Help......73 2.0 won't idle when hot.......

[DF]

Idles good till oil temp reaches 230 plus (runs great under load).....I have messed with the idle screw and the mix adj and even disconnected the 270k heat temp resistor. I am wondering if my ecu is bad - 022 906 021E ? Hopefully this is the correct unit for the car. Virtually everything except the ecu has been replaced including mps. Heeeeelllllp!!

[bowlsby]

Removing the resistor to the TS2 effects the mixture, I forget which way, I think it would be rich without it, which might explain why ot runs good at higher rpms.

Try reinstalling the resistor to even out the mixture?

[Eat-um-up]

My 914 did the same thing. I couldnt get it to idle when hot too. The only way I was able to fix that was by taking it to a porsche mechanics shop and have them give the rngine a tune up. I am just not good enough with fuel systems to make the vut

[pbanders]

Originally posted by DF:
Idles good till oil temp reaches 230 plus (runs great under load).....I have messed with the idle screw and the mix adj and even disconnected the 270k heat temp resistor. I am wondering if my ecu is bad - 022 906 021E ? Hopefully this is the correct unit for the car. Virtually everything except the ecu has been replaced including mps. Heeeeelllllp!!

The 1973 2.0L's have a very specifc combination of components that must be matched in order to work correctly:

ECU:
Porsche/VW: 022 906 021 E
Bosch: 0 280 000 037 (or CU13X exchange)
MPS:
Porsche/VW: 022 906 051 C
Bosch: 0 280 100 037
HTS:
Porsche/VW: 022 906 041 A
Bosch: 0 280 130 017
Ballast Resistor (Wurth):
Porsche: 039 971 762 A

The #1 thing you have to do first is to verify that each of these components have matching part numbers to the list above. BTW, the ballast resistor is 270 ohms, not 270K ohms!! And you MUST check to see if you have the right HTS. The 0 280 130 017 sensor isn't a common part, and they're often replaced with the 0 280 130 012 or the 0 280 130 003 sensors. Both of these sensors have a much higher cold resistance (2300 ohms vs 1500 ohms for the ...017).

It's very unlikely that your ECU is bad. Usually when they fail, they totally crap out and you can't get started or stay running.

Assuming your parts match, from the symptoms you describe, I would suspect that your idle is too rich. Try going back to the factory settings. Reconnect your ballast resistor, and set the ECU idle mixture knob to where there is a notch in the outer ring - this notch may or may not be in the center. Adjust your air bleed screw to the proper idle setting. If the idle doesn't seem to change with adjustment to the bleed screw, then you're too lean. Turn the ECU knob 2 clicks clockwise and try again. Contrary to opinion, I've found that the 2.0L idles and runs best when it's right at the edge of being too lean - which is pretty close to the factory setting.

Brad Anders

[DF]

pbanders - thank you
Well first of all I did find the head temp sensor part number and it is wrong....an 012. All my other parts are right though. Tonight I swapped my plugs and wires to the nology type. Now it misses and lugs down. Maybye my pertronix has gone bad. I also found the aux air regulator wire and thermo time switch were disconnected I reconnected them. I was told to disconnect these by another mechanic...any thoughts???

[pbanders]

Well, a first thought is to start at the beginning. First, check the mechanical state of the engine. You're looking for good compression, over 100 psig and within 15 psig across all cylinders. Next, you need good vacuum, 14 to 18 in.Hg at idle. Make sure the valves are adjusted correctly. Now, check timing and distributor operation. Verify that the vacuum adv/ret mechanism is working correctly. Check timing (I don't think you check dwell with the Pertronix). Check plug condition and gap. Examine plugs for any fouling and note apperance for mixture.

Check battery terminals and both groundstraps to make sure they're ok. Check the FI ground connections at the back of the engine. Examine the FI harness for any broken, loose, or disconnected wires.

Now, at this point, you can start looking at the FI system to see if that's what is causing your problems. The aux air regulator should be connected (assuming it's working correctly), as it supplies additional air needed to overcome engine friction when cold. The thermo switch (the '73 has a thermo switch, only the 75-76's have a thermo-time switch) needs to be connected to operate the cold start valve (check the switch and valve for proper operation).

A tool that can help debug FI mixture problems is an Air/Fuel meter. CB Performance sells a pretty nice one. I have an old Heathkit meter that I used to debug my system, it was very helpful.

Brad Anders