Hot start piston knock
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3rd Gear
Joined: May 2025
Posts: 3
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Hot start piston knock
13 Z06 new to me: On cold start, all normal. On hot start, sluggish start with what sounds like a piston slap ? (very short as if occurring only on a single crank rotation), then runs normally. No error codes, knock sensors functioning normally, no timing issues, battery good, starter probably good, car all stock (with exception of a fresh air intake). Any others out there with similar issue (reputable dealer ran through the issue with GM... no solution yet)?
Last edited by sk13z; Jun 28, 2025 at 10:54 AM. Reason: New info
Pro
Joined: Oct 2024
Posts: 722
Likes: 398
From: WI
13 Z06 new to me: On cold start, all normal. On hot start, sluggish start with what sounds like a piston slap ? (very short as if occurring only on a single crank rotation), then runs normally. No error codes, knock sensors functioning normally, no timing issues, battery good, starter probably good, car all stock (with exception of a fresh air intake). Any others out there with similar issue (reputable dealer ran through the issue with GM... no solution yet)?
Burning Brakes
Joined: Jun 2022
Posts: 750
Likes: 255
From: Florida
13 Z06 new to me: On cold start, all normal. On hot start, sluggish start with what sounds like a piston slap ? (very short as if occurring only on a single crank rotation), then runs normally. No error codes, knock sensors functioning normally, no timing issues, battery good, starter probably good, car all stock (with exception of a fresh air intake). Any others out there with similar issue (reputable dealer ran through the issue with GM... no solution yet)?
Thread Starter
3rd Gear
Joined: May 2025
Posts: 3
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Further info: Found a reputable shop to look at car. Tune was aftermarket (not previously known), knock sensor DTC was off, factory tune had to be placed back as well as pulling some timing on start.
Thanks
Thanks
Thread Starter
3rd Gear
Joined: May 2025
Posts: 3
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Heads not done, but lifters making some noise. When I purchased the car, did not know of the aftermarket tune on the car, as well as some DTC's turned off.
Last edited by sk13z; Jun 28, 2025 at 11:10 AM. Reason: correction
Safety Car
Joined: Oct 2007
Posts: 4,222
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From: South Florida
pistons rise in temp very slowly. The particular knock they could make is only gradually reduced over say, 20 minutes, as they rise from 80*F or whatever to over 400*F (say 0.5kg piston aluminum, change in temp 177*C(80*F to 400*F), specific heat of Al is 900J/kg*K, Q = 71kJ, Idle is 5-7kW so waste heat about 3 to 5kW, only less than 1% of residual heat reaches pistons so 0.03kW to 0.05kW its about 23 to 30minutes to heat the pistons at idle), and may not ever completely go away during operation at neutral loads, such as high idle, decreasing to idle, and during transitions positive to negative torque (start of engine braking and light on/off load).
