I swapped a 5.3/4l60e into my 68 short stepside this summer and it seemed to run great, I tuned it myself with hptuners and had it within 1-2% stft everywhere. Since it has turned cold I have been going way lean when engine is at idle, especially at first start up before it goes into closed loop. I took a video of it to show everyone what is happening. As soon as it gets to 18.0 or so afr it starts breaking up, so I know the wideband is accurate.

https://www.youtube.com/watch?v=wwJi9_2FY1c

Any ideas?

 

Possibly intake manifold gaskets???

 

Honestly I'm really hoping that. I have a knock sensor that went out so I am pulling my intake anyway.

 

FWIW If the gasket is the Red/Orange type and its smushed down, there no good.

Replace with the new Green/Blue type. They were revised to stand up to the higher ethanol content in the newer gas blends.

 

Pretty sure it's the red/orange type. I'm hoping this is the problem.

 

You will know as soon as you take them off.

If there smashed down and nearly flush with the plastic there no good. This happened on mine after I swapped the knock sensors. I didn't buy gaskets and the following week I had to change them out as it started throwing lean codes.

 

You could always spray carb and choke cleaner around any areas that you think could possibly be a vacuum leak. Just spray the areas and monitor your STFT's or if still in open loop your wide band.

 

I've been meaning to have the wife come out with me and watch the wideband but with the weather it just hasn't happened.

 

1Fastbrick, you said

"FWIW If the gasket is the Red/Orange type and its smushed down, there no good.*

Replace with the new Green/Blue type. They were revised to stand up to the higher ethanol content in the newer gas blends."

Does anyone happen to know the part number for the newer gaskets? I am about to order them and don't want the old ones.

 

The GM# 89060413 Not sure about felpro or any other.

Every once and a while you can score an Ebay deal for about 1/2 price of the dealer list.