Causes of Backfire

[jss06c6]

Gents, I think a discussion as to the causes of backfire might be worthy of a new thread (unless this is covered in an older thread). I suffered a backfire which blew off the PCV connection and the Gas Tank Purge Solenoid connection underneath the plenum. I've had the car since 2013 and have never had a backfire. It had been sitting for a couple of weeks in my shop and I needed to move it. When I started it up, it immediately backfired. I waited about 30 seconds and started it again. Fired right up, but belching lots of smoke out the back (rich fuel!) and then settled into a high idle (1200 rpm). Drove it, no problems at all, but had high idle. That's another thread..

So, I was thinking about what could cause the backfire. When I took the intake horn off, I could clearly see where pressure exhausted around the TB valves. Only way this could happen would be that a cylinder fired when there was fuel present and an intake valve was at least partially open. So how can this happen.. I'm guessing that the LT-5 employs a batch fire logic at start up (opposing cylinders get spark at the same time) rather than sequential spark
& fuel injection as newer engines do. If this is the case, how is backfire prevented from happening again when some raw fuel is sitting in a cylinder from engine shut down?

Do we tune this out? I know Marc employs an Antibackfire change in his tune (this is in my car) when headers are installed (like mine).

Listening to learn from the experts.. Seems to suggest with better control from the ECM (MS3PRO or other), we would be able to properly eliminate this issue.

[jss06c6]

Found this in another section.. this explains some of the cause..

"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."

I always keep the starter going until the engine fully fires up..

[lfalzarano]

I’ve lost 2 injectors due to backfire. As mentioned, I also keep the starter going until the engine completely fires up! Never had a backfire since!


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[Ccmano]

Yup, that’s it. Especially when the car has been sitting for a while. Fuel and oil pressure take slightly longer to build while cranking. Just keep cranking don’t stop. Btw the LT5 does have a batch fire ignition system as I understand it.
H
:cheers:

[Flyman 27]

Good to know! Thanks for the info.