Thoughts on backfire?

[KILLSHOTS]

As many of you know, I just went from stock catted manifolds and straight pipes to coated OBX headers (no cats) and Corsa. Sent the PROM to Marc and he removed the DFCO. Yet, this thing is backfiring like an SOB!!! It backfires loudly on nearly every upshift. Frigging embarrassing!

Bill Boudreau data-scanned the car and it is running unusually lean on deceleration. Marc says this is common with FIC injectors and says Accel are better about this. I spent enough cash installing the new FIC and I'm not changing to Accel just to "see what happens". Marc says he can add fuel on decel but isn't sure if this will fix it.

What am I missing? Do I have an exhaust leak? And if so, can an exhaust leak really cause such raucous backfiring? Can I really be the ONLY guy running headers and Corsa without cats whose mufflers are about to blow clean off the car?

TIA!

[mike100]

two things that helped mine-
1) disable the air injection when running catless (I kept my tubes). The air injection leans the post combustion pipe mixture.
2) add catalysts to muffle the pop.

I have the FIC Lucas type injectors. Turning off DFCO worked for me, but there is the possibility of a backfire during the warm up event when the air injection pump is still running...otherwise only goes off once in a blue moon. Cats help a lot.

[Karl]

The consensus says FIC injectors are not very good. I am about to replace mine with the accell 150821 units. I do not have any backfiring but I am in need of new injectors.

When I purchased the car last year the prior owner had FIC injectors installed. I guess they didn't last long with the ethanol in todays fuel.

[XfireZ51]

I would absolutely make certain there are no exhaust leaks. Decel produces high vacuum and improper exhaust system sealing will suck fresh air into the exhaust. And it does not need to be at the collector. Could be further back and it would still cause a popping.

A few questions:

1. Did Bill note if the BLMs are leaner in one bank versus the other during decel?

2. The datalog should show the injector pulse width. Does it look like injectors are shutting off?

3. What about the O2 sensor voltages on both banks? What are they during decel? Are they different from each other?

4. Can you tell where in the exhaust the popping is happening? At the header or further downstream?

[KILLSHOTS]

Originally Posted by XfireZ51

I would absolutely make certain there are no exhaust leaks. Decel produces high vacuum and improper exhaust system sealing will suck fresh air into the exhaust. And it does not need to be at the collector. Could be further back and it would still cause a popping.

A few questions:

1. Did Bill note if the BLMs are leaner in one bank versus the other during decel?

2. The datalog should show the injector pulse width. Does it look like injectors are shutting off?

3. What about the O2 sensor voltages on both banks? What are they during decel? Are they different from each other?

4. Can you tell where in the exhaust the popping is happening? At the header or further downstream?

The backfiring is definitely happening at the mufflers. Marc mentioned the injectors and asked about the O2 sensors, as well. But the more I think about this, the more it seems like it must be exhaust leaks. 3 weeks ago, the car had straight pipes (except for cats) and was EXTREMELY loud. Also, the DFCO had not been disabled at that point, yet there was virtually no backfire. The injectors were changed in February, so they're not the problem. Bill drove the car thoroughly several times after install and probably would have noted loud backfiring if it was happening then, which tells me the situation is worsening over time. I'm thinking exhaust clamps are loosening with heat cycling. What do you think?

[XfireZ51]

Originally Posted by KILLSHOTS

The backfiring is definitely happening at the mufflers. Marc mentioned the injectors and asked about the O2 sensors, as well. But the more I think about this, the more it seems like it must be exhaust leaks. 3 weeks ago, the car had straight pipes (except for cats) and was EXTREMELY loud. Also, the DFCO had not been disabled at that point, yet there was virtually no backfire. The injectors were changed in February, so they're not the problem. Bill drove the car thoroughly several times after install and probably would have noted loud backfiring if it was happening then, which tells me the situation is worsening over time. I'm thinking exhaust clamps are loosening with heat cycling. What do you think?

Chris,

That's an important observation. Leak is the first thing I would suspect and inspect for. Decel would cause fresh air to be drawn into hot exhaust. Would also explain lean mixture. Which side is it or is it both? Yes I've had clamps stretch.
First time I ran into this was after putting MagnaFlow muffs on my 84 Xfire. Doing some tuning, and I was constantly needing to add fuel in the "decel region" of the VE table. That's around 20-30kPa. Until one day I walked up behind the car and noticed the left muffler hanging low. Fixed that, and then I needed to take a ton of fuel out. So the O2 sensor can be affected even by a leak quite a bit downstream of it basically making the situation worse.

[Karl]

I had a few noticable exhaust leaks where the collector and the exhaust meet up. I used red high temp silicone RTV sealant to fix it.

[KILLSHOTS]

Originally Posted by XfireZ51

Chris,

That's an important observation. Leak is the first thing I would suspect and inspect for. Decel would cause fresh air to be drawn into hot exhaust. Would also explain lean mixture. Which side is it or is it both? Yes I've had clamps stretch.
First time I ran into this was after putting MagnaFlow muffs on my 84 Xfire. Doing some tuning, and I was constantly needing to add fuel in the "decel region" of the VE table. That's around 20-30kPa. Until one day I walked up behind the car and noticed the left muffler hanging low. Fixed that, and then I needed to take a ton of fuel out. So the O2 sensor can be affected even by a leak quite a bit downstream of it basically making the situation worse.

Thanks Dom. Not sure on which side, or whether it's isolated to one side or the other; I wasn't able to discern that on my drive. I'll just need to get under there and start wrenching to see if that works.

[Kevin]

Originally Posted by Karl

The consensus says FIC injectors are not very good. I am about to replace mine with the accell 150821 units. I do not have any backfiring but I am in need of new injectors.

When I purchased the car last year the prior owner had FIC injectors installed. I guess they didn't last long with the ethanol in todays fuel.

I haven't heard this consensus....

[Z06scentair]

Originally Posted by Kevin

I haven't heard this consensus....

I'm with Kevin, always had good luck with them!

At least a half dozen sets installed over the years.

Brad