Check engine light

So my CEL has come on twice since I've owned this car. both times have been after about 25 minutes on the highway and 75mph or so. Both times the light went back off within 10 minutes or so. I checked the codes this time and all I get is code 12.... then code 12..... nothing else. Any ideas? It seems to run fine - just the same as it always has... I dont drive it hard or floor it... theres no reason with the 305... it'll get there when we get there LOL.

That code comes up as a distributor pulse issue or in diagnostic mode. If no misfire, perhaps there is a short in the ALDL wiring that is putting it into diagnostic mode.

booster wrote:

That code comes up as a distributor pulse issue or in diagnostic mode.  If no misfire, perhaps there is a short in the ALDL wiring that is putting it into diagnostic mode.

what i read is that it'll display code 12 when in diagnostic mode (when i have the little thing plugged in that essentially jumps the terminals like you would do back in the old days) and then it'll display the trouble codes, then finish with 12 again.... all im getting is the 12 and nothing then 12....

1991BlueBertha wrote:

booster wrote:

That code comes up as a distributor pulse issue or in diagnostic mode.  If no misfire, perhaps there is a short in the ALDL wiring that is putting it into diagnostic mode.

what i read is that it'll display code 12 when in diagnostic mode (when i have the little thing plugged in that essentially jumps the terminals like you would do back in the old days) and then it'll display the trouble codes, then finish with 12 again.... all im getting is the 12 and nothing then 12....

Yeah, I noticed that also, but if there are no other codes diagnostic mode would probably not have anything to display so would go right to the final flash of 12.  Just a guess though.

You might want to try putting it into diagnostic mode manually and see if it does the same thing.

Usually, a CEL coming on while driving at highway speeds means that the computer is not recognizing a leaner mixture when the EGR valve operates. The valve may be sticking, the passage may be partially blocked, or the solenoid stuck, but not long enough to set a code. It could also be the O2 sensor not reading the leaner mixture instantly. It is difficult to diagnose, because it only happens at speed on the road.

Fred Kiehl wrote:

Usually, a CEL coming on while driving at highway speeds means that the computer is not recognizing a leaner mixture when the EGR valve operates. The valve may be sticking, the passage may be partially blocked, or the solenoid stuck, but not long enough to set a code. It could also be the O2 sensor not reading the leaner mixture instantly. It is difficult to diagnose, because it only happens at speed on the road.

any suggestions on unclogging or unsticking the EGR valve, Fred? Any snake oil, seafoam type things that might help?

I do not have anything. You can check the valve for operation by pushing on the membrane. You can check the solenoid by putting 12 V across it while the engine is running. If you have vacuum, it will work, and the engine will stumble. If it does not stumble, with either test, then it is probably the port. If it stumbles, the parts are working. Check for vacuum leaks.

In saying that it is usually the EGR that causes the CEL to come on when driving, that does not mean that it is the only thing that will turn it on. Low voltage will do that as well.