454 tbi idle surge

[Tiger Joe]

truck is a 92 k3500 dump truck. 454 tbi, nv4500. truck has 132k miles.

backstory- we have owned the truck about 6 years tuned up/maintained properly, no issues.
i put in an adjustable FPR, running like 13-13.5 PSI, and a TBI spacer. been on the truck for a few years working fine. also prior to our ownership someone had converted the truck to a heated O2 sensor.



recently, we noticed the fuel mileage of the truck was absolute garbage. its a big heavy dump truck, so it was never great, but the last 2 tanks we calculated like 6.8 mpg. it was just downright bad.

we decided to spend a few bucks and rebuild the throttle body, and put new injectors in it. on top of that, when we went to change the fuel filter, the fuel lines were rusty and broke. spoiler here- i suspect our poor mpg was due to a leaky return line the whole time, we just didnt find it until we started this.

so got the truck back together last weekend, after startup the idle was surging up and down, so i did the IAC relearn procedure and that seemed to work. i did not replace the IAC or TPS, just removed and reinstalled. after the relearned all seemed well.

fast forward to today- drive the truck and after about 10 min, the check engine light came on, and when you push the clutch in to stop the idle is surging up and down again. pulled the code and got 13- which says O2 sensor.

seems like convenient timing that O2 sensor went bad?
anything else i should be looking at? should i do the IAC relearn again, or consider replacing it? it does look like the original GM sensor.

also, i have one of those chinese knock off tech 2 scanners. i think that would work on a 92 to let me look at data correct?

[kwmech]

You have a vacuum leak somewhere. What is/was really common for leaking is the EGR separator cast iron plate under the valve body. That gasket in the front corner, where the blow hole is, would warp. Also check the intake mounting bolts and see if they are tight. If loose time to pull the intake and put new gaskets in

[Tiger Joe]

so just to close the loop here for anyone searching in the future.

my two issues were completely unrelated.

O2 sensor light was due to a damaged wire.

surge at idle was due leak at the IAC. when i rebuilt the throttle body it looks like the new o ring in the kit was a little too big compared to old. must have torn slightly putting it together.