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anonymous38
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Car runs rough on idle

Mon Jun 18, 2018 7:33 pm

I have a 2003 pontiac grand am 2.2 L. It runs really rough on idle in all gears whether it is warm or cold. It pulls out of it when the accelerator is pushed and you don't feel it again until you left off of the pedal. Changed the plugs and harmonic balancer because it was wobbling. There are no codes being pulled and there is no loss of power. I can accelerate pretty fast with no problems. What could cause this?
 
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Re: Car runs rough on idle

Mon Jun 18, 2018 7:35 pm

Welcome to TR! I'm not a car person so I can't really help with your problem though. Maybe a vaccuum leak or a clogged air filter?
 
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Re: Car runs rough on idle

Mon Jun 18, 2018 7:53 pm

For a car that old, it wouldn't hurt to run a bottle of injector cleaner through the fuel system. Could also check to see if the IAC valve (idle air control, it's a little solenoid-type bypass around the throttle plate) has gotten dirty and/or is sticking. Sometimes a problem with any of the airpath sensors (MAF/MAP, temperature, etc.) can cause idle issues, too.
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Re: Car runs rough on idle

Mon Jun 18, 2018 8:17 pm

Carbon buildup in the cylinder heads caused my daily driver to misfire.
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Re: Car runs rough on idle

Mon Jun 18, 2018 8:19 pm

I haven't checked into the specifics of your model. I had an early-90s Honda that would get that way when the Exhaust Gas Recirculation valve would get jammed up with carbon buildup. I had a late-70s Toyota that would get that way whenever there were vacuum leaks.
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Re: Car runs rough on idle

Mon Jun 18, 2018 8:47 pm

I had this problem on my supercharged Cobalt SS and on the mechanics recommendation, did the injector cleaner thing; It got worse. They "cleaned" the mass flow sensor, it got worse. They replaced the mass flow sensor and that did the trick. Cleaning that sensor was almost as expensive as buying a new one; lesson learned.
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Re: Car runs rough on idle

Mon Jun 18, 2018 10:11 pm

The times I've had issues like that it has always been some sort of vacuum leak.
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Re: Car runs rough on idle

Mon Jun 18, 2018 10:20 pm

Definitely sounds like a vacuum leak if it's good under load.
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Re: Car runs rough on idle

Mon Jun 18, 2018 11:17 pm

With no codes coming up, only a few things can cause that and a vaccum leak is the most likely. With the car being 15 years old, the plastic vaccum hoses tend to get brittle and break along with rubber one dry rotting and falling apart. I would start the car and listen to the engine and see if you can hear a hissing noise...that is always the easy way to track down a leak. Once you have the area it is coming from narrowed down, you can start checking the lines and see which one it is.
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Re: Car runs rough on idle

Tue Jun 19, 2018 12:02 am

anonymous38 wrote:
I have a 2003 pontiac grand am 2.2 L. It runs really rough on idle in all gears whether it is warm or cold. It pulls out of it when the accelerator is pushed and you don't feel it again until you left off of the pedal. Changed the plugs and harmonic balancer because it was wobbling. There are no codes being pulled and there is no loss of power. I can accelerate pretty fast with no problems. What could cause this?


How do you "idle in all gears"? At idle a petrol-powered car is basically operating with a mostly closed throttle plate to minimise fuel used. Gunk can accumulate in the throttle body between the plate and surrounding which causes rough idling. Easy, cheap preventive maintenance is to clean out the throttle body with some carby cleaner sprayed onto a rag (never spray into throttle body as it can migrate down to the MAF sensor and cause you a world of pain). Mechanics tend to not do this sort of thing because they are lazy and the problem manifests over time. Be sure to disconnect battery before doing this procedure.

Together with other suggestions about air filter, injectors etc...
 
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Re: Car runs rough on idle

Tue Jun 19, 2018 12:25 am

Pancake wrote:
How do you "idle in all gears"? At idle a petrol-powered car is basically operating with a mostly closed throttle plate to minimise fuel used.

I assume he means releasing the accelerator while in motion, which then closes the throttle plate, and can expose the effects of a dirty IAC valve, vaccuum leak, or various airflow sensor problems as several posters have indicated.
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Re: Car runs rough on idle

Tue Jun 19, 2018 2:03 am

ludi wrote:
Pancake wrote:
How do you "idle in all gears"? At idle a petrol-powered car is basically operating with a mostly closed throttle plate to minimise fuel used.

I assume he means releasing the accelerator while in motion, which then closes the throttle plate, and can expose the effects of a dirty IAC valve, vaccuum leak, or various airflow sensor problems as several posters have indicated.


