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Tim's Auto Repair
Tim's Auto Repair, Mechanic
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Experience:  Have owned a repair shop for 25 yrs.
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I had my 1987 Mazda B2000 "tuned up" recently...new plugs,

Resolved Question:

I had my 1987 Mazda B2000 "tuned up" recently...new plugs, wires, distributor cap, gas filter...
Since then, when the engine is at idle and I turn on the AC, the idling revs up and down. At higher RPM's while driving, it doesn't do this nor does it do it with the AC off. The guy who did the tune up didn't see a loose wire or hose but he hasn't checked really thoroughly. What is wrong?
Submitted: 5 years ago.
Category: Car
Expert:  Tim's Auto Repair replied 5 years ago.

hi

well you do need to double check to make sure you did not disconnect or break a vacuum line when you did the tune up,,but ,the problem may be the iac[idle air control]valve,,this is located on the throttle body,,sometimes you can remove and clean these with carb cleaner and they will work fine,,though you may need a new one

thanks

Idle Air Control (IAC) Valve

  • OPERATION
  • TESTING
  • REMOVAL & INSTALLATION

 

OPERATION



See Figure 1

The Idle Air Control (IAC) valve controls the engine idle speed and dashpot functions. The valve is located on the side of the throttle body. This valve allows air, determined by the Powertrain Control Module (PCM) and controlled by a duty cycle signal, to bypass the throttle plate in order to maintain the proper idle speed

.FIG : Fig. 1: Typical Idle Air Control (IAC) valve which is mounted to the throttle body-cutaway view shows air bypass direction

image



Navajo Models

See Figure 2

  1. Turn the ignition switch to the OFF position.
  2. Disengage the wiring harness connector from the IAC valve .
  3. Using an ohmmeter, measure the resistance between the terminals of the valve.


Due to the diode in the solenoid, place the ohmmeter positive lead on the VPWR terminal and the negative lead on the ISC terminal.

  1. If the resistance is not 7-13 ohms, replace the IAC valve.


FIG : Fig. 2: IAC valve wiring harness connections

image

Pick-up and MPV Models

See Figure 3

The Idle Air Control (IAC) valve is an integral component of the Bypass Air Control (BAC) valve.

  1. Turn the ignition switch to the OFF position.
  2. Disengage the wiring harness connector from the IAC valve .
  3. Using an ohmmeter, measure the resistance between the terminals of the valve.


FIG : Fig. 3: Checking the resistance of the IAC valve on the BAC valve assembly

image

  1. The resistance should measure 10.7-12.3 ohms at 68° F (20° C) for 3.0L equipped models and 7.7-9.3 ohms at 73° F (23° C) for 2.6L engine equipped models.
  2. If the resistance does not measure within specifications, replace the BAC valve.

REMOVAL & INSTALLATION



Navajo Models

See Figures 4, 5, 6 and 7

  1. Disconnect the negative battery cable.



Click image to see an enlarged view

Fig. Fig. 4: To remove the IAC valve, first disconnect the negative battery cable, then the IAC wire harness plug



Click image to see an enlarged view

Fig. Fig. 5: Next, remove the two IAC valve attaching bolts ...



Click image to see an enlarged view

Fig. Fig. 6: ... and pull the valve from the intake manifold

  1. Disengage the wiring harness connector from the IAC valve.
  2. Remove the two retaining screws, then remove the IAC valve and discard the old gasket.


To install:


Click image to see an enlarged view

Fig. Fig. 7: When installing the valve, always discard the old gasket and install it using a new one

  1. Clean the IAC valve mounting surface on the throttle body of old gasket material.
  2. Using a new gasket, position the IAC valve on the throttle body. Install and tighten the retaining screws to 71-106 inch lbs. (8-12 Nm).
  3. Attach the wiring harness connector to the IAC valve.
  4. Connect the negative battery cable.

Pick-up and MPV Models

See Figure 8

The Idle Air Control (IAC) valve is an integral component of the Bypass Air Control (BAC) valve.

  1. Disconnect the negative battery cable.
  2. Disconnect the water hoses (if equipped) and air hose from the BAC/IAC valve.

FIG : Fig. 8: Removal of the BAC/IAC valve assembly-3.0L engine shown

image

  1. Disengage the wiring harness connector from the BAC/IAC valve.
  2. Remove the mounting fasteners from the valve assembly.
  3. Remove the BAC/IAC valve assembly, along with the mounting gasket and O-ring from the vehicle.


To install:

  1. Remove any foreign material from the contact surface.
  2. Install the BAC/IAC valve assembly, with a new mounting gasket and O-ring, onto the engine in correct position.
  3. Install and tighten the mounting fasteners to 14-18 ft. lbs. (19-25 Nm) on 3.0L engines, and 22-30 inch lbs. (2.5-3.4 Nm) on 2.6L engines.
  4. Plug in the valve wiring harness connector.
  5. Connect the air hose and water hoses (if equipped) to the valve assembly.
  6. Connect the negative battery cable.
  7. If equipped with water hoses, verify that the valve assembly does not leak.


Customer: replied 5 years ago.
The instructions above for the operation and installation of the IAC valve are for my Mazda specifically?
You've included a section for a Navajo model; is that a Mazda? If these instructions are for Mazdas I guess I can take these printouts to the guy who did the tune up and refer him to the section headed "Pick up and MPV Models? If you would clarify this I would appreciate it.
Expert:  Tim's Auto Repair replied 5 years ago.

hi

yes,,these are for 2 different models..the 2.0 that you have is on here too....yes please print these out,,,also make sure that it isnt the ac compressor itself kicking in and out .causeing your problem

thanks

Customer: replied 5 years ago.
I was already to click accept and you mentioned in your reply "also make sure that it isn't the ac compressor itself kicking in and out causing the problem"....
I'm not sure what you mean here....the ac compressor comes on and that is when the engine idling starts to rev up and down...my understanding is that when the ac compressor starts up it is normal for the engine to idle higher to compensate for the increased engine drag of the ac. But it shouldn't idle up and down, up and down...it's like rrrrrm....rrrrrm....rrrrrm......rrrrrm
Can you explain what you meant by the compressor itself possibly causing this problem? The compressor would raise the idling level while on but it shouldn't idle up and down. Do you mean that the up and down idling could be a problem with the compressor?
Expert:  Tim's Auto Repair replied 5 years ago.

ok

if you get low on freon...the compressor will go on and off..if the compressor is staying on..and it idles up and down,,it will be the iac

thanks

Tim's Auto Repair, Mechanic
Category: Car
Satisfied Customers: 13230
Experience: Have owned a repair shop for 25 yrs.
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Tim's Auto Repair
Tim's Auto Repair
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Have owned a repair shop for 25 yrs.