Check Engine Light Question

[peterpilot9]

1990 300 na. Completed a plenum pull with injector replacement and cleared all codes. Car runs great from idle to redline. AC blows cold, new inspection and all is well.

Except. When I drive the car for a few minutes above about 45 MPH, the check engine light illuminates. Still runs the same with no issues. When I turn the engine off, the check engine light goes out and will not come back on when I turn the key to the ON position. I can't get the light again unless I repeat the driving pattern. It seems to clear every time I turn off the engine.

So, is it OK to do the trick with the ODB1 port and try to check the code while the engine is running? Can you even check codes with the engine running?

I want to make sure I don't do anything to screw up the ECU. Friend of mine says this is known as a "soft code" but I can't find a reference to this at any source.

Thanks for any advice. It is appreciated.

[ThisIsSparTTa]

By soft source, he probably means a sensor is getting a bad reading, but not that the ECU thinks it has failed which it would probably see as an open or closed circuit.

I'm not sure about the manual procedure with the engine running, but you can go on eBay and grab a Consult to USB cable for $30. ECUTalk is free software that will let you read codes at any time, and will also you give you sensor readouts, which can also be highly useful. I'd recommend grabbing it for future use.

[peterpilot9]

Thanks. That sounds like a good plan.