I have a Ford "ute" about the same vintage but had to look up what an IAC valve was as my car doesn't have one. I only really play with my car and thought manually actuated throttles kind of stopped being manufactured in the 90's?!? Isn't everything since then computer controlled throttle servos? Why would you not do that as soon as cars had ECUs?!? Bizarro world!!!
 
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Re: Car runs rough on idle

Tue Jun 19, 2018 11:22 am

Pancake wrote:
I have a Ford "ute" about the same vintage but had to look up what an IAC valve was as my car doesn't have one. I only really play with my car and thought manually actuated throttles kind of stopped being manufactured in the 90's?!? Isn't everything since then computer controlled throttle servos? Why would you not do that as soon as cars had ECUs?!? Bizarro world!!!

In the US market the transition occurred sometime in the mid-00s, depending on the brand and mdoel. An '03 (Pontiac) Grand Am with the 2.2L was an old model and likely has the older hardware layout (I haven't looked). FWIW Pontiac was axed from GM's portfolio in the 2009 bankruptcy, there wasn't a lot of new investment going into that brad before then.
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Re: Car runs rough on idle

Tue Jun 19, 2018 12:05 pm

Starfalcon wrote:
I would start the car and listen to the engine and see if you can hear a hissing noise...that is always the easy way to track down a leak. Once you have the area it is coming from narrowed down, you can start checking the lines and see which one it is.


You can do the smoke (see where it gets sucked in) or carb cleaner (if the engine races, that's roughly where the leak is) tricks too if you can't actually hear it.
 
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Re: Car runs rough on idle

Tue Jun 19, 2018 1:41 pm

Pancake wrote:
Why would you not do that as soon as cars had ECUs?!?

Throttle response.
 
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Re: Car runs rough on idle

Tue Jun 19, 2018 3:20 pm

synthtel2 wrote:
Pancake wrote:
Why would you not do that as soon as cars had ECUs?!?

Throttle response.

I hear you, but the throttle response on a servo-driven throttle isn't perceptibly worse than cable driven - with the bonus that proper fueling is FAR better since the ECU can set the proper pulse widths and ignition timing as the throttle opens and not in response to the throttle opening.

Short answer: there's no real difference.

Glorious wrote:
Starfalcon wrote:
I would start the car and listen to the engine and see if you can hear a hissing noise...that is always the easy way to track down a leak. Once you have the area it is coming from narrowed down, you can start checking the lines and see which one it is.


You can do the smoke (see where it gets sucked in) or carb cleaner (if the engine races, that's roughly where the leak is) tricks too if you can't actually hear it.

Also, if you have a short length of flexible tubing/hose, use it like a stethoscope to find any leaks. Works wonders.
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Re: Car runs rough on idle

Tue Jun 19, 2018 6:43 pm

Waco wrote:
synthtel2 wrote:
Pancake wrote:
Why would you not do that as soon as cars had ECUs?!?

Throttle response.

I hear you, but the throttle response on a servo-driven throttle isn't perceptibly worse than cable driven - with the bonus that proper fueling is FAR better since the ECU can set the proper pulse widths and ignition timing as the throttle opens and not in response to the throttle opening.

Short answer: there's no real difference.

I have yet to drive a car with an electronic throttle that truly delivers on that (not that that sampling includes much in the way of high-performance stuff, to be fair). They've got their advantages, but the extra delay / lowered rate of change is perceptible.

The ECU can set fuelling and timings according to changing conditions far faster than a suddenly-opened throttle plate translates into MAP. Trying to set them directly against throttle opening would result in knock when the throttle is snapped shut suddenly, would drop timings too quickly on accel, and would go very rich on early accel before the lean dip accel enrichment is designed to counter (and the inverse on decel). It's far more accurate to use MAP for the heavy lifting and throttle position for tweaks than to try to emulate the intake manifold in software well enough to make throttle position the main load sensor, despite MAP on the face of it not being as responsive.
 
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Re: Car runs rough on idle

Tue Jun 19, 2018 7:54 pm

synthtel2 wrote:
Waco wrote:
synthtel2 wrote:
Throttle response.

I hear you, but the throttle response on a servo-driven throttle isn't perceptibly worse than cable driven - with the bonus that proper fueling is FAR better since the ECU can set the proper pulse widths and ignition timing as the throttle opens and not in response to the throttle opening.

Short answer: there's no real difference.

I have yet to drive a car with an electronic throttle that truly delivers on that (not that that sampling includes much in the way of high-performance stuff, to be fair). They've got their advantages, but the extra delay / lowered rate of change is perceptible.

The ECU can set fuelling and timings according to changing conditions far faster than a suddenly-opened throttle plate translates into MAP. Trying to set them directly against throttle opening would result in knock when the throttle is snapped shut suddenly, would drop timings too quickly on accel, and would go very rich on early accel before the lean dip accel enrichment is designed to counter (and the inverse on decel). It's far more accurate to use MAP for the heavy lifting and throttle position for tweaks than to try to emulate the intake manifold in software well enough to make throttle position the main load sensor, despite MAP on the face of it not being as responsive.


