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Discussion Starter #1
I have an 85 El Camino SS with the 305. Car runs decently, but when I get on the highway and just get to cruising speed the check engine light comes on. If I am running around town, spot to spot, stop and go, no issue.

I replaced the pcv valve a few weeks ago. Any ideas for me to check on?

Thanks for your input in advance.

DT-
 

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Well, the first hot bet is the oxygen sensor. But you can go to AutoZone or some such place and they will read the codes.
 

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The check engine light coming on tells you that the ECM is seeing something it doesn't like !! The ECM will leave a code for the problem or 2 !! You can get the codes without the scan tool or you can go to one of those local auto parts store that will scan it for free!
You can do it with just a simple paper clip. Locate the ALDL (Assembly Line Diagnostic Lead). It is usually under the drivers side of the dash. It may have a cover over it, and may even say "Diagnostic Connector".
This is the ALDL. The two terminals that you need to be concerned with is in the picture. It is terminals "A" and "B". Simply turn the Ignition to "Run" (Do not start the vehicle) and put the paper clip in the two terminals. Now watch your CEL. It will start flashing. It will look like Morse Code, but it isn't, and is real easy to interpret.



The first code that will flash will be a code 12. It will be "flash, pause, flash, flash, pause, pause," and repeat the cycle three times. The ECM will flash each code three times, and move on to the next. If you have no codes stored, it will keep flashing code 12. Code 12 simply tells you the ECM is not receiving any pulses from the distributor (since the car is not running). If you pull codes with a scantool and the car is running, this code will not show up. Its no big deal, it lets you know the ECM is in the "Field Service Mode" You will also notice the cooling fans come on, this is normal.​
 

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ZERO MPG
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I have an 85 El Camino SS with the 305. Car runs decently, but when I get on the highway and just get to cruising speed the check engine light comes on. If I am running around town, spot to spot, stop and go, no issue.

I replaced the pcv valve a few weeks ago. Any ideas for me to check on?

Thanks for your input in advance.

DT-
Mine does the same thing only on the freeway it pulled code 24
Vehicle speed sensor
poor elect. connection. Open or short in circuit, Defective VSS, Defective ECM-a FAULT IN THIS CIRCUIT SHOULD ONLY BE INDICATED WHEN THE VEHICLE IS IN MOTION.
 

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Discussion Starter #6
Guys thank you so much for the assistance~

I will check these out over the course of the week
 

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ZERO MPG
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Guys thank you so much for the assistance~

I will check these out over the course of the week
When you figure it out please let me know what it was so I can fix mine.:beer:
 

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If you find that it is indeed the O2 sensor, I suggest buying an OEM one. Installed Bosch O2 sensors on my 4.3 S10 and it kept throwing a code until I went back to GM ones. Others may not have had any issues with non-GM sensors, but that was my experience.
 
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