Backfire

[Stommer]

I need some advice. I have a 92 coupe auto w/ gutted cats and straight pipes back to Vortex mufflers. My problem is that when I really hammer on it, say 2nd gear, up around 80 or so then let off the gas I get backfire. What could be causing this? It can't be to healthy for my car. Other than that she runs like a Swiss watch. Any help would be very appreciated. Thanks. John

 

[Eagle85C4]

Check your timing... and check to see if any codes have shown up in the ECM....let us know...

 

[Stommer]

backfire

Eagle85c4, Timing is fine. No codes. I'm clueless. Stommer

 

[Stommer]

Eagle85c4, Would the addition of hotter plugs and possibly a hotter coil make any difference????

 

[vettepilot]

Stommer,
Please clarify one thing for us if you would, is it a true "backfire" through the intake or is it popping through the exhaust?
Since we are talking about a 92 LT1, it has the opti-spark system. Really quite different in layout from the previous C4 systems, all the parts and pieces are there, just arranged much differently. .
vettepilot

 

[Eagle85C4]

I would not think so....I remember years ago to make a car backfire you would let off the gas and even pull the choke on a bit to throw gas into the exhaust....you may have a leaky injector. I have added Bosch +4's in mine as well as the Accel 8.8 wires, Super Coil and cap and rotor...never had a problem...

Does it idle ok?
Have you done a vacuum check?
Check for loose connection in the distributor and to it.
On the passenger side next to the Blower Motor is the ESC(electronic spark control) box...check the connections there.

A great trouble shooting page can be found at www.chevythunder.com

Great site ...all about TPI

 

[Stommer]

Vettepilot, To answer your question, it sounds like its coming from the exhaust. Car runs great other than the backfire. I must tell you though it never happened until I changed the exhaust. Stommer

 

[vigman]

Lack of back

Since the cats are gutted and you back off the throttle.. the mix for a fraction of a second is to rich and the POP or Burp.. is the last chunk of to rich fuel ( exhaust ) igniting in the exhaust chamber... probably around the rear of the main gutted cat... since that would be the hottest metal area in the pipe system
( after a full throttle burn).

A hotter plug MIGHT reduce SOME OF IT but you will be trading one problem with another.

Consider it normal the way your setup...


Vig!

 

[Stommer]

Vigman, Thank you for your input. How about a hotter coil??? Would that make any difference? Stommer

 

[vettepilot]

One thing I would check for is a slightly loose connection allowing fresh air into the exhaust for a "final burn" from what was already mentioned above... an injector that might not be fully closed between cycles. If it began only after the exhaust system change that would be where I would begin looking. Sometimes a small leak at a connection is all that's needed to allow fresh air, (oxygen) into the system that otherwise would not support combustion because of the high levels of CO2.
Good luck let us know what you find.
vettepilot

 

[Stommer]

Vettepilot, I'm on my way out for the day, I will have a chance later this afternoon to take a look for possible leaks. Thank you for your insight. Stommer

 

[Stommer]

Vettepiolot, Did a complete ck. of the exhaust and found no leaks of any kind. I'm hoping I don't have to bring it in to my chev. dealer. I'm sure that will be a costly analysis. Stommer

 

[cntrhub]

Remember your basic physics. "For every reaction, there is an opposite and equal reaction." So generally speaking, if you are on the throttle, you're pushing gas out the exhaust. If you lift, there is air being sucked back in the exhaust system. Like Vic said, you have this build up of unspent fuel inside the exhaust system. Eventually, there is going to be an ideal situation, where there is enough unspend fuel, air, and a flame front, (hot spot) to ignite fuel vapor inside the exhaust system. This is the "backfire" you are hearing.
A different spark plug won't help. You'll still have unspent fuel inside the exhaust system, weather you have a cold or hot plug. The problem is unspent fuel after the exhaust port.

 

[ArtZR1991]

I agree with Vig, and I also believe the absence (relatively) of back pressure is allowing some intake charge thru unburned when you release the throttle. if it's what I think it is, I would say you may be able to tweek it out, but I would not consider it to be a problem, assuming the engine has been adjusted for the new exhaust setup. Engines with headers and straight thru exhaust (like my Cobra) do this kind of thing.

Regards,

 

[vigman]

Well here's a nutty thought

If you back off slowly... does it still happen?

And as you jump OFF the gas does it happen MORE?

A hotter coil/plugs will not fix the situation but it MIGHT lessen it ( a small percentage..like 1 less burp ).

The ONLY way to fix it is to put the cat back in!

OR

You could stop/ plug the air injection going to the main cat...with less Air...it might not fire as bad......

Left feild theory

The other thing to consider here is after you shut the engine down after a hot run,
( using open/ straight pipes as an example )

The cold air in rush after shutdown will chill the valves to quickly.. causing silght heat stress ( cracks ) on a microscopic level, which over time will cause a valve to snap & drop... OUCH!

Now if your running stainless valves...it's less of a deal...

As you reduce the back pressure you give ambient air a chance to get back into the exhaust system......

Where do you think all that water comes from when you fire up on a COLD morning!

You might want to consider this....

Any exchange for preformance costs something ( as you are finding out )

And in my line of work...

There no such thing as a free lunch!


Vig!

ps
In High school, a Honda SL 100 gave it's life to me on order to learn this lesson ( Oh yeah 2 pairs of shoes... since I didn't have a ride after I hack sawed the muffler off.. and dropped an exhaust valve)

 

[Stommer]

Vig, The only time it happens is when I'm really on the throttle and have to back off real quick. I must tell you guys this is my first vette and I am not that verse on how to get the most bang for my limited buck(wife). A friend of mine told me the first thing he would do is change the exhaust. Just the way I explained to you all earlier. I hope I didn't mess anything up. My vette is a cream puff!! Stommer