I haven't driven a car with that sort of "BLARP!!! BLARP!!!" idle throttle response manufactured before maybe the '80s when carburettors were a thing. Because emissions control. When presented with a load (eg first gear) the engine is going to be bogged down trying to build up revs anyway. Downshifting is one case where I find it takes the engine more time than desirable to build revs (on my manual transmission). But, again, I don't think it's the throttle servo speed but - emissions control. Anyway, the approx. 260 HP and 280 pound-feet torque of my car makes for hilarious entertainment with an unloaded bed on a light-ish truck with low-ish gears.
 
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Re: Car runs rough on idle

Tue Jun 19, 2018 9:05 pm

Electronic throttles make cruise control much less complicated. Even if the lag is a thing, not many care enough to notice but they would notice no cruise control. Not that it can't be done mechanically but it's far less elegant and not as reliable.

So, is the OP trolling is and we are just enjoying the conversation now?
 
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Re: Car runs rough on idle

Tue Jun 19, 2018 10:40 pm

DragonDaddyBear wrote:
Electronic throttles make cruise control much less complicated. Even if the lag is a thing, not many care enough to notice but they would notice no cruise control. Not that it can't be done mechanically but it's far less elegant and not as reliable.

I've owned or driven at least five vehicles that had the old-school vacuum-driven cruise control connected to a mechanical throttle, and all worked fine (one, a 1998 Camry, still does). What am I missing?
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Re: Car runs rough on idle

Wed Jun 20, 2018 6:27 am

ludi wrote:
I've owned or driven at least five vehicles that had the old-school vacuum-driven cruise control connected to a mechanical throttle, and all worked fine (one, a 1998 Camry, still does). What am I missing?


I guess nothing. It's just more complicated. As mentioned, vacuum hoses can become brittle and are prone to breaking. I very much dislike troubleshooting carbureted vehicles from the late 80s because there were so many vacuum hoses. There are also systems that have a mechanical tie in. I think my mom had a Jeep like that and my motorcycle is like that, where some kind of electronic device twists the throttle body.
 
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Re: Car runs rough on idle

Wed Jun 20, 2018 7:30 am

You haven't truly learned to hate vacuum hoses until you have to diagnose non-functional four wheel drive actuation on an early 2000s Chevy. :evil:

The manifold vacuum goes to an electronic switch on the transfer case, which opens a valve, which brings vacuum back up to the engine bay (while crossing dangerously close to the exhaust), to a vacuum cylinder, which pulls on a cable, which pulls a pin that engages the front differential.

Plus, the HVAC diverter system is also vacuum driven, and GM had a lovely issue where the vacuum valve on the transfer case would blow a seal, so the (pressurized) transfer case would force transmission fluid into the HVAC selector, which would ooze out from under the dash. :roll:
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Re: Car runs rough on idle

Wed Jun 20, 2018 9:18 am

That's the early era of O2 sensors for ECUs, and with electronic fuel injection replacing carburettors and chokes, the idle smoothness is down to either poor injection/ignition (check fuel lines, injectors, sparks and leads) or poor air/fuel mixture (check O2 sensors and ECU error codes).

Might be worth visiting a shop with an in-house sparky so that you can look at the cheapest thing to fix first (ECU and O2 sensors). Once they rule that out, the other possibilities will undoubtedly cost more to fix.
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Re: Car runs rough on idle

Wed Jun 20, 2018 9:21 am

SuperSpy wrote:
You haven't truly learned to hate vacuum hoses until you have to diagnose non-functional four wheel drive actuation on an early 2000s Chevy. :evil:

The manifold vacuum goes to an electronic switch on the transfer case, which opens a valve, which brings vacuum back up to the engine bay (while crossing dangerously close to the exhaust), to a vacuum cylinder, which pulls on a cable, which pulls a pin that engages the front differential.

Plus, the HVAC diverter system is also vacuum driven, and GM had a lovely issue where the vacuum valve on the transfer case would blow a seal, so the (pressurized) transfer case would force transmission fluid into the HVAC selector, which would ooze out from under the dash. :roll:


Sounds like my girlfriend's previous Mitsubishi 4x4 light truck with the "Super Select" system. The 4WD vacuum actuation module (comprising a couple of vacuum solenoid valves) was located right under the chassis near the right wheel arch, completely unprotected and would get covered in crud. I got lucky - cleaned it out and gave it a good dose of WD-40. 4WD operational. But you really have to wonder at what sort of hare-brained engineer would think "well, this seems like a perfectly good spot to put this unprotected not particularly robust component!". How does stuff like that happen?

