Onboard computers pcm/ecm. . . all of them

[kjprag01]

This is going to end in total ridicule but i have to ask. Today i was changing a head light and while disconnecting the battery I noticed my carbon canister had a vacuum line completely missing. Clearly, I replaced that line fixed my headlight and continued about my day. Then i did the most dangerous thing, I got to thinking, why didn't my check engine light come on, how long has that line been missing, (which looked like, it just slowly rotted away) and Can the onboard computers fail and the vehicle still operate normally?

 

[swynx]

This is going to end in total ridicule but i have to ask. Today i was changing a head light and while disconnecting the battery I noticed my carbon canister had a vacuum line completely missing. Clearly, I replaced that line fixed my headlight and continued about my day. Then i did the most dangerous thing, I got to thinking, why didn't my check engine light come on, how long has that line been missing, (which looked like, it just slowly rotted away) and Can the onboard computers fail and the vehicle still operate normally?

when i got my pickup i also had one of those lines missing, but my box was also melted, ive since then deleted the box and 99% of the vacuum/egr lines with no issues. typically the only issue that comes with this is a high idle, or laggy response.

typically when the computers do "fail" they go into a limp mode i suppose you would say, i cant remember if it would be open loop or close loop. (i think open loop) anywho, there is a set of default perimeters the ecu would run off of to get you home. unless of course the ecu really did fail and just quit working then you would likely be stuck on the side of the road.

 

[85_Ranger4x4]

Computer probably just thought there wasn't much vapor in the canister as it was sucking outside air instead of the vapor.

Newer ones monitor the canister closer (control it with valves)

 

[RonD]

If you want to check your PCM just unplug a main sensor, like MAF sensor, start engine, give it a minute to run its tests and see if CEL comes on.
If it does turn off engine plug MAF back in and restart, CEL should not be on again, but the MAF code will be stored in memory, as it should be, so just remember that next time you read codes.

Yes, the EVAP system was not heavily monitored in 1995, you could get an EVAP code but not always.
On later models when you get an EVAP code it can take days to get code to clear even after fixing it, so be happy

 

[Andy D]

Similar experiences on the Rat with rotted off evap hose. No CEL . I thought the bulb was broken until I saw it on the panel with the key on..