Rough Idle Mystery Unsolved...so far...

[TBGal]

I purchased my current TB about a month ago with a little over 60,000 miles on it. It is in good shape, but it has a slight rough idle.

After reading about throttle body cleaning on this site, my buddy came over and helped me take the tb out and clean it, and it was dirty. We did do the battery disconnect for the PCM re-learn. I also did the fuel system cleaning with Chevron Techron Concentrate in the gas tank.

I am still having a bit of a rough idle problem, though. It doesn't matter if the truck is cold or warmed up, nor does it matter if it is in gear or not or the air conditioner is on or not...there is a bit of a rough idle regardless.

Umm, I want to provide you all with as much info as I can so you all can help me troubleshoot. The air filter, an AC Delco, is new. The oil has been changed. I only put top tier gas in it, but I can't account for what quality of gas was put in it prior to my ownership.

The rough idle is there. The RPMs at idle are steady at 650; however, because I don't have a scan tool, there might be something that is going on that is not showing accurate on the gauge. During rough idle, the steering wheel does not shake. I am not having a misfire or any codes...so far. The performance of the vehicle is pretty good. It's pretty responsive, not having a problem with hesitation, and it passes the 0-60 test with flying colors.

I hope all that info helps to narrow down what might be the problem.

If you all could help me troubleshoot and throw out some ideas, I'd appreciate it.

Thanks

 

[McGMT]

I purchased my current TB about a month ago with a little over 60,000 miles on it. It is in good shape, but it has a slight rough idle.

After reading about throttle body cleaning on this site, my buddy came over and helped me take the tb out and clean it, and it was dirty. We did do the battery disconnect for the PCM re-learn. I also did the fuel system cleaning with Chevron Techron Concentrate in the gas tank.

I am still having a bit of a rough idle problem, though. It doesn't matter if the truck is cold or warmed up, nor does it matter if it is in gear or not or the air conditioner is on or not...there is a bit of a rough idle regardless.

Umm, I want to provide you all with as much info as I can so you all can help me troubleshoot. The air filter, an AC Delco, is new. The oil has been changed. I only put top tier gas in it, but I can't account for what quality of gas was put in it prior to my ownership.

The rough idle is there. The RPMs at idle are steady at 650; however, because I don't have a scan tool, there might be something that is going on that is not showing accurate on the gauge. During rough idle, the steering wheel does not shake. I am not having a misfire or any codes...so far. The performance of the vehicle is pretty good. It's pretty responsive, not having a problem with hesitation, and it passes the 0-60 test with flying colors.

I hope all that info helps to narrow down what might be the problem.

If you all could help me troubleshoot and throw out some ideas, I'd appreciate it.

Thanks

Dunno if you have seen the posts about it however, a big culprit of a rough idle is the engine mounts...

 

[bore_pig]

So by rough idle do you mean the engine sounds rough, or the car seems to shake when idling?

 

[TBGal]

Doesnt sound rough - shakes a little.

Possible motor mounts? With this few miles? Ugh. Not a job I can come anywhere near tackling with my friend - that's waaaaaay beyond my skill level and probably beyond my buddy's level too. My current skill level is at changing the air filter. lol

What is a cost estimate of getting motor mounts changed at the dealership?

 

[McGMT]

Doesnt sound rough - shakes a little.

Possible motor mounts? With this few miles? Ugh. Not a job I can come anywhere near tackling with my friend - that's waaaaaay beyond my skill level and probably beyond my buddy's level too. My current skill level is at changing the air filter. lol

What is a cost estimate of getting motor mounts changed at the dealership?

Don't go to a dealer, it is really not THAT bad of a job but go to a reliable shop, not the dealer.... Any part can fail before it is even installed really.... What you need to do is inspect them and see if they look to be collapsed or damaged... Im sure someone can give you instructions on how to MAYBE tell if they are going bad... "Friends don't let friends go to stealerships"

 

[bore_pig]

Does sound like a motor mount. Have you noticed that the shake/vibration gets worse when you first start to move in reverse?

 

[TBGal]

Does sound like a motor mount. Have you noticed that the shake/vibration gets worse when you first start to move in reverse?

Eh, it doesn't seem worse in any particular gear.

There is always a little bit of a rough idle, though intermittantly it is worse.

