Coolant temp sensor

SLYDELL

MEMBER
1985 PONTIAC TRANS AM
175,000 MILES

Hi sorry I am just getting back to you, I couldn't sign on for some reason.I copy and paste my question I had asked you.

Hi, I have this problem with the coolant temp sensor.I replace the coolant temp sensor and the pigtail, but as soon as I put the pigtail on the new cts my car run rough and start missing or shake.I replace the cts again, same thing, we check for broken wires all the way to the pcm, found none, when I install the pigtail again on the cts, it make my car run bad again, I had the pigtail and sensor check at auto zone, its all good. Could this be my pcm going bad? I just dont understand this. Theres no open wires or split wires going from the pcm to the cts.I have been trying for 2 weeks figuring this out, cant get it. Do you know whats causing this? It only run bad when I connect the harness pigtail to the cts. I hope to hear from you soon. And thanks. Glen ps. My car has the electronic rochester carb. No codes, please help me, now its a month trying to fix this issue.

2 Replies

CARADIODOC

MECHANIC
33,881 POSTS

This problem likely has nothing to do with the coolant temperature sensor circuit. The clue would be the engine runs okay when it's still cold. The Engine Computer modifies fuel metering based on readings from the oxygen sensor but it can't use those readings until the sensor is warmed up to 600 degrees. The computer goes into "closed loop" where it uses those readings when the coolant temperature reaches about 180 degrees. At that point the computer can become confused by an excessively rich or lean condition. A vacuum leak is a common cause of running too lean.

Also keep in mind ignition timing advance may be cancelled when you unplug the coolant temperature sensor. The spark won't have such a big gap to jump between the ignition rotor and distributor cap terminals. When you reconnect the sensor, the ignition timing advances and it takes more voltage to fire the spark plugs. You could simply need new spark plugs and wires. The coolant temperature sensor can not cause misfires on its own. A fast and simple way to tell if the sensor and circuitry are working properly is to connect it with an accessible jumper so you can measure the voltage while it is plugged in. With it unplugged you will find 5.0 volts on the wire. With it plugged in you must find between 0.5 and 4.5 volts. The higher the temperature, the lower the voltage you will find.