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K. Wiggins
K. Wiggins, Chevy Technician
Category: Chevy
Satisfied Customers: 2785
Experience:  ASE Certified Technician AA in automotive Technology, 15+ years in the trade
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Check engine light comes on at idle then shuts off going

Customer Question

Check engine light comes on at idle then shuts off going down the road. Rpms go up and down like it is shifting when keeping constant speed. Just replaced plugs and wires thinking that was it but it's doing same thing still. Check engine light codes were 21 and 41 but checked today and it was only throwing 41
JA: Does the light stay on, or does it blink?
Customer: It will stay on at idle like when I'm at a stop light and then shuts off when I get up to speed again
JA: What is the model/year of your Chevy?
Customer: 1985 chevy k10 5.7l carbureted
JA: Are you fixing your vehicle yourself? What have you tried so far?
Customer: Yes. Well I have a mechanic that works on old trucks and he showed me how to read the codes. I've replaced spark plugs and wires, cleaned air filter, and replaced ignition control module under distributor cap
JA: Anything else you want the mechanic to know before I connect you?
Customer: Yea also the reason why rpms are going up and down like it is shifting when keeping same speed level ground
Submitted: 7 months ago.
Category: Chevy
Expert:  K. Wiggins replied 7 months ago.

The code 21 (TPS issue) can cause the mixture control solenoid to go rich/lean resulting in engine speed changes. The code 41 can also do this. You will need to use the charts below to figure out what the issue is. An older style GM OBD1 code scanner with live data will make this much easier to figure out as you can see the TPS and MAP values while driving.

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