Tuesday, May 15, 2012

High Idle on Chevy Blazer?

Problem -could be vacuum leak/EGR valve/Idle air control valve/throttle position sensor-

A fuel injected engine's idle speed is controlled by the IAC motor. IAC means Idle Air Control.
IAC motor is a controller for the plunger or valve that opens and closes a vacuum port in turn controlling the engine idle speed.
At certain level the  IAC motor cannot control the engine idle speed.
If you get high idle but not rough idle then its not Faultily EGR

Totally not EGR. EGR causes very rough idling and/or stalling, certainly not high idle speed.

The IAC on the other hand, controls how much air gets by the throttle plate when the trottle plate is closed. thus if the IAC is dirty or improperly set, too much air get by. the computer in turn, sets the fuel mixture to match the air the going in to keep the proper ratio.

The first thing to be checked is idle control motor.
There is a small electric motor to open and close a valve that regulates the vaccuum in a fuel injected engine (the vaccuum controls the idle). The motor  fails because of carbon buildup and has to be replaced, and the idle will get back to normal.

If the motor looks ok,then inspect the Idle air control valve.The valve may be clogged by carbon build up and cause this problem.

There is really no way for carbon to get into the IAC valve.You will get a oily buildup on it and in the throttle bore which is caused by reverberation in the intake and the pcv system.

Also you can  check the IAC with an ohm meter.For this  measure the resistance across terminals A & B and C & D ( top two and bottom two terminals). the resistance should be 40-80 ohms, if not, replace it.

If IAC valve checks ok, then you know a more thorough cleaning is needed.

Also while re-installing the IAC that the distance from the IAC mounting flange to the tip of the pintle is less than 1-1/8". If its more than that, press the pintle in, till its less than 1-1/8".

Please confirm this "Are you getting a "Service Engine Soon" light on dash coming up".You should be because the powertrain control module monitors idle speed.Get the error codes scanned that are stored in cars computer.I suggest having a tech who is familiar with using a scanner take a good look at all your Blazers engine sensor data values while it is idleing high and also make darn sure there is no mechanical issues (binding throttle cable,misadjusted cruise cable,dirty throttle bore,etc.)causing the throttle to stick.Do you have a floormat that is interfering with the throttle pedal?Just had to ask because I have seen something as simple as this be overlooked before.

Also check the diffuser on the underside of the throttle body plate. It is held on by rivets. The diffuser can come loose and keep the throttle body open, keeping the idle high. I had this happen to my Blazer, and ended up replacing the plate with a cfm-technologies plate.

IN one our cases at our garage the procedure below solved the problem:-----
There is a clip that secures the cruise control cable that gets broken and this partially jams the throttle cable. Its pretty dangerous because the throttle could've just as easily binded at high throttle as a slightly high idle.

Inspecting this clip will be helpful.

Also other possibility can be dirty MAF( mas air flow sensor or f there is any kind of vacuum leak.

Try this:---  Spray the mass air flow sensor down with "mass air flow sensor cleaning solution" It comes in an aerosol CANS for pretty cheap. Just don't touch it.

In case of vacuum leak. Listen carefully under the hood for a hissing sound and track it down. We notice  this kinds of problem  after changing  cap and rotor. Turns out that we forget to put the vacuum line back on back there that actuates the A/C selector switch.
If vacuum leak is there it can cause a high idle. So to confirm whether there is vacuum leak or not check for broken or dilapidated vacuum hoses on and around the engine.
Vacuum hose  connects to the engine intake manifold or throttle body .Some manufactures  design larger vacuum transfer hose that connects the intake manifold to the IAC (idle air control) motor. If broken or crimped these vacuum lines can cause the engine to lose vacuum which will allow the engine to idle high.

This details will help.

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