1gt2nv

iTrader: (5)

I had a lm1 cat back and sold it. I bought a used magnaflow cat back. The car runs just fine, drives fine even at wot. But if I have it in park or neutral and rev the engine, it will sound like a misfire. It pops and breaks up at about 3 grand. But if I slowly raise the rpm's, it sounds just fine. Could the muffler just be shot? Maybe plugged up or something? The tips had alot of build up in them when I bought the catback. My car never did any of this with the LM1 cat back.

bad_408_vert

iTrader: (6)

I noticed it with my magnaflow around the same rpm range also but while accelerating not on a rev. What muffler did you use?

1gt2nv

iTrader: (5)

I'm not sure sure what muffler it is. I bought it used. It's a magnaflow cat back. The muffler is where the stocker was. Two pipes on the pass side and one on the drivers side.

99Bluz28

iTrader: (35)

Check for a exhaust leak where the Y-pipe and I-pipe join, if not there work you way forward until you find it.

1gt2nv

iTrader: (5)

I looked for leaks. There are none. Felt around with my hand and theres also no water on my garage floor after I let the car run for awhile. I could always look some more though.

2win_SS

iTrader: (1)

Mine LM1 will not really POP! but it makes a noise when I'm around 2-2.5k rpm and change gears... Only when's it cold tho... But my buddy BORLA also does this. I thought nothing of it

2win_SS

iTrader: (1)

Anyone have sum info?

my_99Z

iTrader: (4)

I have long tubes an off road y pipe and magnaflow catback and don't get any popping. I also had the magnaflow on it before doing the long tube headers and never had any issue with popping or backfiring.

Blackhawk777

I suggest maybe going over the usual things. Plugs, wires, stuff like that. Mine with the lm1 was doing that until I gave it a tune up.

2win_SS

iTrader: (1)

Sweet. I'll check it out

hiram007

iTrader: (19)

Yup, my LM1 did the exact same thing, when cold and when shifting, I didnt have a tune when I installed my LT's though so I'm pretty sure that was it. All my wires and plugs were good.

2win_SS

iTrader: (1)

^^^ did this noise stop after you tuned the ECU?

StainlessWorks

If it only pops when you let off, then I'm betting the muffler is OK and its in the tune.

Take a look at your Deceleration Fuel Trim.
When stock, the computer dumps fuel every time you let off and I'm betting that's the popping you are hearing.

If it is breaking up while giving it gas, a muffler change shouldn't be enough to skew the tune.

leon phelps

Fuel trim. get a street tune. had the same problem with a ls1 swap. the street tune fixed it and stopped me from throwing hundreds at the problem. you will be best off paying for it from a shop and not having a gypsy doing it.

2win_SS

iTrader: (1)

Sounds good. Thanks fellas

hiram007

iTrader: (19)

Never did a tune and I ran it like that for 7 years lol. It was mostly a weekend car for about 5 of those years and sat for about another year. It was only the last year that I really began to put money into it. Planned on getting the tune but then sold it to my brother who totaled it. The tune however, would have taken care of it as was mentioned by others without a doubt.

1gt2nv

iTrader: (5)

Still trying to figure out this pop sound. I got a new tune and it still does it. Checked for exhaust leaks and there are none. Think the muffler could be bad? It was used, so I have no idea how many miles are on it. I found a part number for the muffler that magnaflow uses on there cat backs. #14267. I might just buy a new muffler. It only does this while I rev the car up. Not when the revs come down.

1gt2nv

iTrader: (5)

Dyno sheet says AFR is min: 12.3 Max was: 14.0 and Avg was: 12.7
Could it be to rich? Think I might have a leak where the y pipe and I pipe meet. I'm going to weld it and see if that helps. Any ideas?