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Eric
Eric, Automotive Repair Shop Manager
Category: Buick
Satisfied Customers: 24229
Experience:  20+ yrs. experience as repair shop manager and technician.
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95 Buick Century: the gas in park the engine will rev..governor

Customer Question

I daughter has a 95 Buick Century. When pressing on the gas in park the engine will rev up and down, up and down. Kind of reminds me of a lwan mower engine with a governor. Any idea on why this wont rev up smoothly and stay revved when pressing steady on the gas.
Submitted: 4 years ago.
Category: Buick
Expert:  Eric replied 4 years ago.

Hi,

 

The erratic idle is normally due to a faulty IAC(idle air control) valve, which is located on the throttle body:

 

Click image to see an enlarged view

Fig. Fig. 2: Throttle body unit on an SFI vehicle

 

REMOVAL & INSTALLATION



  1. Remove the air cleaner.
  2. Disconnect the electrical connection from the idle air control assembly (IAC).
  3. Remove the idle air control assembly (IAC) from the throttle body.

 

Before installing a new idle air control valve, measure the distance that the valve is extended. This measurement should be made from motor housing to end of the cone. The distance should be no greater than 11/8in. (28mm). If the cone is extended too far damage to the valve may result. The IAC valve pintle may also be retracted by using IAC/ISC Motor Tester J-37027/BT-8256K. It is recommended not to push or pull on the IAC pintle. The force required to move the pintle of a new valve should not cause damage. Do not soak the IAC valve in any liquid cleaner or solvent as damage may result.

  1. Be sure to identify the replacement idle air control valve and replace with an identical part. The IAC valve pintle shape and diameter are designed for specific applications.




Click image to see an enlarged view

Fig. Fig. 1: Model 300 TBI unit Idle Air Control valve; there are two types, one with a collar and one without



Click image to see an enlarged view

Fig. Fig. 2: The Model 700 TBI unit's IAC valve is basically the same as that for Model 300

Shiny spots on the pintle or seat are normal and do not indicate misalignment or bend in the shaft.

To install:

  1. Install a new O-ring and lubricate with engine oil onto the IAC valve.
  2. Install the new idle air control valve with the attaching screws using a threadlocking material Loctite®262 or an equivalent.
  3. Tighten the screws to 27 inch lbs. (3 Nm).
  4. Reconnect all electrical connections.
  5. Replace the air cleaner assembly.
  6. The base idle will not be correct unit the ECM/PCM resets the IAC.

 

Be sure to clean the IAC valve O-ring sealing surface, pintle valve seat and air passage. Use a suitable carburetor cleaner (be sure it is safe to use on systems equipped with an oxygen sensor) and a parts cleaning brush to remove the carbon deposits. Do not use a cleaner that contains methyl ethyl ketone. It is an extremely strong solvent and not necessary for this type of deposits. Shiny spots on the pintle or on the seat are normal and do not indicate a misalignment or a bent pintle shaft. If the air passage has heavy deposits, remove the throttle body for a complete cleaning. Replace the IAC O-ring with a new one.

Resetting Procedure

  1. If a repair has been made, reset the IAC before retesting.
  2. Turn the ignition switch OFF for 10 seconds.
  3. Start the engine for 5 seconds.
  4. Turn the ignition switch OFF for 10 seconds.
  5. Retest IAC system, as necessary.
Customer: replied 4 years ago.
Thank you for the info - would this IAC valve be faulty and not set up any fault on the "check engine" light - I have no fault light coming on
Expert:  Eric replied 4 years ago.

Hi,

 

Yes, it is entirely possible for the IAC valve to be failing and not set a fault code to turn on the check engine light. I have replaced many of them that never set any fault codes that were bad

Customer: replied 4 years ago.
I'm confused the repair part item is called an idle air control valve - the idle seems fine - the problem is when I try and rev the engine up to higher rpm's - that is when it runs erratic and seems to rev up down. Does this item also control higher rpms air as well
Expert:  Eric replied 4 years ago.

Hi,

 

The IAC valve has a pintle that moves in and out and helps control the engine rpms when revving the engine. When the pintle is not moving correctly or sticking, the engine rpms become erratic when revving the engine.

Customer: replied 4 years ago.
Eric,
OK thanks for your responses - I will pay you for your time and answers - looks like this part will run around $75 - hope it cures the problem - if I continue to have the same problem after installing the new IAC will you still be available for further questions

Charlie B
Expert:  Eric replied 4 years ago.

Hi,

 

Yes, I am here everyday until about 8pm MST, and you can re-open this question at any point, even after accepting, and it is directed straight to me.

Eric, Automotive Repair Shop Manager
Category: Buick
Satisfied Customers: 24229
Experience: 20+ yrs. experience as repair shop manager and technician.
Eric and 6 other Buick Specialists are ready to help you
Customer: replied 4 years ago.
That did not work - same exact symptoms - when I rev the engine it stills pulses up and down - now I am out $80
Expert:  Eric replied 4 years ago.

HI,

 

Ok, sorry about that. Your description of the symptoms is dead on for a faulty IAC valve.

 

If you still have the valve, look at it and tell me if the pintle( the end of it) is black with carbon deposits.

 

Is this just an issue when revving the engine in park, or is it noticable when accelerating in drive(under a load)?

Customer: replied 4 years ago.
The old one is black with carbon deposits - it is not only an issue when revving in park - after installing the new IAC I took it around the block to warm it up a bit and it kind of backfired and sputtered a little - didn't take it on the highway to get the higher speeds like the kind of high rpm I would get when in park - took it back to my driveway put it in park and tried to rev it up - did the same exact thing it revs then slows - if I keep constant pressure on the gas it revs and slows in a steady kind of pulsing action - like I said earlier it reminds me of the way a small engine with a governor acts when trying to get higher rpms out of it.
Expert:  Eric replied 4 years ago.

Hi,

 

did you measure the distance the new pintle extended and make sure it didnt extend beyond 1 1/8" from motor housing to end of pintle cone, per above directions?

Customer: replied 4 years ago.
I did not try to extend it out - the instructions that came with it implied that it should not be extended further than that when installing it
Expert:  Eric replied 4 years ago.

Hi,

 

As long as it wasnt extended beyond 1 1/8" you are ok on that.

 

I am checking a couple of other things on this and will get back with you shortly. havent forgotten about you, just had a couple of family issues to take care of

Customer: replied 4 years ago.
OK - I am still here - hope you can come up with some other ideas - I am going to borrow my freinds pressure guage tomorrow to check cylinder pressures. Not sure if that is even relevant however I did pull the plugs and the number 3 cylinder plug was oily.
Expert:  Eric replied 4 years ago.

hi,

 

Sorry it took so long, but my older manuals are all tucked away as I dont come across many of the pre 96 models anymore.

 

if the #3 cylinder has low compression, that is going to be a separate issue.

 

Do you know anyone that has a scanner with live data capability? I would like to get the IAC counts, as we could be looking at a pcm not sending correct voltage or info to the IAC. Also, the O2 readings would be helpful in knowing what the ratio is as to whether it is running lean or rich when this occurs.

 

In the meantime, spray the base of the intake manifold and the throttle body base with carb cleaner while someone is revving the engine. If the idle smooths out, let me know

 

 

 

 

Customer: replied 4 years ago.
Dont know anyone with a scanner - I will try spraying as you suggested but that wont happen till later today after my freind gets off work

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