After I cleaned the engine bay of the new Biarritz I drove the car out of the garage. The car started beautifully, but for some reason it would not idle down and run on fast idle speed - even when it was fully warmed up.
The only way I could get it to normal idle speed was to manually release the fast idle cam on the carburetor. For some for me unknown reason it did not release itself - even when the car was fully warmed up...

As I´m not exactly an expert when it comes to carburetors, it took me a while to figure out what was going on and what the problem could be.

I got myself the Rochester M4MC / M4ME Carburetor Manual online and started reading. The trouble shooting section did not mention my problem and symptoms though, at least I learned a lot by reading through the manual...

The carburetor in my 1978 Eldorado is a Rochester M4ME # 17058531 (late Federal).

After a lot of reading I suspected that the electric choke thermostat must be the problem.
I removed it to test it like it is described in the manual.

Electric Choke Diagnosis and Checking

In order to have good engine performance during warm up and be able to pass Federal exhaust emission standards, the electric choke must function properly .

Possible reasons for the choke not operating properly are listed below.

1. No engine oil pressure.
2. Malfunctioning oil pressure switch.
3. No current to oil pressure switch due to:
a. Burned out 15 amp "radio -idle stop" fuse.
b. Broken wire to switch (18 brown/double white).
4. No current between choke coil and oil pressure switch due to:
a. Broken 18 light blue lead wire.
b. Wire terminal not locked on coil terminal.
c. Ground circuit incomplete between grounding plate of choke assembly and housing.
5. Failed choke coil assembly.

A voltmeter or continuity light may be used to check the circuits for continuity to the oil pressure switch and the choke coil.

If it is suspected that the choke coil assembly has failed, the following check may be made.

1. Remove coil from carburetor and cool to room temperature (above 60°F).
2. Attach a jumper wire between positive battery terminal and terminal of coil assembly. Attach a second jumper wire between negative battery terminal and grounding plate of choke coil assembly.
3. The tang of the coil should rotate 45°in 54 to 90 seconds.
4. If coil fails to rotate or exceeds the above timing specification, replace coil assembly.
5. If coil is within above timing specification then coil is good and problem is elsewhere.
6. Reinstall coil and set to proper index (Fig. 52).
If coil is cooled off sufficiently the choke valve will close when throttle is opened slightly. Attach a jumper wire between the positive battery terminal and choke coil terminal. Choke coil should warm up and the choke blade can be observed opening, indicating a good and properly grounded choke coil assembly. At room temperature the choke blade should be wide open in approximately 90 seconds.
If the choke does not operate properly after the coil has been proven satisfactory , check out the other possibilities that prevent current from getting to the choke coil.

After I performed the test procedure above it was clear that the choke coil was the problem.
I ordered a new one from www.rockauto.com and it arrived within 3 days. The manual mentions the original part #17059968 (the original one on my car showed #17059969 though - but every parts list I checked does not list this number...?).

I got a replacement from Airtex # 2C1045 and it seems to fit.
I installed it today and the choke works as it should again.

This was the original choke thermostat on the car . #17059969

The Airtex replacement part in the car.

The original part and the new part - slightly different at the tang of the spring - but it works and fits perfectly...

original and new part

The Airtex replacement choke thermostat

Tags:1978, Eldorado, 1978 Cadillac Eldorado Biarritz, 1978 Cadillac Repair Tip, Rochester, Carburetor, Choke, Thermostat, Electric Choke Thermostat, 17059968, 17058531, High Idle, Restoration, 17059969