Faulty "CHECK ENGINE SOON" light
Thread Starter
Intermediate
Joined: Mar 2008
Posts: 31
Likes: 0
From: Santa Clarita Ca.
Faulty "CHECK ENGINE SOON" light
My 90 L-98 has started exibiting the check engine soon light after driving for 15 to 20 miles. I pluged a tester unit and found no hard codes, all appears normal. After the engine is turned off and fired up again the light does not come on until the above stated miles are on the odometer.
Racer
Joined: Jun 2006
Posts: 361
Likes: 6
Is this highway miles or city? The egr temp switch will through a soft code if the ECM doesn't see what it wants when it tests the switch. Usually this happens at a certain temp and speed, can't remember off hand. It resets once you turn off the vehicle. This was happening to mine when I first got it.
Do a search for EGR Temp Switch, there's a bunch of info on the board.
Do a search for EGR Temp Switch, there's a bunch of info on the board.
Melting Slicks
Joined: Jan 2009
Posts: 3,181
Likes: 286
From: Puyallup Washington
Most likely the "Check Engine Light" is not faulty. It is being told to turn on. You should hook a code reader up to the car and go for a drive and see what code gets set when the light comes on.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
Sounds like a non-functioning EGR system. The ECM looks for a change in manifold pressure when it commands EGR. If it doesn't see that pressure change it turns on the SES light. On the earlier C4s, the ECM looks for the EGR switch closing.
The coolant temperature has to be over a threshold value and you have to be going faster than another threshold value for another threshold value of time for the ECM to command EGR. That's why the SES light turns on after a few minutes of driving.
The coolant temperature has to be over a threshold value and you have to be going faster than another threshold value for another threshold value of time for the ECM to command EGR. That's why the SES light turns on after a few minutes of driving.
Racer
Joined: Jun 2006
Posts: 361
Likes: 6
Sounds like a non-functioning EGR system. The ECM looks for a change in manifold pressure when it commands EGR. If it doesn't see that pressure change it turns on the SES light. On the earlier C4s, the ECM looks for the EGR switch closing.
The coolant temperature has to be over a threshold value and you have to be going faster than another threshold value for another threshold value of time for the ECM to command EGR. That's why the SES light turns on after a few minutes of driving.
The coolant temperature has to be over a threshold value and you have to be going faster than another threshold value for another threshold value of time for the ECM to command EGR. That's why the SES light turns on after a few minutes of driving.
Instructor
Joined: Feb 2008
Posts: 177
Likes: 5
From: Toronto Ontario
The code that triggered the light to come on should be stored in the ECM memory until the battery is disconnected or the code is cleared by a scan tool. If no codes are present after the light has come on this seems to be a problem in it's self.
Use the simple paper clip method to determine if codes are present or not. If no codes are present after the CEL has been on then understanding why that's the case is a problem to investigate.
Use the simple paper clip method to determine if codes are present or not. If no codes are present after the CEL has been on then understanding why that's the case is a problem to investigate.
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