2007 4.3L runs rough when cold better after warmed up

[thx1138v2]

I just bought a 2007 GMC Sierra 4.3L Classic. It runs rough until it warms up. It stutters and surges until it gets warm and then smooths out and runs much better when warmed up but still dies on occasion when stopped at stop light.

 

This LU3 engine is new to me. I previously drove a 1994 GMC with a VIN Z engine for 30 years.

 

Any help will be appreciated.

[mjm-1957]

Make sure the EGR valve is closing. Carbon on the pintle can prevent it from closing and cause what you are describing.

[richard wysong]

Codes?

[thx1138v2]

7 hours ago, richard wysong said:

Codes?

No codes but I probably need a better OBDII scanner. My '94 was an OBDI and I never did get it working with the scanner I have.

 

Recommendation?

[thx1138v2]

9 hours ago, mjm-1957 said:

Make sure the EGR valve is closing. Carbon on the pintle can prevent it from closing and cause what you are describing.

I didn't think of that one. I did think of the CTS and the IAC. I'll look into it. Thanks.

[richard wysong]

yeah a scanner that does live data would be helpful, It sounds like the problem is worse in open loop

[thx1138v2]

On 8/2/2023 at 5:55 AM, richard wysong said:

yeah a scanner that does live data would be helpful, It sounds like the problem is worse in open loop

I'm shopping for another scanner. Any suggestions?

 

I've heard the "open/closed loop" terms for years and still don't fully understand them. I'm a computer programmer by trade so it may be that is what is confusing me. Could you please explain the phrase to me.

 

BTW, after determining that all of the problems were caused by carbon build up, I put a can of SeaFoam in the gas yesterday and drove about 60 miles. It seams to have cleared it up for now. I'll monitor it though.

 

Any good/bad comments about SeaFoam in the gas as opposed to the crankcase?

[thx1138v2]

I just got some interesting info from Ace Hardware, of all places. I took my key there to get a copy made and the guy told me that the key has a transponder in it that communicates with either the fuel pump or the ignition system, depending on the make/model.

 

I don't know how the key gets its power but I suspect it draws a slight charge from the charging system. I suspect the truck sat for a long time before I bought it and the charging system for the key ran down and it was dropping out as I drove it. The longer I drove it the stronger the charge got so it has a stronger transponder signal now and its running just fine.

 

Does anyone else have any info on this? I'm surprised but the symptoms match a poor transponder signal.

 

I'll put a new Coolant Temperature Sensor (CTS) in it anyway to hopefully improve the gas mileage. I don't know, in light of the above info, if the SeamFoam is making any difference or not.

[thx1138v2]

ACE Hardware couldn't make the key I have so I went to the dealership. It turns out that my key does NOT have a transponder in it so the dealer made me a plain key and it works fine.

 

I haven't had any problems since I had the new key made but it definitely didn't cure anything. So I guess I need to replace the EGR valve. I can't find it on the internet You tube videos or stills. I may have located it on the driver's side right above the valve cover but I'm not sure that is it.

 

I think what I'll do is remove the air inlet duct and take a picture of what I think is it and post it here.

 

I'll do that in the morning because it's already 100F here and is supposed to get to 107F today.

[thx1138v2]

Well it looks like the EGR system has been removed. At least that's my take after looking for it for 2 days.

 

I did go ahead and take a couple of photos. I should definitely get better gas mileage with the new CTS.