how does a 3.6 know it's a cold start?

[blucille]

I've got a 1989 964 c4 3.6 in my '79 SC. the car runs great except cold starts. I've replaced the rubber boots between the intake runners and plastic inlet ports, no change at all.

The car will start and die right away, probably 20 times. after 10 or 15 tries I can feather the gas and keep it running, and by the time I get up to 20 starts it'll run, but not well. After a few minutes of driving, it's flawless.

Is the cylinder head temp sensor how the ECU determines it's going to richen the mixture for a cold start, or are there other electronics I should investigate?

[yelcab1]

You got it. Head temp sensor.

[blucille]

the CHT is a 2 wire connection, if I'm not mistaken, I did replace the CHT recently, is it possible (does it matter) if the pins are reversed?

[blucille]

A little update.

I swapped the pins on the CHT connector, no difference.

Once I got past all the stalling, I sprayed some carb cleaner at the 4-5-6 plastic intake ports and the idle would change. I wonder if one might have a crack. It’s hard to pinpoint exactly where the problem is. I’ll try again when it cools off.

[blucille]

Well, I went thru a smoke machine test and found a very insignificant vacuum leak up by the throttle plate, I corrected that and my cold start issues are exactly the same. No other vacuum leaks.

If anyone has some explanation of the cold start on these motors, I’d love to learn more.

[phelix]

Check the resistance of the sensor with the engine hot and cold; it should be quite different.

[Mixed76]

I don't know the 3.6 well but it should have an auxiliary air valve (idle air control valve- there are several different names) near the throttle body that adds additional air for cold start. Look for that- is it plugged in, does it work, hoses connected etc.



Sent from my Pixel 7 using Tapatalk

[cliyde]

Originally Posted by blucille

I've got a 1989 964 c4 3.6 in my '79 SC. the car runs great except cold starts. I've replaced the rubber boots between the intake runners and plastic inlet ports, no change at all.

If the Head temp sensor in not at fault, in this order I would try the following:

1) O2 sensor disconnect start
2) Check the ICV/ISV functionality, take it off and spray some brake cleaner through it.
3) Check Closed throttle idle switch works properly, verify ohms at closed throttle and the open circuit when throttle pressed.
4) Clean your AFM and ensure the air temp sensor is functioning correctly. Check AFM meter for worn contacts with barn door activation. Do a forum search, tons of info on how to move the contact traces over on fresh track if the resistance doesn't check out initially.
5) Valve adjustment

Good luck!!

[blucille]

Thanks for all the suggestions.

I made some progress. I think.

I have a couple of spare CHT sensors with the wrong plugs. They all measure the same resistance at today’s temperature, including the one in the car.

And. The car started like crap this morning. I unplugged the O2 sensor. No difference.

And

I unplugged the CHT and no difference.

Here’s another good one.

I had been running the factory air flap, it ran exactly the same as with an FVD hot wire air mass meter.

[blucille]

I consider this problem solved.

I found the CHT sensor wire where it met up with the other sensors and then fed into the harness under a large piece of shrink wrap had been compromised.

Peeled back the shrink wrap and spliced in some new wires to the sensor connector and everything seems perfect.

Thanks for all the help.