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1998 Chevy Blazer IAC valve

Tiny
CHEVYBLAZER1327
  • MEMBER
  • 1998 CHEVROLET BLAZER
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 127,000 MILES
I have a 98 chevy blazer with the 4.3 l vortec. The service engine light is on and according to checker and autozone and a reputable shop its the IAC valve.

I have replaced the valve but still have the engine light on and the diagnostic is still the IAC is there something else I need to check? I did check and replace some of the vaccum line but this didnt fix it. Can the mass air flow sensor cause the continued surging?

The engine idles good but when I drive it at 40 mph it seems to run by itself and as I come to a stop it has to be at a complete stop before the car will kick back down.

What do I need to check and are there any places I can go on the net for diagrams.
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Monday, January 25th, 2010 AT 9:48 PM

9 Replies

Tiny
SCOTTY DEE
  • MECHANIC
  • 592 POSTS
Tell me the actual trouble code number and I will get you started.
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Wednesday, January 27th, 2010 AT 7:39 AM
Tiny
CHEVYBLAZER1327
  • MEMBER
  • 5 POSTS
I think the code was P0502 or P502.
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Wednesday, January 27th, 2010 AT 10:32 PM
Tiny
SCOTTY DEE
  • MECHANIC
  • 592 POSTS
The Vehicle Speed Sensor Assembly (VSS Assy.) Provides vehicle speed information to the Vehicle Control Module (VCM). The VSS Assy. Is a Permanent Magnet (PM) generator. The PM generator produces a pulsing Idle Air Control (IAC) voltage as rotor teeth on the transmission output shaft pass through the sensor's magnetic field. The IAC voltage level and the number of pulses increase as the speed of the vehicle increases. The output voltage varies from a minimum of 0.5 volts IAC at 100 RPM to more than 100 volts at 8000 RPM. The VCM converts the pulsing voltage to vehicle speed. The VCM uses the vehicle speed signal to determine shift timing and Torque Converter Clutch (TCC) scheduling.

When the VCM detects a low vehicle speed when there is a high engine speed in a drive gear range, then DTC P0502 sets.

Do you have a scan tool and a DVOM?
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Wednesday, January 27th, 2010 AT 10:40 PM
Tiny
CHEVYBLAZER1327
  • MEMBER
  • 5 POSTS
I dont know what a DVOM is and the only access I have for a scan tool would be what Checker Auto offers. Other than that no.

What do I need to be looking for?
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Wednesday, January 27th, 2010 AT 10:52 PM
Tiny
SCOTTY DEE
  • MECHANIC
  • 592 POSTS
DVOM (Digital Voltage Ohm Meter). There is a series of tests that should be performed to determine resistance, opens, or short to ground values at the VCM.

It's going to be a wiring problem, and bad VSS sensor, or a bad VCM. Without doing the proper testing it would be a guess.

It may be worth your time and money to have a reputable full service facility do the diagnosis and give you an estimate of the repair.

I hope I have pointed you in the right direction.

Scotty
www. Allaboutcarcare. Com
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Wednesday, January 27th, 2010 AT 10:58 PM
Tiny
CHEVYBLAZER1327
  • MEMBER
  • 5 POSTS
I have checked the VSS, and found it was coroded. I checked the wiring and vaccum and have found no problems. Is the VCM the computer module on the passenger side of the vehicle? If so I have a module from another vehicle same year, but I have hooked it up and was unable to get the car to work, it would turn over but not start.

Any help would be great. So far everything else has been great help.

Thanks
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Friday, January 29th, 2010 AT 9:31 PM
Tiny
SCOTTY DEE
  • MECHANIC
  • 592 POSTS
If you can give me the exact code/s I can point you in a flow of diagnosis. Do you have just a basic code reader?
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Friday, January 29th, 2010 AT 9:36 PM
Tiny
CHEVYBLAZER1327
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The new code after I replaced the VSS is now reading P0507, and originally it read P0502.
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Friday, January 29th, 2010 AT 9:45 PM
Tiny
SCOTTY DEE
  • MECHANIC
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If you have a P0507, and it's the only code, you clear it, and it keeps coming back, the problem is engine mechanical and NOT electrical/sensor/s.

Possible causes.
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.
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Friday, January 29th, 2010 AT 9:54 PM

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