car runs very rough when its cold throw code 25 off and on which is the plenum temp sensor after the car is warmed up it runs greatt...


what is the problem???

 

im gonna go with the temp sensor....start there and work you way back, its not gonna throw a code for no reason

 

should i replace it or is it going to be a shorted out wire??

 

What a code 25 fundamentally means that the ECM is seeing an unreasonably high temperature. This would explain why it runs so bad cold because when its actually cold the ECM thinks its pretty hot and calculates the fuel requirements as such. Specifically the conditions te set a code 25 are listed below:

ECM reads a temperature greater than 150 C according to the MAT sensor
The time since the motor has been started is greater than one minute
The vehicle is moving
All above conditions are met for at least 12 seconds

Now the way the temp sensor works is the resistance is inversely related to the temperature. This means as temp increases resistance drops. So a short could cause the ECM to see it as a very high temperature reading. You can measure the resistance of the sensor and get an idea if its reading correctly. And thats where i would start personally.

 

thanks

that makes perfect sense


so how would i get the reading of what the sensor is sending???

 

Well first i would measure the resistance of the sensor itself directly with an ohm meter and if that checks out that means the sensor is good. If the sensor was good I would then reconnect the sensor and try to measure the resistance from the ECMs connector. By doing this we are trying to read the sensor as the ECM does. Since weve already measured the sensor by this point and know its good any issues we see with the resistance value now would be the result of the wires connecting the ECM to the sensor. If this checks out we have an intermittent problem or a bad ECM.

 

imma get to it tonight....what should the sensor itself ohm at??

 

Well it depends on the temperature lol. I would do a search as the charts that compare resistance vs temperature are out there because it can read over a huge range everything from 100,000 ohms at 40 F to 70 ohms at 255 F so the range is pretty wide lol.