Anyone familiar with '80s GM engine/emissions controls?

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Old 10-26-12, 09:34 PM
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Anyone familiar with '80s GM engine/emissions controls?

Hey all,

I've got an '86 Trans Am with a 305 V8 with the Quadrajet feedback carburetor and the 700R4 transmission. 53,150 miles. The carb had been leaking so I had it rebuilt, and then I had codes 13 and 34 for no signal on the O2 sensor and for poor vacuum on the MAP sensor. The latter turned out to be an unplugged vacuum hose to the MAP sensor, at the carb, where it had either come loose or where I hadn't hooked it up. I checked the wiring for the O2 sensor and found no obvious faults so I simply unplugged it and reconnected.

When I road tested it the stumbling and bog that I had been getting was gone. When I was getting ready to get off the freeway though, backing off the throttle, the Service Engine Soon light came back on again for about fifteen seconds until I slowed to almost a stop. Once it shut off it didn't come back on again for the remaining six miles of the road test. I have not yet had a chance to check for codes again.

Any suggestions, besides checking for codes?
 
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Old 10-27-12, 04:25 AM
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If it did it on deceleration, I would suspect an EGR valve malfunction, since it activates in negative pressure. Just a stab in the dark.
 
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Old 10-27-12, 09:31 AM
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I'll take a look. Previous owners had the intake manifold replaced by a dealership service department so I'd hope that EGR isn't an issue, but I'm also not too familiar with how GM set up their emissions controls. I did realize that my idle speed is a bit high, about 1100RPM, so I'm going to bring that down first to see what happens.

This car isn't too much fun to work on. It's a spaghetti nightmare of vacuum lines and the engine is set very far back from the front of the car and is very low in the bay.
 
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