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3.8 Rough Idle

6K views 17 replies 9 participants last post by  lvdukerider 
#1 ·
Hey everyone, new to this forum and have a question about one problem I've been having. I have a 3.8 with 98k, since I got it I have been dealing with a rough idle.

I have replaced plugs, checked all the plug gaps. Checked for vacuum leaks. Intake filter is clean, no intake leaks(that I can find). It never stalls and the tach doesn't have any noticeable fluctuation but when sitting still for more than a couple seconds at idle it will start getting rough, and shakes a little bit. Don't think it's misfiring.

Don't know what else to try, I thought it was something I just had to live with until I test drove another bk1 3.8 and noticed that car idles perfectly. Now again I'm chasing this problem. Appreciate any input! Sorry for the long post.
 
#5 ·
My 2012 3.8 idled pretty rough till the dealer found the left engine mount (hydraulic) was collapsed. They replaced it under warranty, and no more rough idle.
 
#6 ·
I tightened up my engine leash the other day because it had loosened up a little, and the idle got noticeably better. Still rough but not as much vibration so I'll check the engine mounts as that would make sense. As for checking the timing, I'll get the timing cover off eventually. Maybe order a timing belt kit as they seem to be fairly cheap.
 
#10 · (Edited)
After reading all the replies, and in light of 98K I say do some preventative maintenance. Remove your LPFP in the tank, clean the filter sock and replace or upgrade your LPFP. At your mileage it cant hurt, and its real cheap.

Then after doing that run about 3 full tanks with big bottles of techron.

I agree if it was timing it wouldnt just effect idle. Vac leaks would be the primary culprit but you have insisted you've checked that.

We are left with fueling, heck yyou might just try the techron first, but cars that age suffer low pressure fuel pump issues all the time.
 
#11 ·
OP hasn't specified, but I think he has a BK1.

"...I thought it was something I just had to live with until I test drove another bk1 3.8... "

In that case, he only has the 1 in-tank fuel pump. I agree with you, it's probably a sneaky vac leak somewhere. I've got one I can't find and have nearly identical sounding symptoms...
 
#12 ·
He stated in the OP that he test drove another BK 1 and found it vibration free. I'm still thinking the motor mounts. Mine did not vibrate when moving and also quit when revving the engine up to about 1000 rpm. Only shook at idle.


Also did you check for any engine codes? Might also be a cam position sensor. If the timing was off it could also require a tweak of the ECU by the dealer.

Also we do not have timing belts. We have chains. Another thing, does it vibrate with the AC on? Mine catches a bit of vibration when the AC is on and the motor is idling. Turn it off and it's vibration free.
 
#15 ·
Hey everyone, first thanks for all the input. Anyway, yes mine is a bk1. I have checked for codes and nothing. Turning the AC on does make it vibrates a little more, but nothing crazy, and it still does it when it's off. I did valve cover gaskets a little while back and cleaned the throttle body then so I think that's good. I suppose I will go ahead and clean that filter on the pump and if they aren't super expensive probably drop a new one in there, like someone said it won't hurt. Also I'll try that techron stuff.

As of now I've checked the engine mounts and they look ok. I'll update you guys when I change the pump and see if that helps.
 
#16 ·
Hey everyone, new to this forum and have a question about one problem I've been having. I have a 3.8 with 98k, since I got it I have been dealing with a rough idle.

I have replaced plugs, checked all the plug gaps. Checked for vacuum leaks. Intake filter is clean, no intake leaks(that I can find). It never stalls and the tach doesn't have any noticeable fluctuation but when sitting still for more than a couple seconds at idle it will start getting rough, and shakes a little bit. Don't think it's misfiring.

Don't know what else to try, I thought it was something I just had to live with until I test drove another bk1 3.8 and noticed that car idles perfectly. Now again I'm chasing this problem. Appreciate any input! Sorry for the long post.
check gasket in plenum(upper manifold), what is strange is that is not detectable by leak test. replaced mine, problem went away
 
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