Question about 1998 Chevrolet C1500

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98 Chevy 5.0 fast idle

I have had some problems with my 98 chevy truck with a 5.0. It started out where I had rough idle when I would come to stop lights, seeming like it would load up with fuel. I took it in and they said I had a bad MAP sensor and post cat 02. I took it home and replaced them and cleared the codes. Then I got these codes, P0151, P0300, P0507, P1153. I checked the right bank plugs and they were fouled, replaced but didn't fix the rough idle. Replaced the right bank 02 and cleaned the idle control valve, now runs much smoother, but still a fast idle in park 1200-1300 rpm. It will idle fast in gear coasting as well. Code P0507. Any ideas on the fast idle??

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  • jdohlman Mar 02, 2009

    Already done, I cleaned the entire throttle body.

  • v8intercepta Mar 03, 2009

    I have the same problem with 1998 chevy truck 5.7 liter. I cleaned the throttle plate and replaced the throttle position sensor but it still idles too high. I may try the idle air control valve next. I do not have a check engine light. Does that mean the O2 sensors are ok?

    I noticed that my engine temperature is low. I did replace the thermostat a long time ago with a 165 deg t-stat. I wonder if it is stuck open. Could a low coolant temperature cause a high idle? Also ive noticed when coasting I can put the transmission in neutral and eventuallyt the idle speed will reduce some.

  • Anonymous Mar 22, 2014

    when the vehicle was diagnosed,a code for the O2,sensor bank 1 sensor 2 , had the sensor replaced cleared the codes,took the vehicle for a test drive,it came back even worse , so the cap and rotor was replaced, the fuel filter was also replaced,still the problem is there, now i'm told all 3, 02 sensors are tripping a code but the engine light has not come on at all ,and it si working,it lights up as soon as the key is engaged to sart the blazer but goes out immediately as soon as the blazer is running,when the idle drops down after the engine has been started then the rough idling begins


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Try cleaning the throttle plate with the right cleaner . from a auto parts store.

Posted on Mar 02, 2009

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