High Idle

[bh357]

I have a 2003 GT with a bit over 160k miles. Lately the rpms haven't been dropping all the way when I let off the gas to slow down. It'll drop to just under 2000 rpm. However, if I give the throttle a blip with the clutch engaged, the rpms will drop down to normal, and it'll idle that way just fine. No blip of the throttle, and it'll want to idle at the higher speed.The only DTC I've had is an occasional 420 code, which has been ongoing for a very long time. The idle issue just cropped up a couple weeks ago. There are no other driveability issues.

[bh357]

In the correct forum this time I have a 2003 GT with a bit over 160k miles. Lately the rpms haven't been dropping all the way when I let off the gas to slow down. It'll drop to just under 2000 rpm. However, if I give the throttle a blip with the clutch engaged, the rpms will drop down to normal, and it'll idle that way just fine. No blip of the throttle, and it'll want to idle at the higher speed.The only DTC I've had is an occasional 420 code, which has been ongoing for a very long time. The idle issue just cropped up a couple weeks ago. There are no other driveability issues.

[joatmon]

I looked in the matrix repair manual and for high idle it says the causes could beidle speed control (ISC) valve circuit problems-PCV hose-Open or short in ISC valve circuit-ISC valve is stuck or closed-Open or short in A/C switch circuit-Air induction system-ECMor - ECM power source circuit- Neutral start switch circuit (AT only)- Back up power source circuit

[RIT]

Sounds like the throttle cable is sticking, and may need to me lubricated. If it were an electronic problem, then blipping the throttle would probably have no effest, so it must be mechanical. i suppose the IACV could be sticking, but i would start with the throttle cable.WD40 is your friend. Or maybe some Gumout on the cable 1st to remove some grease/dirt that may have built up on it.

[Raven]

Try lubricating the throttle linkage around the throttle body. it may be just sticking.

[ToolGuy]

Honestly at 160K miles it could be a few things. The first and cheapest thing I would do is have your throttle body cleaned out. There is a flap in it for lack of a better word and if its path gets a gummed up from dirt and it does get dirty in there, that could cause this. Clean the Idle Air Control motor as well which is also in the throttle body. If you have never had the TB cleaned then it is over due and you will be surprised at the dirt that is in there. You can do it yourself and by had but in your case I would have it taken off the car or done by a professional. And what is a 420 code? Usually there is a letter before it. Let me know and I can look it up.

[joatmon]

420 is either P0420, which is likely the catalytic converter, or the car has been smoking weedI remember on cars with carbs, not fuel injection, having all kinds of idle kick down issues. Odd to have it on these cars, so it's plausible that something on the intake/throttle body is sticking or gunked up.

[ToolGuy]

LOL!!!At 160K this car is due for a new cat and 02 sensor! The light is evidence of this. The fuel mileage could be hurting too or will be as the cat and 02 get even older. In fact I would not wait for them to fail and replace both now but that is just me... Even if the cat was replaced like mine for the rotten egg smell TSB.

[Raven]

PO420 is about the most common code there is. It can be many things but usually is caused by a lazy O2 sensor, either the first or second one. The computer is not seeing the proper differences between the two sensors and triggers the CEL. There is no reason for the cat to die at any mileage, they can last literally forever.

[ToolGuy]

Well if the code is there it needs to be addressed. Was not there when the car was new so it is detecting something and letting the driver know now. I personally do not ignore lights on my cars and get them fixed... The most common failure of a cat is it gets plugged from a failed O2 or the inside starts to crack, come apart, rattle around inside and eventually plug the exhaust. Happened on my wife's car years ago so I cut if out and I know others that the cat failed under warranty and needed to be replaced. The vehicles had poor performance but gave no codes. Older cars cat would rust apart as well I am talking 80's cars though. Had my Vibe's cat replaced under warranty do to the rotten egg smell. Not sure what caused the smell but GM said to replace the cat. Been fine ever since too.Catalytic Converter FailureCatalytic converter failures typically fall into one of four categories:1. Thermal failure (overheating)2. Plugged substrate3. Thermal shock4. Physical damageThermal failure is most often caused when excessive raw fuel comes into contact with the catalyst, and "burns" in the converter instead of in the engine. The high quantity of fuel generates temperatures well in excess of the capacity of the converter, causing meltdown of the ceramic monolith. The melted ceramic could block the exhaust path, leading to a significant loss of engine power. Visible symptoms include heat-related discoloration of the converter shell.Potential causes of thermal failure include: misfire, malfunctioning oxygen sensor, fuel delivery issue, improper choke setting/operation, and ECU malfunction.A plugged or contaminated substrate can be the result of an overly rich air/fuel mixture, radiator sealant, and oil or antifreeze entering the exhaust flow. The resultant carbon deposits restrict the operation - and ultimately the flow characteristics - of the converter by coating the unit's reactive surface. This degrades the converter's ability to perform its chemical conversion process, leading to potentially illegal levels of HC, CO, and NOx.Root causes of this problem are a malfunctioning O2 sensor, plugged or inoperable fuel injectors, piston blow-by, leaking head gasket, broken or frozen choke or carburetor float, excessive cranking time, and repeated incidences of running out of gas.

[joatmon]

Quote, originally posted by Raven »PO420 is about the most common code there is. It can be many things but usually is caused by a lazy O2 sensor, either the first or second one. The computer is not seeing the proper differences between the two sensors and triggers the CEL. There is no reason for the cat to die at any mileage, they can last literally forever.Let me know if I am wrong, but a number of us here have had the P0420, and while some have had it be intermittent or never posted what they did to fix it, I can't find any mention of anybody actually fixing it other than by replacing the cat. Well, there was RIT in http://forums.genvibe.com/zerothread?id=19728 who said that changing the spark plugs made his P0420 go away, but then later, in http://forums.genvibe.com/zerothread?id=31560 he says having the cat replaced under warranty fixed it. Still, I can't see where the P0420 could cause the sticking high idle, I'd think the two were separate issues.