thinks the car is warm already. Based on this, and the fact that the switch is not closed when cold, do you agree it sounds like my problem is the 15 deg. switch?

Matt,

Sounds like you found a problem. I think the 78F switch is VW part and all you need to do is change the connector. Or you can wire up a toggle switch on the dash with a "Choke" label...

Why would you recommend the higher temperature switch? Is it to keep the richer mixture longer for better cold performance?

Matt,

If your car is stored in a heated garage, it may never be below 59F. The 78F switch is marketed and sold by a Porsche dealer that sold and serviced these cars when new and knows what they are talking about.

Ok. Thanks for your help.

For the VW switch, any idea what the part number or model is?