2000 Chevy Monte Carlo 3.4l

The problem that I currently have is that the car will not idle smoothly, it bounces between 600 and 800 rpm. I have replaced the spark plugs, wires, fuel filter and thought I checked the hoses. Just got done changing the upper and lover manifold seals, during which I replaced or cleaned what ever was suggested by the place I purchased my parts through. I am at a loss, I have begun to notice that there are small fluctuations on the highway when I am using the cruise control and am on flat land. I would like to solve this problem with out going to a mechanic, this being the cheaper option, if anyone has any suggestions I would appreciate it.

Replace every vacuum hose. At 15yo, they are all suspect and not expensive.

The problem might be caused by a faulty Idle Air Control valve hunting for the proper idle position.


The IAC valve controls the engine idle speed whenever the gas pedal isn’t depressed. A faulty IAC valve can also cause fluctuations in engine RPM’s at highway speeds.


Both Busted and Tester have good comments but my first question is: Is your “service engine soon” light on? If so, the codes it has stored will give you a place to start. If Testers idea is valid it should turn on that light. Good luck.

I have yet to see a faulty Idle Air Control valve turn on a Check Engine light.

The IAC valve isn’t an emissions control device. It’s an engine control device.


My guess, something is allowing a varying degree of extra air and/or gasoline into the engine. When your foot is off the throttle and the engine is idling, that shouldn’t happen. Here’s some ideas:

  • vacuum hose leaking
  • vacuum operated device leaking
  • throttle valve is not staying in exactly the idle position
  • idle air control not holding it’s nominal position
  • intake manifold gaskets leaking
  • egr valve sticking or otherwise not operating correctly
  • vacuum switches failing, like if you have one that increases the idle speed when turning the steering wheel, turning on the headlights, etc.
  • fuel injector(s) leaking or not metering the fuel correctly b/c they are dirty
  • fuel pressure not being adequately regulated

Idle problems are one of the most challenging problems, for a mechanic, but especially for a diy’er. Because there are so many different possible causes. The best strategy is to choose the one that is the cause as the first thing to test. If you can’t do that, just go through them one by one. Based on what you say, probably the first thing I’d do is clean the throttle body. If at all possible, I’d remove it from the engine and give it a thorough bench cleaning. Then re-install it w/a new gasket.

“I have yet to see a faulty Idle Air Control valve turn on a Check Engine light.”

I have


They even have their own codes… P1504 to P1511.

Where’s the generic codes?

Those are manufacturer specific codes.

And there’s only two of them?

That shows how closely they monitor IAC circuit.


thanks for the advice GeorgeSanJose I will work my way down that list now that you say something I replaced the upper and lower manifold seals, it sounds like there may be a leak on the upper one.

1504 to 1511 is only two? What’s that, “new math”?
What makes you say these are “manufacturer specific”? EPA considers them generic codes.




Any P1xxx or P2xxx code is technically a manufacturer code, yet a generic scan code may retrieve them. That said, the cheapo scan tool may simply retrieve P1504 and say “manufacturer code” or something to that extent. The professional level scan tools will correctly interpret the codes

A P0xxx code, for example, is a generic code


They’re listed as generic codes.
But I accept your explanation. Whether they’re generic or manufacturer is irrelevant to the problem at hand. If a scanner pops a P1xxx, it can be looked up in the listing of generic codes and diagnosis can continue from there.

And I still maintain that 1504 to 1511 contains more than two numbers.
Two numbers would be described as 1504 AND 1511.


The P1XXX codes are vehicle manufacturer specific codes or enhanced codes.

These are codes the manufacturer can add at their own discretion. And a code P1XXX on one vehicle may not have the same definition on another make of vehicle with same code as these are not standardized or generic codes.

I misread your post because i didn’t have my glasses on when I read it.

So since these codes aren’t generic codes, the manufacturer can decide if they want to monitor the IAC valve circuit, or not.

As with a faulty crankshaft position sensor rarely setting a code, so too does a faulty IAC control valve rarely set a code.

Now go have beer and relax. It’s the weekend.



My suggestion would be to purchase a cheap vacuum tester and plug it in. That will show instantly if there’s a vacuum leak or a mechanical issue such as compression, weak valve spring, etc; any of which can cause an iffy idle.

My preference has always been to weed out the possibility of a mechanical malfunction before heading to the parts bin.