Engine sputters, stumbles and misses when hot, hard to start hot, runs great cold

767Jockey

Original poster
Member
Apr 5, 2012
30
My TB is a 2005 4x4 Trailblazer with a Vortec 4.2L six. It's got about 110K miles on it. The engine sputters, stumbles and misses when hot, and it's a bit hard to start hot, however it runs great when the engine is cold. The problem is most pronounced when accelerating away from a stop. It'll sort itself out after a few seconds, but never runs quite right after it warms up, there is always a barely perceptible surging when driving even at highway cruise speeds. The engine idles just fine, either hot or cold. It's worst when accelerating from a stop.

I have replaced the air filter and spark plugs, that didn't help. I am told that because the car is a 2005, there is no separate fuel filter, that it's integral in the tank. It has never been changed, as I am the original owner and have never done it.

If anyone has any ideas or troubleshooting that I can perform I would greatly appreciate it. I don't know where to start on this one.

Thanks!
 

TangoBravo

Member
Dec 5, 2011
208
Mine is an 04 and I have a fuel filter on my frame rail, no filter in my tank. I checked rockauto.com and two other parts stores and they do not list an in tank filter for an 05. just the same one on the rail like I have. I would first start by replacing that. Also what spark plugs did you use?
 

Tarnished

Member
Dec 3, 2011
74
767Jockey said:
My TB is a 2005 4x4 Trailblazer with a Vortec 4.2L six. It's got about 110K miles on it. The engine sputters, stumbles and misses when hot, and it's a bit hard to start hot, however it runs great when the engine is cold. The problem is most pronounced when accelerating away from a stop. It'll sort itself out after a few seconds, but never runs quite right after it warms up, there is always a barely perceptible surging when driving even at highway cruise speeds. The engine idles just fine, either hot or cold. It's worst when accelerating from a stop.

I have replaced the air filter and spark plugs, that didn't help. I am told that because the car is a 2005, there is no separate fuel filter, that it's integral in the tank. It has never been changed, as I am the original owner and have never done it.

If anyone has any ideas or troubleshooting that I can perform I would greatly appreciate it. I don't know where to start on this one.

Thanks!

Yep, us 05 guys and newer were cheated out of a fuel filter. I installed an aftermarket one on mine.
Hopefully the more knowledgeable guys chime in.. it could be any number of things. I've never had to service my in-tank screen before.
 

Ed H

Member
Oct 18, 2012
167
If it runs fine cold (open loop), but not hot (closed loop), it might be a sensor going.

Do you or a friend have access to a scanner or the Torque app?
 

767Jockey

Original poster
Member
Apr 5, 2012
30
Ed H said:
If it runs fine cold (open loop), but not hot (closed loop), it might be a sensor going.

Do you or a friend have access to a scanner or the Torque app?

No access to either. Anyone have any ideas what sensor may cause this? Rather than drop large coin at the dealer, it's far cheaper to replace sensors one at a time until I hit the right one. They're usually not terribly expensive.
 

767Jockey

Original poster
Member
Apr 5, 2012
30
767Jockey said:
My TB is a 2005 4x4 Trailblazer with a Vortec 4.2L six. It's got about 110K miles on it. The engine sputters, stumbles and misses when hot, and it's a bit hard to start hot, however it runs great when the engine is cold. The problem is most pronounced when accelerating away from a stop. It'll sort itself out after a few seconds, but never runs quite right after it warms up, there is always a barely perceptible surging when driving even at highway cruise speeds. The engine idles just fine, either hot or cold. It's worst when accelerating from a stop.

I have replaced the air filter and spark plugs, that didn't help. I am told that because the car is a 2005, there is no separate fuel filter, that it's integral in the tank. It has never been changed, as I am the original owner and have never done it.

If anyone has any ideas or troubleshooting that I can perform I would greatly appreciate it. I don't know where to start on this one.

Thanks!

I forget which they were, they were the exact same as originally installed in the car. My old ones went over 100,000 miles!
 

CaptainXL

Member
Dec 4, 2011
2,445
767Jockey said:
it's far cheaper to replace sensors one at a time until I hit the right one.

I normally err on the side of caution regarding this line of reasoning. Sensors aren't cheap. Since you installed new plugs the O2 sensor should be changed along with them at 100K. It's fairly standard practice as O2 sensors can get sluggish over time.

767Jockey said:
Anyone have any ideas what sensor may cause this? Rather than drop large coin at the dealer

It's highly recommended to bring it to Autozone or Advance Auto and have then pull any codes as a preliminary check. We don't know the problem anymore than you do.

When they hookup the scanner check the coolant temp and see if it goes into closed loop. They will be able to see that. This scan will check for codes and test your coolant temp sensor.
 

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