Mechanical Temp Gauge -- Engine won't start!!

I have a '91 S-10 Blazer 4.3L

Last night I installed an aftermarket mechanical coolant temp gauge (made by Sunco).. replacing my stock temp sensor with the one on the gauge. Everything installed great.. But!

When I tried to start the engine, it wouldnt run! It just kept turning over. We reconnected the stock temp sensor, and my brother barely got it to start with the pedal to the floor. After it ran for about 5 minutes, it sounded fine so we decided to drive it to the shop. It got there fine, had power, and sounded great. We shut it off at the shop, tried to start it, and again it wouldn't start!

Has anyone had this problem? The shop says it may be because the stock sensor is not in there.

Also, is there any other place where I could mount the gauge sensor, so I can keep the stock sensor in its original port?

Any help would be greatly appreciated!

it might need the sensor to run of a loop. try driverside head there should be a 3/8 squre head plug thats where i would put a mech guage.

there should be 2 different sensors on the engine. 1 sensor is the ect (engine coolant temp) and that sensor tells the ecm the coolant temperature so it can calculate fuel quantity output to the engine. the other sensor is for your coolant temp gauge in the instrument cluster. make sure you replaced the coolant temp gauge sensor and not the ect sensor. with out that sensor hooked up your ecm thinks your coolant temperature is -40F which would make it run extremely rich.