So, she now drives a Holden 4x4 light truck which cut out intermittently and wouldn't start occasionally. Which I diagnosed to these really clever quick-release battery clips they put in new cars. Why??? How was unscrewing a simple sturdy bolt EVER a problem???

Mental retardation isn't confined to software "engineering" much to my disappointment. I always hoped mechanical engineers would be of a higher standard.
 
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Re: Car runs rough on idle

Wed Jun 20, 2018 9:56 am

Amateurs. Rank amateurs. Behold the vacuum hose routing diagram for a 1985 Civic with the CVCC carburetor.

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Re: Car runs rough on idle

Wed Jun 20, 2018 10:18 am

Captain Ned wrote:
Amateurs. Rank amateurs. Behold the vacuum hose routing diagram for a 1985 Civic with the CVCC carburetor.

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Re: Car runs rough on idle

Wed Jun 20, 2018 10:25 am

Captain Ned wrote:
Amateurs. Rank amateurs. Behold the vacuum hose routing diagram for a 1985 Civic with the CVCC carburetor.


My mom had an 80's Toyota "trud-cell." That looks about right. When we pulled the carb we used her endless box of nail polish to color code the ends with the carb.
 
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Re: Car runs rough on idle

Wed Jun 20, 2018 10:33 am

I've done a carb rebuild exactly once in my life. Not an experience I'd care to ever repeat. Ditto front struts (the old school kind where you needed to compress the spring and disassemble everything).
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Re: Car runs rough on idle

Wed Jun 20, 2018 11:03 am

Chrispy_ wrote:
That's the early era of O2 sensors for ECUs, and with electronic fuel injection replacing carburettors and chokes, the idle smoothness is down to either poor injection/ignition (check fuel lines, injectors, sparks and leads) or poor air/fuel mixture (check O2 sensors and ECU error codes).


Uhh, not exclusively, it absolutely can be vacuum leaks like many have suggested or the IAC valve like ludi indicated.

IAC valves are typically like ~50 USD, and it's typically two bolts and simple gasket.

Vacuum leaks can be more expensive, depending on how/where, but they can also easily be things like ~10 USD semi-rigid piping/hosing piece, like for EVAP etc... Those can be as simple as picking a part off the shelf (not behind the counter i.e. these pieces fail/degrade all the time!) at the auto parts store and just pop the darn thing on.

Anyway, both of those can absolutely cause a rough idle, and they should absolutely be where you start, not the O2 sensors. (I mean, if the code reader explicitly talks about "O2 high", is the sensor bad, or are you lean cause of the leak? [conversely if you fix what you think was a leak, it might start saying low--not necessarily because it's bad, but because your ECU has been overcompensating etc...])

EDIT: In my experience, the codes give you very rough diagnostic information, but you have to think about the system in totality and try to figure out what that might possibly mean. Unless it's outright obvious already, you very rarely find immediately straightforward answers.
 
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Re: Car runs rough on idle

Wed Jun 20, 2018 11:21 am

Glorious wrote:
Chrispy_ wrote:
That's the early era of O2 sensors for ECUs, and with electronic fuel injection replacing carburettors and chokes, the idle smoothness is down to either poor injection/ignition (check fuel lines, injectors, sparks and leads) or poor air/fuel mixture (check O2 sensors and ECU error codes).


Uhh, not exclusively, it absolutely can be vacuum leaks like many have suggested or the IAC valve like ludi indicated.

IAC valves are typically like ~50 USD, and it's typically two bolts and simple gasket.

Vacuum leaks can be more expensive, depending on how/where, but they can also easily be things like ~10 USD semi-rigid piping/hosing piece, like for EVAP etc... Those can be as simple as picking a part off the shelf (not behind the counter i.e. these pieces fail/degrade all the time!) at the auto parts store and just pop the darn thing on.

Anyway, both of those can absolutely cause a rough idle, and they should absolutely be where you start, not the O2 sensors. (I mean, if the code reader explicitly talks about "O2 high", is the sensor bad, or are you lean cause of the leak? [conversely if you fix what you think was a leak, it might start saying low--not necessarily because it's bad, but because your ECU has been overcompensating etc...])

EDIT: In my experience, the codes give you very rough diagnostic information, but you have to think about the system in totality and try to figure out what that might possibly mean. Unless it's outright obvious already, you very rarely find immediately straightforward answers.



The IAC will compensate for idle loads like the air conditioning (compressor) kicking on. If the IAC was bad, you would likely get a stall in this case. If you don't stall when the A/C comes on, IAC is prolly OK.

A vacuum leak is by far the most likely problem (especially with no codes). You are lucky the car lasted this long without a substantial crack in a vacuum line. Do the carb cleaner trick mentioned by another post. Usually the hose leak is not readily visible.

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