jweather73

First of all (yes, I did my search like all good little member does) and found a lot of good info... But what I would like to do is to state my problem and what I found to be the possible solutions and see if you believe it to be correct, or if I may be missing something.

(84-GSL, 45k on engine rebuild, mostly stock, gutted cat.)

The problem is, for example, I'll be cruising down the Dallas Autobahn (the Tollway) and I'm typically in 5th. Well, I can accelerate pretty good in 5th since it's a short gear (which I wish I could change) but anyway, I can floor it (or really near) for oh, about 15-20 seconds and get to the speed/area I need to be, and when I let off the gas, in a few seconds I'll have what sounds like a major explosion under my car! (meaning, it's not just the typical puttering it does when the shutter valve is out or disconnected.) It's LOUD!

This all started when I gutted my cat out this weekend. Basically I have a completely empty cat "shell"... my air pump is disconnected too (though still mounted on the engine). Shutter valve is bypassed, and my exhaust needs two new gaskets -- one at the e. manifold, and the other where one pipe meets the other on one side of the cat. So, yes, I have an exhaust leak which I know can cause it to backfire. Not having a cat can cause it too... bad plugs as well. I really doubt it has anything to do with the timing since it just started after gutting the cat (unless I'm just totally clueless and it has everything to do with it.)

Anyway, am I missing something else? Anything else you can suggest (besides a new muffler which will happen when I can afford it).

Thanks in advance!

-Jeff

kleach18

When you gut the cat its gonna do that...Whats wrong with that, you get some huge flames now(thats what happenes when you hear that loud pop)...

kleach18

I've been told that when piston engines back fire it can be bad, but to the best of my knowledge it doesn't hurt our cars..... The way fuel is burn't(or not burnt) in a rotary is why it does that... Whatever fuel is unburnt is pushed out into the exhaust(when you had the cat it kept it from igniting in your exhaust system)

If I'm wrong somebody please correct me....

GT3_racer

That whats happening, flames out the back, might I suggest a warning sticker, and I have a nice tall diff for you, want to work a deal?

cdrad51

WARNING: IF YOU CAN READ THIS, IT'S ABOUT TO GET REAL HOT

Tom

MY car does not exhibit that at all. All emmisions removed, RB header, duals all the way back. Very little sputtering, no backfires..

The only time it would backfire is if the idle was adjusted too high (around 1,000 rpm+) and you shut the car down.

cdrad51

My car idles currently at 1500 rpm, thanks to the dashpot, and when I shut it off it doesn't backfire. In fact, the car doesn't backfire at all no matter what I do, and I have all emisions removed and a cat replacement pipe. And it's running real rich. Go figure.

Tom

Just goes to prove these cars have temperments all their own..

LongDuck

Well, you already identified this in your original e-mail, but the exhaust leaks from your gaskets falling apart are more than likely the cause for the backfiring, combined with an overly rich mixture from running it up and then backing off.

When you run the engine up to high RPM, this causes your carb or EFI system to induce an overly rich mixture so that the engine has enough fuel to burn during acceleration. In other words, to get the engine to spin faster, you need more fuel to burn to accelerate the engine parts and attain that high rpm. When you get to the high rpm setting and HOLD it there, the fuel/air mixture from the EFI or carb stabilizes to close to stoichiometric balance, and all the fuel gets burned.

When you get to high RPM and BACK OFF quickly, this causes all that excess fuel to run out through the exhaust ports without being completely burned. Normally, this isn't a problem, since you have that hot cat back there to burn any unburned fuel in an enclosed container. This is a very controlled burn, and in fact, the cats do not get fresh air on deceleration to help avoid backfiring which can blow out exhaust systems and cause fires.

Instead, on your engine, the gaskets that need to be replaced are allowing fresh, O2 rich air from the outside to enter your exhaust system, where they mix with the fuel and create a combustible mixture. This combustible mixture ignites on some hot part of your exhaust system and results in a resounding blast down the tailpipe.

I want to caution you that this only takes place for so long before you blow out your muffler or damage your exhaust system somewhere. About 6 months ago, an SE owner posted a picture of a muffler that was split down the side from an exhaust system explosion.

That's too close to the fuel tank for my comfort,... I don't know about you. HTH,

cdrad51

god I had to look that one up. Here it is for anybody else who hadn't seen it before:

stoichiometric:
Having the exact proportions for a particular chemical reaction


stoichiometric ratio:
In a spark-ignition engine, the ideal air/fuel mixture ratio of 14.7:1, which must be maintained on engines with dual-bed and three-way catalytic converters.

clashkpr28

was actually just going to post this question but i saw the thread.... i have a strait pipe and my car backfires in 5th just like jweather's... anything i can do to prevent or stop this from happening?

thanks

Brianhsval

Hell, mine has NO cats and backfires in ALL gears if I let off of it to quick. Matter of fast that is why I have NO cats left. Blew them out! I try not to let off to quick anymore, seems to help alot.

jweather73

Apparently the problem was, in fact, the burned up gaskets. I replaced them for the 3rd time since I got the car (a little over a month ago)... the 1st time was because they were just rusted out. The 2nd time was about 2 weeks ago when the cat was getting totally clogged and burned them out way too fast. Obviously, last night was the 3rd time... I replaced the e.manifold gasket (broke and replaced a bolt on the damn thing too) and one on the non-welded side of the cat (opposite of where the pre-silencer used to be.

So, apparently, it was the "breath of fresh air" that caused the tiny explosions in my exhaust system.... too bad I didn't have an after-market muffler.... would have been nice to see the flames!

sweetege

Yea that pic might have been me i was messing around with the gas and boom sounded like a shotgun. definetely hurt my ears. and i had a half-foot wide gash the whole length of my muffler. it was a cheap muffler but it still ripped it a new one. and before that i was doing the same gas thing (yea i do'nt learn) and that was when i had my cats. well..... they got gutted and slung all the interior of them into my stock muffler and bam straight pipe once again. so i've finally learned. i'm gettin a RB header & presilencer and a magnaflow ss packed mufler.

198512A

my air pump is disconnected too (though still mounted on the engine).




Sorry to thread jack, but is this safe to do, or would it be better to make a blocking plate rather than leaving te stock pump on?

LongDuck

To this last question, if the pump isn't turning (belt removed, etc.), then you're not getting any airflow to the exhaust ports or the cats, so it really won't matter if you have that huge chunk of metal in there, or not.

The AP is only functional to provide fresh air when it's spinning, so if it's not, I'd remove it. You'll save yourself about 10lbs of weight and the clutter on that side of the engine, but if you need to pass emissions inspection, put it in the garage for a future date.

The cats need the air to work, but if your pump's not running, it's not going to matter other than plugging the cat inlet somehow. HTH,

198512A

Thanks, no New Mexico has no smog tests, no cats, so I will make backing plate. But isnt the metering pump in that area? maybe it needs that air to cool? I am not sure but I thought I would ask.

LongDuck

Nope, the OMP does not get any air from the AP - you're safe to remove it all and replace with the blocking plate if you want.