Resolved 1G - Check engine light doesn't stay on

[97egl]

Sorry if this has been answered before but searching for check engine light issues brings up so many threads that I couldn't find anything

Anyways on the 1G (OBD1) cars does the check engine light normally "reset" every key cycle.

I drive the car for a couple mins. It throws code 31 for the knock sensor (it's unplugged). Then I get to where I'm going and shut the car off and when I start it back up the check engine light doesn't stay on until it sets the code again. I swear on my other cars the check engine light would stay on once it got a code until the issue was fixed.

It makes me worried there is a problem with the ECU

 

[DSMPT]

If my memory serves me correctly, it's normal. When the knock sensor goes bad, disconnected or something is wrong in its circuit including inside ECU, each time when you off the key the CEL 31 would disappear and it would come back in a while after you start the engine or if you rev the engine.

 

[motomattx]

Are you possibly missing your backup 12v to the ecu? any blown fuses?

 

[97egl]

Are you possibly missing your backup 12v to the ecu? any blown fuses?

Pin 103 does retain power with the key off. I've got some blown fuses but those are due to pop up relay issues and window switch problems.

I may just unplug the knock sensor on my 90 and see if the light acts the same way. But everything electrical on the dang 90 is different so I don't know that I'd trust that it's supposed to act the same way. Probably should pull the ecu and look at the caps just cause they're 30 years old....

 

[steve]

If you haven't replaced them already you should do so ASAP regardless of what they look like.
Same for anybody with an automatic to the TCU.

1990 ECU's are special, they like to burn when the 47uf cap goes dry.

 

[waltah]

If you look up the code in the Mitsu shop manual for your car it will tell you what the CEL is supposed to do. All the different possibilities (I think?) exist. Some codes once the light is on it stays on until it's reset. (With scan tool or by disconnecting battery.) Some it'll come on but go off if the condition goes away ... some go off more quickly than others. And some will reset per trip I think.

 

[97egl]

Not sure how it was even still running. One of the capacitors was rolling around inside the case. I had planned to change the caps eventually as other maintenence was more important at first so I hadn't even checked them yet since the car ran fine. Tossed a spare epromm ecu in that I had bought. Light is staying on now but may be that it's stored more codes since the ecu isn't broken.

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[steve]

Not a 90 but same problem, dried out and shorted.