Rough idle [Archive] - Forum


View Full Version : Rough idle

12-24-2011, 09:35 AM
Well, i just purchased another grand am of my own. its an 05 gt with the 3.4 ofcourse. its only got 40,500 miles on it. during the test driving and everything i didnt notice the idling problem. i test drove 3 other grand am's with slightly more miles and i didnt take them because of a sloppy 1-2 shift wot shift, while this one was nice and crisp and seemed to pull harder. anyways, onces its warmed up its more of an issue, while its cold it smooth since the idle is higher.

while in drive at a redlight, its a constant drone/viberation feeling in the whole car. if u slightly hit the gas with the brake on and boost it up an extra 100rpms it smooth. then if u let off from doing that it dropsdown to like 500 quick and shakes and repeats its unsually rough feel. i dont beleive its skipping as the exhaust note sounds clear at idle with no pops. no check engine lights.

i replaced the fuel filter, pcv valve, air filter and removed the IAC and gently cleaned that the best i could and removed and sprayed the MAF down with throttle body cleaner as well as cleaning the bore of the TB. i put it all back together and it really made the car mad, it had a horrible droning in the exhaust idling while i was warming it up and it was very rough and horrible feeling. i turned it off and recycled the motor on it seemed fine but still had its rough feeling idle and didnt change a bit.

even in park idling its rough, seems to be very consistant, smooth then rough then smooth. i have owned 3.4's and all were silky smooth even with 190,000 miles.

not using coolant that i can tell, doesnt appear to be leaking, oil is clean.

i gave it a drink of water thinking maybe there is some build up on the valves, that almost seemed to help but problem is still the same.

im out of ideas and the chevy dealer i bought it from is screwing me and saying its normal, i know its not. any suggestions?

12-24-2011, 10:18 AM
Have you scanned it of any misfires? My suggestions would have been some of the things you already listed, such as IAC and throttle body. I guess check spark plugs and plug wires. Is it only a rough idle or is it idling higher than normal?

Last summer, I was at the drag strip and after my last race my rpm's shot up to 2100. My car would cruise up to 65 without me even touching the pedal. I replaced every sensor and checked everything and turns out the 65mm TB was the issue somehow. My stock TB fixed the problem.

12-24-2011, 11:06 AM
i did check for misfires with the scanner. at idle it doesnt count any misfires, but if i blurp the throttle quick holding the brake in drive ill get a couple on everything cylinder, sometimes as high at 12-15 on cylinder 5 and a diff one i cant remember. the one plug i did pull looked ok but seemed to be gapped a lil more then .060. wires didnt seem to be arching i checked them when it was pitch black out and didnt see anything obvious.

12-24-2011, 06:16 PM
Hmm. I guess any other input I have is to swap all plugs, double check every plug wire by pushing down onto every plug to make sure they are all on tight. I can't really think of anything else. Sorry bud

Actually, check for vacuum leaks. Most of the vacuum lines are on the upper intake manifold.

01-22-2012, 11:40 AM
nothing has changed yet. been in the shop more then ive owned it. def the worse grand am ive ever owned. upper intake gasket was leaking, replaced. fuel trims were pulling fuel way back upstream o2 sensor was bottoming out, replaced upstream o2. according to them the fuel trims were back how they should be but i didnt actually see afterward to verify myself. but before they would be pulling back fuel -14, -15. they said they checked the cat i still thing it could be an issue if i give it a good hot supper and come to a stop i can smell cat, not rotten eggs really just the smell of hot metal like a hot cat. they replaced the plugs and supposidly did a compression test on all 6, 150 even across the board. reflashed the pcm. im a tech myself but very limited to diag on this because im a chrysler tech with chrysler scan tools and cannot communicate well with gm computers.

overall, hasnt changed a bit. still runs rough at idle. it will be smooth when i first come to a stop sometime for a light then after 2-3 seconds i can start feel it doing its pulsing/surge sound and vibrating of the seat/console and door panels.... the chevy garage says its normal and i call major bs and im pretty upset with the quality of service how ive been treated, i HATE being the customer.

any other directions on where to look? it doesnt count misfires at idle at all. runs rough in drive as well as park and neutal.

also will add i also noticed its rough enough where the car doesnt even really wanna roll back while in drive if on a slight incline. all my other gt's were very smooth and would easily roll right down the hill. then one just pulses. wants to roll then comes back.

01-22-2012, 11:47 AM
also forgot to add that ive noticed an exhaust drone/engine bogging tone from like 1700-2200r's when im on the highway, never noticed any of my other ga's do this but they were also 4 doors, this being a 2 door it could be normal?

01-22-2012, 12:55 PM
Can you see what the rpm is at idle on your scanner? Stock idle speed should be 600, so if it's dropping down to 500 that would be a problem right there. The stock tachometer isn't very accurate though so you need a scanner to see exactly where it's at.

For the fuel trims, -15 is NOT bottomed out. iirc the max adjustment for the fuel trim is almost 30% +/- ... I know I've seen a few cars with way more than 15%. That is still more than ideal though and could point to a problem. If the O2 sensor was fouled it could be incorrectly adjusting the fuel trim which could cause the exhaust temps to go up, or dump unburned fuel making the cat run hot and eventually get clogged. Normally the O2 should last for almost 100k miles though, so I don't know why it would be bad after only 40k unless there is some other problem that contributed to it fouling.

I would try replacing or re-gapping all the spark plugs first just to make sure there are no issues there. Then I'd run a bottle of fuel system cleaner through the tank to make sure the injectors are spraying properly.

