Jurrel

The car in question is a 1984 2.8L V6 T-5 Firebird, no mods.

The problem is that usually, but not always when shifting up or down from 1-2, and rarer though it still happens 2-3, you can hear the exhaust pop as the engine revs come down. To me this does not seem normal. I do know that in the 2.8 V6 Manuals that there is a Deceleration valve, in the SMOG/A.I.R setup, to stop/limit backfiring from what I can figure out. Could that be bad or could this be tied back into the ignition? I have replaced the alternator and the coil recently, looking at the plugs and wires, those likewise are in good shape. BTW this engine has at least 250,000 miles on it.

Also as far as I know, there are no exhaust leaks.

92RSSlowmaro

Mine does the same thing. Its because my cat is shot. Might be a different case for you, but im 99 percent sure thats what causes the "cackling" noise when I release the throttle at high RPMs.

Hit your cat with a rubber mallet, and if you hear anything rattle around insite, its probably dead.

2_point8_boy

yeah, mine does that, but once the motor is all warmed up, it goes away. Does your's do the same once it's warm. I think it's normal and that there isn't really anything to do about it.

Jurrel

Well, to be honest as a test I gutted my cat, and it is still doing it. I will be replacing it with a high flow in about a month. We will see then if it goes away or if as 2_point8_boy says, its normal on the cars, esp at this age. As to if mine does it when warm, I beleive it does but at this point not to sure, haven't paid close enough attention then.

Thanks for the replys!

89cmrodriver

mine does that too, my cat is bad. it does it till it gets warm

Gumby

Yep, I have no cat and it does that. Also the higher the octang rating of gas, the more it does it. ultra 94 is sweet at night.

Matt

camaro_junkie

Mine does that also. I think it might be either the ignition or the cat. I also have a T5 and it backfires when I shift up. I recently replaced my decel valve and EGR solenoid and the backfires don't happen as frequently. I think my cat is plugged because the car doesn't have a lot of power especially when it warms up.

Hey Gumby, is octang that orange powder you put in your gas to speed up your car? Just bugging you.

Gumby

As funny as that may be. Your plugs will turn a bright orange color if you run it to high. Half a can of 104+ in a 2 gallon motorcycle will do just that.

Matt

camaro_junkie

Can an exhaust leak before the O2 sensor cause a backfire?

Gumby

Yep, cause the computer may run the car richer from a bad reading.

camaro_junkie

Ya, I was thinking that fresh air could get in through the leak and cause a high O2 reading (but then I thought this couldn't happen because the exhaust gas was being forced out), but I wasn't thinking that exhaust gas leaking out would cause a bad reading. Good point.

AGood2.8

You guys all have exhaust leaks somewhere in the system- most likely pinhole corrosion leaks in the exhaust tubing. My cat is bad (rattles when first started- goes away once car is warm) and I do not get even the slightest crackle or backfire ever. My exhaust sound very smooth.

Remember when everyone was posting the exhaust clips? I could hear all of the crackle leaks then.

Check your gaskets and tubing carefully- especially the bend up over the rear axle.

camaro_junkie

You should see my I pipe. First off, it's not mandrel bent. Second, I replaced it when I replaced my fuel pump because I had to cut the old one off. Then I had to hammer the new one into the old muffler. It's got numerous restrictions from that. The whole things going. I'm getting a complete Dynamax system including a high flow cat. Hopefully Pacesetter will get together a header and Y-pipe package too.