Rough idle when coming to stop

tylerrmack · August 1, 2016

Did your engine light come one with the fouled plug?

Yes light came on p0300 random/multi misfire. No other codes. Had to have it towed because it wouldnt drive. Dealer said just random bad plug. Dont know how often that happens, seems more like "we dont know" With my limited knowledge I hooked up an OBD sensor and looked at some readings. 2 things i noticed were o2 sensors dont seem to coordinate well bank 1 vs bank 2 especially at idle and fuel pressure at idle is about 42psi with lots of flucuations and drops down in the 20s. I drove another 2016 sierra although it was a 5.3 and fuel pressure was more steady around 64 with mild fluctuations. Dealer didnt seem to know what to do with the comparison numbers or what it meant. They tried to look up some numbers but couldnt find anything. Figured i would yalk to them again at oil change or like i said earlier take it to an independent shop.

darkfox1 · August 2, 2016

I had a weird issue the other day. I kept getting a "check gas cap" warning. After a few days of ignoring it (I just knew I had put my gas cap on properly) it threw a tantrum and tripped a CEL. I pulled over and opened the fuel door and the cap fell out!

Since resealing it and checking that its firmly attached every time I fill up I haven;'t noticed the rough idle.

Seems unlikely to be related.. but..??

My truck has 7000 miles on it now. It seems to do this far less often than it used to.

mookdoc6 · August 3, 2016

I am at 7000 miles too maybe more.....Anyway mine has not done that rough idle business since it was brand new! Thank the lord too!

Loupls · August 5, 2016

Ok heres the deal me and the service foreman went for a drive after they had installed new motor mounts and the shims and still have the same idling issue rough in drive as soon as u put in park it goes away or in reverse. The loaner truck that I have been driving does not do this same body and engine. The service foreman says he will continue to try and solve the issue, but when is enough enough 4 trips to the dealer making payment on something I cant even drive.

Now the dealer says its my ahead and its now with in specs, dropped my son off at karate last night went into the store and came out and the truck would not start. It took a little while, calling on star, my wife to pick up my son but finally figured out they have linkage in shift lever messed will not go into park according to the instrument panel. It started in neutral and has an issue with going into park so its back to the dealer so tired of this.

Rosedog · August 5, 2016

At the dealer again for my scheduled android auto install. Had the look at the idle issue and... They need the truck for a day to scope and do an idle adjustment. They also said it might be the mounts needing to settle in too since it is brand new.

Rosedog · August 6, 2016

So before I go in and get my idle readjusted (told its a day job), has anyone else had this done yet and if so did it fix it?

Thomcat · August 6, 2016

May be a long shot but a quick and easy test. See if the idle smooths with the foot off the brake in Drive and gets rough only when you step hard on the power brake.

An air leak in the induction system downstream of the MAF sensor can upset the fuel mixture and cause a rough idle.....if the check valve on the brake booster does not seal properly or if the diaphragm is damaged it will cause a hidden air leak that can lean out some cylinders and probably not set a code.

Rosedog · August 8, 2016

May be a long shot but a quick and easy test. See if the idle smooths with the foot off the brake in Drive and gets rough only when you step hard on the power brake.

An air leak in the induction system downstream of the MAF sensor can upset the fuel mixture and cause a rough idle.....if the check valve on the brake booster does not seal properly or if the diaphragm is damaged it will cause a hidden air leak that can lean out some cylinders and probably not set a code.

Tried your suggestion, didn't change. I'll be taking it in around my first oil change (per their suggestion).

Dale_K · August 8, 2016

I had the first episode of a poor idle after a hard stop this weekend. Light changed to yellow and I braked to a stop more suddenly than I would typically. Mild shudder at idle. Didn't do it again but subsequent stops were more gentle. It's going to be hard to duplicate, no doubt.

I've read the link to the sandyblog GM Techlink page that talks about motor mounts. Maybe it's possible that the motor could shift position slightly during a hard braking episode, I don't know.

I'd like to hear a first hand report from anyone who has had their motor mounts replaced or shimmed.

mookdoc6 · August 9, 2016

I had the first episode of a poor idle after a hard stop this weekend. Light changed to yellow and I braked to a stop more suddenly than I would typically. Mild shudder at idle. Didn't do it again but subsequent stops were more gentle. It's going to be hard to duplicate, no doubt.

I've read the link to the sandyblog GM Techlink page that talks about motor mounts. Maybe it's possible that the motor could shift position slightly during a hard braking episode, I don't know.

I'd like to hear a first hand report from anyone who has had their motor mounts replaced or shimmed.

How many miles? Like I said, " mine did it for maybe 1000 miles or so? Now it has not for thousands?

Dale_K · August 9, 2016

5,000 miles on my truck. I had seen this thread title but never checked out the posts because I didn't think it applied to me. Until now.

There seems to be a mixture of two different issues in the posts. One is about an engine miss and the other is about vibrations but only the OP and maybe a couple of others connect it with braking. I came up with a little theory today for the brake related idle issues - maybe the brake use draws off enough engine vacuum to confuse the ECU about fuel/air mixture. Lots of modern cars have a supplemental vacuum pump but all the ones I'm familiar with involve DOHC motors with variable cam phasing. I don't know if the GM pickups have a vacuum pump. Also the plumbing probably has checkvalves to stop reverse flow. If any of that stuff was wrong it might create an idle problem during brake application but I'm just guessing.

mookdoc6 · August 9, 2016

Not Sure than because I am officially out of this thread rough idle is gone some kind of break in thing for me? Good Luck

dbt1127 · August 10, 2016

Well, my dealer finally took my truck in again, and I think they finally fixed my truck's rough Idle....knocking on wood and hoping I'm not typing this to soon.

I was told they redid the shims on the motor mounts and added a shim to a mount that they had not done previously. I didn't think this was going to work, but I did not experience 1 moment of rough idle on my way to work, or on my way to lunch. Time will tell, but hopefully I can move on and start enjoying my brand new truck. they also measured engine vibrations and said they are now within spec.

This took me over 2 months to get resolved. Keep being persistent. If your dealer is not helping you, call GM and hound the hell out of them. The way the dealer treated me once the GM resolution team got involved was a night and day difference. Good luck

mookdoc6 · August 10, 2016

Great to hear....when it's smooth the truck is a dream!

dbt1127 · August 15, 2016

Well, my dealer finally took my truck in again, and I think they finally fixed my truck's rough Idle....knocking on wood and hoping I'm not typing this to soon.

I was told they redid the shims on the motor mounts and added a shim to a mount that they had not done previously. I didn't think this was going to work, but I did not experience 1 moment of rough idle on my way to work, or on my way to lunch. Time will tell, but hopefully I can move on and start enjoying my brand new truck. they also measured engine vibrations and said they are now within spec.

This took me over 2 months to get resolved. Keep being persistent. If your dealer is not helping you, call GM and hound the hell out of them. The way the dealer treated me once the GM resolution team got involved was a night and day difference. Good luck

I spoke to soon.....UGH!!! Now the talk is to fly an engineer in to look at my truck. What a nightmare