01-22-2012, 03:09 PM
If i remember right idle on scanner is around 680-700 flucuating around quite a bit. I did run seafoam threw the brake booster then dumped the rest of the can about half in the gas tank which was about empty. Then went and beat the hell ourta it and got fuel. Didnt notice a difference. Whats some good injector cleaners gm likes?

01-22-2012, 06:07 PM
Seafoam is supposed to be decent, but you need to dump a full can in a half empty tank then fill it up to mix it, and run the full tank to fully clean the injectors and intake valves etc. I've also heard the Chevron Techron fuel additive is good, and have used that before. Same process.

01-22-2012, 06:59 PM
You may have a sticking injector. That would explain the rich fuel trims and the rough idle but good performance when driving.

Run some of that seafom through the tank and see if that helps clear it up.

01-23-2012, 03:39 PM
Desired idle was 672. It bounces all around 650-735 on scanner. Front 02 is still dropping to almost 50mv then shoots back to 500. I did notice the map sensor to be reading funny. 10 hg in vacuum at idle in park and almost 11.3 in in drive idling. Seems very low to me im used to seeing 18-20 on a normal running chrysler vehicle i would assume that same for gm. So that raises my eye for concern, low vacuum at idle. Iac steps are 12 in park. 18 in drive, seems low to me as well. Check fuel pressure 55psi key on egine off. 50 psi at idle. Book gives no spec for idling psi only koeo, which is in spec. Im stumped.

01-23-2012, 06:28 PM
Idle rpm usually do vary a little. It's not going to sit right on one spot. The stock programmed idle speed is 600 though so I don't know why you'd see 672 as desired. But anyway that amount of variation is pretty normal. If the PCM needed a CASE learn procedure done (crank position for the 24x crank sensor) that could smooth out the idle more, but it shouldn't need that unless the crank pulley or PCM have been replaced. Could be worth a shot though.

The O2 sensor bouncing around is normal too, although it seems to be varying on the low side. 500mv is nominal, with 900mv being rich, and 100mv being lean. It usually bounces back and forth on either side of 500mv, and a sensor that is contaminated will usually switch slowly or not vary enough from 500, and will set a DTC code in the PCM.

The idle vacuum definitely seems low though. Did you hook a vacuum gauge to the manifold to double check? To my knowledge the PCM doesn't output MAP sensor in in/Hg reading, unless your scanner program converts it. The only output I get on my OBD scanners/tuner is Kpa. I have a boost/vac gauge in my car though and even with an aftermarket cam in it I'd still see 14 in/Hg regularly, and 16-18 in/Hg with the stock cam.

01-23-2012, 06:45 PM
Hm. Yes my scanner did convert the map sensor reading i believe. All i saw was hg which id imagine in inches vacuum. I didnt actually confirm it with an actual vacuum gauge as i dont believe i have the proper fitting for the car. Fuel mileage is also poor, and kickdown points seem lower than my old 04, would map have any effect on that as well? Ill have to try to reset the crank. Pretty sure after the pcm was reflashed he did it. Forgot to mention i also checked misfire counters and saw only 2 on cyl 4 for a split second then all stayed 0. Im thinking my problem lies with low vacuum. Burnt valve would cause low vacuum, but a misfire count id imagine...

01-23-2012, 06:55 PM
Seems to get slightly better but is still there once the temps rise above 200 while in sitting traffic or long idling. My old grand am actually always ran about 205-210 no matter what and it ran beautiful. Wonder if maybe the heat is causing expansion of something? Egr would cause low vacuum but id imagine id see a egr code

01-23-2012, 07:02 PM
Yeah, a worn valve could definitely cause low vacuum. I think the vacuum is supposed to bounce up and down when that valve opens and closes, but maybe that's more on carburated engines? I know a clogged exhaust will also make low vacuum as well as hurting gas mileage.

As for the trans kick downs, shifting is somewhat controlled by the PCM's calculation of engine load, and it does use the MAF and MAP sensors as part of determining that, so yes that could possibly throw it off a little.

01-24-2012, 08:49 AM
Aaron is right - vacuum problems can cause these issues. Definitely check with a reliable gauge. I'd be spraying carb cleaner around all potential leakage points to see what perks up.

01-24-2012, 12:25 PM
Checked vacuum with a gauge steady 18 in hg at idle in park, about 17 in drive. Needle doesnt move like a leaking or burnt valve. The map sensor is still reading 9.8-10.3hg inches in park 11 or so in drive. While the gauge on the manifold is steady 18in. Unplugging the map i was hoping it would default to a real value closer to 20. It went to a constant 3in steady with gas or out. Koeo the map sensor reads a realistic reading for today of 29.5

01-24-2012, 02:03 PM
hmm... wonder if the map sensor is bad maybe? The MAF is the primary modifier for fuel enrichment on these cars, but the MAP is a secondary modifier and is also used to check outside atmospheric pressure to calculate air density. If the MAP is reading wrong that could mess up your idle. I've seen two different instances where MAP's with faulty barometric readings made the idle rough.

01-24-2012, 04:05 PM
I think its a good probability it is bad. I found another gm vehicle and that is reading 18hg in vacuum at the map idling. Eitherway its back at the dealership again, if its not fixed after this time im filing for used car lemon law.

02-04-2013, 11:36 AM
How is this going?
I'm encountering a similar issue with my car now and am curious if you got anywhere with this..