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2009 LY5 - 8cyl mode rough idle & hesitation

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85,000 miles LY5 5.3 DOD 4spd, only mod is whacked exhaust post cat converter to side exit.


here's the issue, started getting what felt like a vacuum leak on -20F days idle... like the classic old vacuum hose leak stumbles (but no codes)


I've since learned that the problem does exist at higher rpms, anytime I'm in V8 esp. on part throttle high load it acts like somebody unplugged the brake booster line, lol. I kept it in 3rd to lock out the DOD and you can feel the sluggishness, random hesitation, and sometimes i think it even detects knock even after the truck is plenty warm, but ONLY in 8 cylinder mode....


So, i'm thinking 4 things

1. fuel pump

2. injector

3. fouling 1,4,6, or 7 DOD cylinder

4. vacuum leak?


eliminated fuel pump because it's fine in 4v mode.

if it's 2. it's gotta be on 1,4,6, or 7.

fouling plug - haven't checked.


Leaning on vacuum leak, because:

it's the worst on cold days

i'm not getting misfire codes and a fouling plug should code.

A fuel pump or fuel delivery problem would also code a lean or misfire.

therefore, prob a fluctuating a/f ration due to vacuum leak on 1,4,6, or 7 intake runner at head i.e. intake gasket.


I can only really feel the problem in V8.... not using oil.... usually about 1qt over 3,000 and thats 5w30 Mobil 1 synth


Worth noting, in 4cylinder mode the computer bumps up the vacuum by opening the throttle without you moving the pedal to increase torque offsetting cylinder drop outs, could that mask an intake manifold gasket problem and trick me into a fouling cylinder?

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Just got the Call from Amanda, service rep at Carter Chevrolet in Manchester, CT.... TPS was toast, throttle position sensor. which makes sense, TPS would make it act like vacuum and misfire so thats that, covered under 100,000 mile warranty and it was a tsb.... thanks GM.

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