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Rough idle, no check engine light

I have had an issue with my 2005 pontiac grand prix for a few months. It has a rough idle and hesitates sometimes from a stop. Very rarely it went start unless I give it gas. I’ve changed the coils, plugs, air filter, and the fuel filter. I have no idea what to do.

My fuel mileage is down as well. I also have no codes or pending codes

I’d start by cleaning the throttle body assembly to see if that helps.
Since this is not an intermittent condition, you can also try pinching off / plugging the EGR system and seeing of that helps. If the valve is carboned up and sticking, it’ll cause idling and initial acceleration problems.

Post back.


I have already cleaned the throttlebody. I haven’t checked the EGR because I thought the check engine light would be on for that. Ill give it a try though. Thanks.

You may also want to remove and clean the idle air controller and its associated passage on the throttle body.

I took the egr tubes and valve off and cleaned them. There was no change. This car has an electronic throttlebody and uses it to adjust idle. It does not have an idle control valve. Any other ideas?

Here are some links to idle air controller information for a 2005 Pontiac grand prix. Hopefully they’ll help.

There’s’ a lot of things that could cause this. I expect you already know that. Given what you say, my guess is the air/fuel mixture is too lean at idle. Vacuum system leak or clogged fuel injectors. Ask your shop to test the vacuum system hoses and connections, and all vacuum operated gadgets for diaphragm leaks, esp the brake power booster. Then add a can of fuel injector cleaner to the gas tank at the next fill up.

@Vcab - You just tagged on to thread that has a different vehicle than yours. That only leads to confusion. Please start your own thread so all replies will be for your problem.