 

[TBGal]

Don't go to a dealer, it is really not THAT bad of a job but go to a reliable shop, not the dealer.... Any part can fail before it is even installed really.... What you need to do is inspect them and see if they look to be collapsed or damaged... Im sure someone can give you instructions on how to MAYBE tell if they are going bad... "Friends don't let friends go to stealerships"

I've got a friend who used to do collision repair for a GM dealership. I'll ask him for a referral to a good independent shop for this particular type of job.

 

[The_Roadie]

The passenger side motor mount is the typical one to go bad first, due to heat from the exhaust manifold. It's supposed to be possible to pry up on the motor to take some weight off the mount to see if the vibration changes.

But if it's rough and not a clear 10 Hz vibration, it might just be plugs, a water-filled plug well, or a deteriorating coil. It's early at 60K, but I'd pull the coils and plugs and inspect.

 

[TBGal]

Thanks for the feedback, folks. I am going to look into the things you all have suggested as possibilities.

Because there doesn't seem to be a real pattern to the rough idle, it is hard to explain. I mean, at first I thought it grows rougher after idling a while. But, not always. And sometimes it is rougher at first then smooths out. So, no consistent pattern.

Keeping sharing your thoughts. I will update this thread as I look into things.

Thanks

 

[McGMT]

Thanks for the feedback, folks. I am going to look into the things you all have suggested as possibilities.

Because there doesn't seem to be a real pattern to the rough idle, it is hard to explain. I mean, at first I thought it grows rougher after idling a while. But, not always. And sometimes it is rougher at first then smooths out. So, no consistent pattern.

Keeping sharing your thoughts. I will update this thread as I look into things.

Thanks

I agree with roadie.... But this is starting to sound more and more like a bad mount... Almost like deja vu from the other person that had a bad mount...

Good luck

 

[The_Roadie]

Yeah. Used to be the top five introductory questions included "stalling at lights with the ac on?", "I jumped it and now all the interior lights are dead", and the ever popular "what's this red wire?"

Now as the vehicles age, it's the "erratic idle in drive" syndrome.

 

[tblazerdude]

Vacuum leaks can cause erratic and rough idle. Just something else to check before you spend money at a shop.

 

[Morgoth604]

I purchased my current TB about a month ago with a little over 60,000 miles on it. It is in good shape, but it has a slight rough idle.

After reading about throttle body cleaning on this site, my buddy came over and helped me take the tb out and clean it, and it was dirty. We did do the battery disconnect for the PCM re-learn. I also did the fuel system cleaning with Chevron Techron Concentrate in the gas tank.

I am still having a bit of a rough idle problem, though. It doesn't matter if the truck is cold or warmed up, nor does it matter if it is in gear or not or the air conditioner is on or not...there is a bit of a rough idle regardless.

Umm, I want to provide you all with as much info as I can so you all can help me troubleshoot. The air filter, an AC Delco, is new. The oil has been changed. I only put top tier gas in it, but I can't account for what quality of gas was put in it prior to my ownership.

The rough idle is there. The RPMs at idle are steady at 650; however, because I don't have a scan tool, there might be something that is going on that is not showing accurate on the gauge. During rough idle, the steering wheel does not shake. I am not having a misfire or any codes...so far. The performance of the vehicle is pretty good. It's pretty responsive, not having a problem with hesitation, and it passes the 0-60 test with flying colors.

I hope all that info helps to narrow down what might be the problem.

If you all could help me troubleshoot and throw out some ideas, I'd appreciate it.

Thanks

it sounds like a spark plug issue not a motor mount issue. check your coil pack and clean them, and check the gap in your spark plugs and the condition of your spark plugs. here is a site that tells you what they should look like: NGK Spark Plugs USA

id also recomend using dieletric grease on the coil packs where the spring in the rubber sheathing meet the plug(just a drop will do) i used a q-tip to apply the dieletric grease where the wiring harness connects to the coil pack(a drop of grease there aswell). i used wd40 to clean the coil packs and the ceramic part of the spark plugs.

on my 2002trailblazer ltz the engine was knocking and then one day i started it up a boom she was shaking and hesitating. so the first thing i did was check the plugs and coil packs. and wouldnt you know it one plug was half way screwed out of place so that was causing the knocking and hesitation. so i decided to check them all and clean and use dieletric grease and now boom puuuurrrrrrrs like a cheta ready to pounce on a gazelle for dinner. hope this helps you out.

