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2000 Gmc Sierra Rough idle


Kelsey92504
03-06-2008, 09:40 PM
I have a 2000 Sierra with a 4.8L and when its cold the first start of the day it starts up and idles high when you first start it like it supposed to and then once it warms up a little bit and idles down it acts like it wants to stall or rough idle. Less than a year ago I had the fuel system serviced and they burned all the carbon out of the motor, and I just changed the fuel filter. I was going to change the plugs and wires but I talked to another guy with the same truck and motor he was having the same problem and he said he changed his plugs,wires and fuel filter and he still had the problem. I have no engine light or code. My buddy says he is almost 100% positive it is the temp sensor. Does the 4.8L have 1 or 2 temp sensors? Any help would be greatly appericated. Thank you

2000CAYukon
03-06-2008, 10:03 PM
I would start by cleaning the throttle body. I usually spray some TB cleaner on a clean rag and then open the TB by hand and clean the TB with the rag. If it is really dirty, it should be removed and cleaned. Be careful not to get any TB cleaner in the throttle position sensor.

You don't list mileage but if the miles are high, the plugs and wires may be needed but may not be the cause of the rough idle.

//2000CAYukon

Kelsey92504
03-08-2008, 12:51 PM
I checked the mass air flow sensor and the throttle body and both were spic and span clean. Another member told me to check the Iac but I have to find it. Thanks for your help in advance.

Kelsey92504
03-10-2008, 05:52 PM
I unplugged the Iac and started my truck and it seem to run better. I checked it for resistance and it was reading between 40 and 80 ohms. While the truck was running with the Iac unplugged it seemed like the gas pedal was sticking a little bit. If you hit the gas the rpms would go up and slowly come down, but with it plugged in it didnt do that. With it unplugged I also got no check engine light, should it have came on? Because of the way the rpms would slowly fall with it unplugged does that mean it is dirty or need to be replaced?
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2000CAYukon
03-10-2008, 09:25 PM
I unplugged the Iac and started my truck and it seem to run better. I checked it for resistance and it was reading between 40 and 80 ohms. While the truck was running with the Iac unplugged it seemed like the gas pedal was sticking a little bit. If you hit the gas the rpms would go up and slowly come down, but with it plugged in it didnt do that. With it unplugged I also got no check engine light, should it have came on? Because of the way the rpms would slowly fall with it unplugged does that mean it is dirty or need to be replaced?
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If it is sticking a little bit, it probably needs to be cleaned. I find that I need to give mine a good cleaning every 10-15K miles. Unplugging the IAC may not set a light right away but it would eventally come on. With the IAC unplugged, the PCM may raise the idle since it does not know throttle position.

Give the TB a good cleaning and go from there.

//2000CAYukon

Kelsey92504
03-11-2008, 04:02 PM
I took out the iac, it was covered in black soot as well as the passage. I cleaned the throttle body, passage and iac with throttle body cleaner. Also replaced the pcv valve, it to was dirty. At first after I got it back together and started my truck, it was acting like the iac was bad. When you rev it up, the engine rpms would slowly fall. So I unhooked the negitive battery cable and unplugged the iac and then hooked it back up. Then re-started truck and it was idling good. Took it for a test run and it ran better, more responsive throttle. Don't really know if it did fix the cold idle, because it usually only does that in the morning or late at night. Im pretty sure it did, because u can tell a difference. Thank you for your help. Very much appreciated

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