could use a little advice

[90 cheyenne]

I have a 90 K1500 cheyenne 350. Awesome truck, love it, simple but functional, I have a miss that i cant seem to get rid of though. Really noticeable at idle. I was doing a minimum air rate check on it and when I unplugged the iac motor and started the engine, it was smooth as glass!?! Even in gear,smooth, but plug the iac back in and start the engine, and it misses again. The iac goes through all its steps as far as warmup, so I know its not that. If anybody has any suggestions, they would be greatly appreciated!!!!!

[special-K]

Welcome to the site. I'm not much on modern systems,so can't help you there. Don't worry about the wrong section. People make the mistake often.You'll find it moved to the 88-98 message board shortly

[Square_78]

I ment to say what he said

Welcome to the forum. I didnt see that it was your first post!

cheers
Lonnie

[ChevyTech]

Before you put any more codes in it by unplugging things, check to see what trouble codes are stored in the ECM.

Here is a link to instruction from the sticky at the top of the board:
http://67-72chevytrucks.com/vboard/s...5&postcount=11

Next try running it with the O2 sensor unplugged and see if it runs smoothly.

How does it run cold before the engine warms up?

[90 cheyenne]

Thank you for welcoming me to the site, and thank you for responding so quickly. As per Chevy Techs response, I did unplug the o2 sensor and drove it around town and it seemed to do great. Idle was smooth. Can it be ran like that all the time, or will it cause damage somewhere else? The truck has no cat. converter, and is no longer subject to emissions in our state.

[ChevyTech]

With the oxygen sensor unplugged the system can not go into closed loop operation which is where it will get the best fuel mileage and have the most power.

The test, seeing how it ran when forced to stay in open loop, indicates the oxygen senor reading is causing problems. The bad reading could be caused by and exhaust leak letting air in the exhaust, by a cylinder misfire, or by a bad oxygen sensor.

Exhaust leaks:
Air getting in the exhaust system can affect the o2 reading. In open loop operation (cold) the ECM ignores the o2 sensor reading, but in closed loop (warm) the ECM uses the o2 reading to determine if the engine is running lean or rich. If the exhaust system leaks ahead of the o2 sensor, (or just behind it), the pulses in the exhaust will draw in air and cause the o2 sensor to get an incorrect reading. A vehicle with an A.I.R. will have the same problem if the air management valve allows air to be injected in the exhaust manifolds during closed loop operation.
Exhaust flow moves forward and backward in the exhaust system more then many people realize. A term used in discussion of this is “exhaust scavenging”.

If it has an exhaust leak fix it and see how it runs.
If there are no exhaust leaks, and the truck does not have Air Injection Reaction, then I would test the oxygen sensor or try a new Oxygen sensor.

[90 cheyenne]

Thank you Chevy Tech, ill check for exhaust leaks first and then go from there. I appreciate it.