 

[jaktomas1]

Dunno if you have seen the posts about it however, a big culprit of a rough idle is the engine mounts...

I'm experiencing the same idle issue with my 05 TB with the exception that it only happens when in gear at a stop. In neutral or park idle is smooth and steady.

 

[McGMT]

I'm experiencing the same idle issue with my 05 TB with the exception that it only happens when in gear at a stop. In neutral or park idle is smooth and steady.

Sounds like a frequency vibration. Thing is about these engines is that they run so very smoothly that any small vibration is really evident...

 

[The_Roadie]

I'm experiencing the same idle issue with my 05 TB with the exception that it only happens when in gear at a stop. In neutral or park idle is smooth and steady.

10 Hz vibration in gear that goes away in neutral is THE definitive test of it being motor mounts.

 

[Morgoth604]

I'm experiencing the same idle issue with my 05 TB with the exception that it only happens when in gear at a stop. In neutral or park idle is smooth and steady.

have you tryed having sumone put the truck in drive while you watch the engine and then put it back into nutral. the engine may move back and fourth if the motor mounts are going or have have gone bad. if the engine doesnt move significantly then id say clean your plugs and coils really good with wd40, and then use dieletric grease on all the wire connections and do test agian with sumone in the truck shifting from nutral to drive and back to nutral. there is one other thing you can check is the tightness of the motor mounts. to do this just grab the right sized sockect wrench and see if the bolts move if they move tighten them up to manufactors specs. good luck.

 

[TBGal]

Thanks, folks, for the suggestions. I have enlisted help for this weekend to look into the plugs and coils as well as investigate the motor mounts and for a possible vacuum leak. I'll update this thread when I have some updates.

This week I am going to replace a missing clamp on the air intake tube, which I noticed was missing (whoever replaced the air filter probably forgot to put both back on), try to feel around for the intake manifold bolts to check for tightness (as I see mentioned on this forum is commonly found to be loose), and get my battery checked by AutoZone (as it seems to be running at about 11.5).

 

[McGMT]

have you tryed having sumone put the truck in drive while you watch the engine and then put it back into nutral. the engine may move back and fourth if the motor mounts are going or have have gone bad. if the engine doesnt move significantly then id say clean your plugs and coils really good with wd40, and then use dieletric grease on all the wire connections and do test agian with sumone in the truck shifting from nutral to drive and back to nutral. there is one other thing you can check is the tightness of the motor mounts. to do this just grab the right sized sockect wrench and see if the bolts move if they move tighten them up to manufactors specs. good luck.

There is not appreciable difference when these engine mounts go bad... They just quit dampening and the vibration starts..

 

[TBGal]

A little update...

I got the battery checked. It did need replacing. No suprise there, as I confirmed it was the orignal battery on the vehicle. I said "hello" to the little red wire, as the old battery came out and the new Interstate Battery went in.

The alternator tested high voltage, both yesterday at AAP and today at the battery shop, tested without load and with load.

Question: tomorrow when the alternator is replaced, should I go ahead and have the drive belt replaced too? It is in good condition now, but I am thinking that because it's gotta come off in the process of the alternator replacement anyway, and it is not an expensive part, just go ahead and get it done now?

 

[TBGal]

Bad voltage regulator

Last Night
No load: 15.40V
Loaded: 15.16V

Today
No load: 14.99V
Loaded: 15.5V

 

[McGMT]

Bad voltage regulator

Last Night
No load: 15.40V
Loaded: 15.16V

Today
No load: 14.99V
Loaded: 15.5V

serp belt? Yes, if you have the loot go ahead, can't hurt, keep the old one in the back as a spare just in case.

That voltage is a little high, but it is going to vary with the conditions and hey, it is charging, so we can't fault that, now if it was 16-19 volts, then I would stop driving and replace the alternator.

 

[TBGal]

serp belt? Yes, if you have the loot go ahead, can't hurt, keep the old one in the back as a spare just in case.

That voltage is a little high, but it is going to vary with the conditions and hey, it is charging, so we can't fault that, now if it was 16-19 volts, then I would stop driving and replace the alternator.

Thanks for the tip about keeping the old serp belt. That is a great idea, and I will do that.

I have had an alt go out on me...many years ago...but, I still remember the experience vividly, and of course it went out on me while I was driving AT NIGHT. I'd rather replace the alt now than have it go out on some night later this week!