Testing out the "Slow Down" system

After having been on a dozen or so drives, it's clear to me that I need to get to the bottom of this "slow down" light issue. A month or so ago when I installed functional catalytic converters, I started getting an intermittent slow down light on the 1-4 cylinder bank. This means that the 1-4 cat was getting hot enough to trigger the alert system.

First I needed to know where was the problem. The first thing to check was the actual alert system itself because I didn't know if the light was a false alert or an actual overheating situation. The alert system consists of a K-type thermocouple in the outlet of each catalytic converter. A thermocouple measures temperature by generating a very small amount of electricity as it heats up. The thermocouple is attached to an ECU, which monitors the voltage and turns on the light when the voltage reaches a set point. My goal was to test the ECUs and test the thermocouples themselves. To do this, my buddy SL brought over an instrument meant for testing thermocouples.



The thermocouple ECUs are mounted to the end plate in the passenger's side footwell. I unscrewed this plate to expose the ECUs and the wiring. First, we tested the ECU operation. My putting the instrument into output mode, we could simulate the microvoltage as output by an actual thermocouple. After the first round of testing, we could see that the ECUs were good and behaved within 10 degrees F of each other. Using the instrument, we piped in a test signal of 36.05mV, which corresponds to a temperature of 1630 F. At this temperature, the slow down light begins to flash at an interval of about 13 seconds. As we cranked up the test temperature, the slow down light flashed faster and faster until it goes solid at 1710 F or 37.84mV. Wow, 1710 degrees. That's hot!

The next test was to check the output from the real thermocouples. We touched tester leads to the thermocouple wires and read 59 degrees F from each side with the engine not running. So far so good. The ambient temperature output was the same from both thermocouples at ambient temperature. Next, we started the engine and let it come up to temperature. Switching from side to side, we could see each thermocouple output increase at roughly the same rate. After a few minutes, we brought the engine up to 3000 RPM and held it there. As the temperature increased past 1000 F, it was clear that the rear bank was reading about 150-200 F higher than the front.

All in all, we spent probably 2-3 hours in testing. Our conclusion is that the slow down system is indeed working as intended and the rear bank is actually running hotter than the front. The next question is, what to do about it?