Rough idle starting cold engine

Miked

Original poster
Member
Jan 1, 2022
7
Va
2007 Trailblazer 4x4 4.2 L engine. When starting the cold engine the idle is rough in gear. Once the vehicle reaches operating temperature the engine runs as smooth as the day it was purchased new.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON

mrrsm

Lifetime VIP Donor
Supporting Donor
Member
Oct 22, 2015
7,639
Tampa Bay Area
...and a Clogged CPAS can also cause Rough Idling, too:

 
  • Like
Reactions: Mooseman

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
How old are the spark plugs? If they were changed, what type? This engine only likes Genuine ACDelco 41-103. Watch out for fakes.
 

Justirv

Member
Nov 15, 2021
1
California
This could be the intake manifold gasket. Cold it can shrink, allowing un-metered air in, causing a lean condition. Once the engine warms up, they expand, closing off the leak(s) and restoring normal A/F ratio.
 
  • Like
Reactions: Mooseman

Miked

Original poster
Member
Jan 1, 2022
7
Va
The battery was disconnected for 1 hour and spark plugs were changed out 2 years ago with AC Delco plugs
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Where did you buy them? I ask because there are fakes floating around Amazon and eBay.

And as mentioned, it would also be worth it to check the intake manifold bolts. They do have a tendency of getting loose.
 

Miked

Original poster
Member
Jan 1, 2022
7
Va
Where did you buy them? I ask because there are fakes floating around Amazon and eBay.

And as mentioned, it would also be worth it to check the intake manifold bolts. They do have a tendency of getting loose.
I can't remember where I purchased them
 

Reprise

Lifetime VIP Donor
Supporting Donor
Member
Jul 22, 2015
2,724
And as mentioned, it would also be worth it to check the intake manifold bolts. They do have a tendency of getting loose.
This is the first thing to check. Cheapest, quickest, least invasive.
The torque spec is fairly light on these bolts, as I recall. Like in 'inch-lbs'. So don't gorilla them.
If you do find one or more loose, it may be useful to put some blue threadlocker on them before tightening. Don't use red!


This could be the intake manifold gasket. Cold it can shrink, allowing un-metered air in, causing a lean condition. Once the engine warms up, they expand, closing off the leak(s) and restoring normal A/F ratio.
This is what I was thinking, myself. To test, get a can of starting fluid. Brake cleaner can also work.
On a cold engine, raise the hood, and locate where the intake manifold joins the cylinder head(s).
Start the engine, and head back underhood. Spray at the juncture of manifold / head. If you notice the idle smoothing up for a few seconds immediately after spraying, you've found the culprit. Note that leaks can be one or many in nature, in different places. So you may have to spray in a few different places around the manifold.
 

Forum Statistics

Threads
23,273
Posts
637,484
Members
18,472
Latest member
MissCrutcher

Members Online