Engine Stumble At Cold Start

[carlom]

I have a 2008 Gmc Sierra K-1500 4.3L V6 auto with 41K miles. The engine has always idled and ran smooth. A few days ago out of the blue at every cold start, the engine stumbles like multiple misfires, it's pretty bad. This happens just while backing out of my garage. It just does this for about a minute or two while it's in open loop. Once it's in closed loop it's fine from then on. There is no check engine light. I checked for stored codes with my scanner, there are none. I cleaned the throttle body from the top only but it made no change. Air filter is perfectly clean.

 

Thanks for any help.

[nickvrebel]

if you feel the difference occurs between open and closed loop you need to hook up a scanner capable of reading the o2 sensors and look for a possible bad sensor that just isn't tripping the light.

[carlom]

if you feel the difference occurs between open and closed loop you need to hook up a scanner capable of reading the o2 sensors and look for a possible bad sensor that just isn't tripping the light.

 

 

Thanks for replying. The problem occurs in open loop mode. The O2 sensors are not used in that mode. In closed loop mode when they are used it runs fine.

[nickvrebel]

great, well it seems you are smart enough to figure this out on your own, have a nice day

[carlom]

great, well it seems you are smart enough to figure this out on your own, have a nice day

 

 

I did figure it out myself. There is "special policy # 10250" extended warranty that covers defective spark plug wires and spark plugs. #1 wire was defective and fouled the plug. I had both replaced for free at a dealer. I guess I didn't have to bother anyone here afterall.

Thanks, I am having a nice day!