Hello everyone!
Does anyone know what would cause my C4 to fluctuate, surge at idle. Especially at a traffic light? I need to put in neutral. Thank You.

 

I would check your IAC (idle control valve). If they are off they will open and close the little pintel causing your car to run rich/lean/rich/lean. This open/close creates a vacuum leak hence the high idle.


Also, please name the car year and type of motor as it helps.

 

Could be an air leak somewhere or your IAC valve is defective or the passages gummed up. The best you can do is take the TB off, take the top plate off, unscrew the IAC, remove the torx bolts holding the IAC housing off and clean the thing. You need to spray the IAC on the pintle gently and wipe it clean GENTLY. DO NOT PUSH OR PULL THE PINTLE or you are buying a new one. Clean the passages and reassemble with fresh gaskets.

As to an air leak or the IAC setting is out for whatever reason, well, you are going to need a scanner to determine that assuming you are an L98 engine.

 

clean tb as aklim suggests. then you should search minimum idle adjustment here on the forum and do that procedure. the min idle adjustment will most likely work. but if not, then as aklim says you will need a scanner to see what positiion your IAC is chilling at.

if my memory serves, you want to adjust the limit screw so that the IAC is chilling at idle at around 35. but you want to verify this.

NOTE, if you have an 85, you cannot use the scanner to checkup on the IAC position because there is no way to view the 85 at "normal idle". Been there done that.

If after all this you cannot get a good idle, do the smoke test on the engine and have a couple good friends there to find tendrils of cigar smoke (you don't have to inhale) that you are blowing INTO the break booster vacuum tube. You may find tendrils of smoke coming out of areas that are vacuum leaks. meaning that at idle, the vacuum power of the engine is sucking unmetered air in through those leaking spots and you end up with a lean condition which increases your idle. remember to use masking tape to tape off the throttle body opening or the smoke just come out the tb intake.

one thing you can do with an 85 to give you a clue is look at the blm and int numbers. if while idling, your blms are well above 128, then you know that the computer is adding fuel because its getting extra air through those vac leaks. the computer discovers the extra air by reading the air:fuel ratio from the 02 sensor.


I did all of this with my 85. in the end, i couldn't see major smoke coming out anywhere, no amount of minimum idle adjustment would work, i replaced my IAC because there was a chance it was just being "lazy" or slow to respond to ECM inputs. Nothing helped. i determined it had to be an intake gasket leak that smoke just wasn't coming out of, and after i replaced those, sealing with hylomar, no more issues.

have a read:https://www.corvetteforum.com/forums...w-to-scan.html

 

It does work ASSUMING everything is perfect.

I use about 20 to 30 counts. If you have turned the screw all the way out and cannot achieve it, you have some other issue lurking. Engine at operating temp and everything but motor shut off. Timing and fuel pressure correct. Say I set it today and tomorrow it is off, reset one more time and if it still does that, something is wrong.

 

^^^ yup exactly. Its worth a shot to go through the min idle adjust first. Reality is with cars this old, its not going to work! lol. Usually, multiple failure points. If it doesn't work then u r 100% correct....there is something else going on.