GM: Care must be taken to prevent hydrolock
Some GM service procedures may advise to de-carbon the engine with GM Upper Engine and Fuel Injector Cleaner to remove valve deposits, but the label on the container does not include instructions.
Following are GM guidelines regarding the use of this cleaner.
NOTE: Extreme care must be taken to prevent hydrolocking the engine when using the cleaner. If too much cleaner is induced at too-low rpm, or if you force the engine to stall by inducing too much cleaner, the engine may hydrolock and bend a connecting rod.
In a well-ventilated area with the engine at operating temperature, slowly/carefully induce a bottle of GM Upper Engine and Fuel Injection cleaner into the engine with rpm off of idle enough to prevent it from stalling (typically around 2,000 rpm). Depending on the engine configuration, induce the cleaner through the throttle body or an engine vacuum hose/pipe.
Turn the engine off after inducing the cleaner and allow the cleaner to soak with the engine off for one hour (do not let the cleaner soak for more than two hours, as remaining deposits may start to harden again).
Add a bottle of GM Fuel System Treatment Plus to the fuel tank and fill the fuel tank with one of the Top Tier gasolines listed in the latest version of 04-06-04-047. See bulletin 05-00-89-078 for more details on GM Fuel System Treatment Plus.
Test drive the vehicle extensively to circulate the GM Fuel System Treatment Plus.
Advise the customer to only use one of the Top Tier gasolines listed in the latest version of 04-06-04-047 to minimize intake valve deposits in the future.