Help with a 97 sierra??????

[ldhwade]

We have a 1997 Gmc Sierra. It has a 305 vortec....8 cylinder....5.0. We are recieving the codes P0305, P0147, P0507.......we know what the codes are and everything we tested came out like it was suppose to. The main problem is the P0507 code.....when in park the Rpm's are too high...but they go down when you take off....but the vehicle trys to run away with itself...without being given any gas. Also there is a jerky idle and a knocking noise that comes and goes. And the MPG has started to be really bad. Any help would be greatly appreciated.

Thanks in Advance

Ldhwade

[maxwedge]

Well I'd worry about the knock right away, you need someone to listen to this for a diagnosis. Anyway the pcm percieves too high an idle for the iac position ,so the iac could could be bad or carboned up or the throttle body and passages could be dirty or a vacuum leak ( less likely).

[ldhwade]

Okay....we did a check on the iac and it tested good. Then we cleaned all the carbon out of the throttle body and passages.....and none of this worked. Are there any other suggestions?????

[92GMCstepside]

You might try the knock sensor, and or maybe a bad tank of fuel i had the same type of problem wiht my 92 gmc

[ldhwade]

Changed out the IAC didn't help. Still is reving high at idle and takes off without being giving any gas. Also now it's pulling code P0121.
Any suggestions.

[swensonb71]

how are your brakes acting? it may be the brake booster diaphram that has a leak in it giving you a vacumn leak causing the idle to be high.

[GMMerlin]

P507
The following items could cause this DTC:

An IAC valve which is stopped and cannot respond to the VCM
A throttle stop screw which has been tampered with
A damaged throttle or linkage
A slow, unstable, or fast idle may be caused by a non - IAC valve system problem that cannot be overcome by the IAC valve. Out of range control IAC scan tool counts will read above 60 if the idle is too low and zero if the idle is too high. Check for the following:

Fuel system too lean and rich
Throttle body - inspect the bore for foreign material.
Refer to Rough, Unstable, or Incorrect Idle and Stalling .
IAC reset is done with the scan tool misc. functions mode.
P121
The scan tool displays the throttle position in volts. It should display 0.20 volts to 0.90 volts with the throttle closed and ignition ON or at idle. The voltage should increase at a steady rate as the throttle is moved toward Wide Open Throttle (WOT). Also, the scan tool displays the throttle angle %. 0 % = closed throttle. 100 % = WOT.

Check the condition of the connector and sensor terminals for moisture or corrosion, and clean or replace as necessary. If corrosion is found, check the condition of the connector seal and repair or replace as necessary
The TP sensor has an auto zeroing feature. If the voltage reading is within the range of about 0.2 to 0.9 volt, the VCM uses that value as a closed throttle. If the reading is out of the auto zero range at the closed throttle, check for a binding throttle or a damaged linkage. If the throttle and the linkage are okay, continue with the diagnosis.