We receive many calls from terrified owners that have experienced a backfire condition on initial start. There is a simple cause of this. You. What happens with the LT5's ignition sequence is the ECM checks the cam and crankshaft positions by their appropriate sensors. The ECM monitors the signal, determines which cylinder is ready to fire and then releases fuel and timed spark which then allows the engine to start. This occurs much in the same way every time we start our cars and frankly, we become so familiar with the exact amount of crank time we eventually anticipate the start up ever so slightly. When we move the ignition cylinder from the "Start" position prior to the engine actually starting, this can cause an out of sequence cylinder to fire with it's intake valves open, causing a "backfire".

This causes several problems. First, the PCV valves (located just left of center of the rear of the engine) become detached from their respective rubber housings. Secondly, most other vacuum hoses on the LT5 become soft and loose after a period of time which allows them to come loose also. The visible result of this is black smoke (MAP sensor line), and poor performance in general.

In the case of a backfire, immediately inspect all visible vacuum lines and connections. It is possible that vacuum connections under the plenum have also become detached. This may be readily apparent in that the auxiliary vacuum pump, located in the right front headlight area, will operate continuously with the key on. In order to gain access to all vacuum hoses, the plenum will require removal. We strongly suggest either replacement of any loose or oil soaked vacuum lines or at least the use of tie strap retainers used tightly to secure them.

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