Thanks for everyone's suggestions and explanations.
I took a look at the stock fuel heat exchanger and didn't see any wires going to it, so I'm assuming it is controlled by an integral mechanical thermostat, not the fuel temp sensor. It looks like the temp signal is only used by the engine control module, as proposed by Marty. There's no telling what adjustments the computer makes based on the fuel temp.

Thanks Terry for pointing me to MDISAV. He actually helped me pick out the car and gave me tips as I was installing the veggie conversion. His 99 E300 hasn't shown the fuel temp sensor error with his Elsbett veggie system, so I'm assuming my temp sensor is the culprit.

Len, were you able to replace just the temp sensor, or is it only available with the fuel shutoff?

The CE light has gone off on my car. I'm going to give it a few more days to see if it comes back. If not, I'll address the leaking delivery valves and save the temp sensor for another day.

Thanks,
Paul

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1998 E300D 1-tank Elsbett Veggie system
1986 190D 2-tank Veggie system -Sold