Coolant temp sensor

SLYDELL

MEMBER
1985 PONTIAC TRANS AM
175,000 MILES

Coolant temp sensor

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.

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Replied on April 25, 2013

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.

Tiny Answered by caradiodoc (expert)
12,469 answers provided

Replied on April 26, 2013

thanks for responding. The engine runs terrible when cold, it only run bad with the cooant temp sensor connected. When disconnected it runs better. With connected it stays in a open loop. Autozone tested the coolant temp sensor and the pigtail, it tested fine. I replace the oxygen senser, plugs and wires, new CTS. New computer, new rotor, everything inside the distributor cap, and it still run bad only with the CTS connected, it runs like its missing, even if I connect the coolant temp sensor to a cts that's off the car it make the car shake and misfire. I sure hope you can help me, a mechanic told me its in the wiring and he wants $600.00 to find a short.
Where do I look for a short? Or does this sensor work with other sensors? This is a carb car not fuel injection.I am so confuse. We even ran 2 wires from the pcm straight to the cts, same problem. When the car is cold or hot, with the cts on it runs bad, off it runs ok.

1 Reply

CARADIODOC

MECHANIC
33,723 POSTS

You need to find a different mechanic who specializes in engine performance problems. You can't have a wiring problem as the first one thinks. If the wires for the CTS were shorted together or grounded the problem would be there all the time whether the sensor was plugged in or not. Same if one of the wires is broken. Connecting the sensor wouldn't make any difference.

You need to take the voltage reading I mentioned earlier. You can expect to find around 2.0 to 4.0 volts. If you do, the entire circuit is working properly and the problem is due to the computer using that information to make other decisions. When the CTS is unplugged the voltage on the signal wire will go to 5.0 volts. That is an unacceptable condition and should trigger a diagnostic fault code and turn on the Check Engine light. The computer will know it can't rely on that reading so it will inject an approximate value and run on that.

The mixture solenoid in the carburetor doesn't have enough range to cause a misfire condition. Even if it stays in the full-rich or full-lean state, it should not cause a misfire, especially as bad as you described. Go back to the basics, spark, fuel, and compression. When the misfire is occurring, introduce some propane into the carburetor's air flow. If it smooths out, suspect a blocked port or passage in the carburetor. Perform a compression test to be sure all cylinders are about the same. Ground out the spark plug wires one at a time to see which cylinders are misfiring. You may need to put the engine on a scope to view the spark firing lines. If they randomly drop out you should be smelling unburned fuel at the tail pipe. That can be caused by a worn bushing for the distributor shaft.