exhaust backfire only when engine cold

**Homer J** · 06-20-2011, 08:28 AM

so, for the past couple of days my car has started to have an exhaust backfire every morning, but through every gear. 1st, 2nd, 3rd, 4th, then i slow down to get on the highway and it stops.

it also does it maybe the fisrt time a shift in the afternoons.

no misfire, no hesitation, just the exhaust backfire when i let off the pedal to shift.

mostly stock, SLP lid and SLP dual-dual exhaust. 47xxx miles.

just wondering if its normal for it to pop when it a cold start?

**98TransAmWs-6** · 06-20-2011, 08:32 AM

Originally Posted by Homer J

so, for the past couple of days my car has started to have an exhaust backfire every morning, but through every gear. 1st, 2nd, 3rd, 4th, then i slow down to get on the highway and it stops.

it also does it maybe the fisrt time a shift in the afternoons.

no misfire, no hesitation, just the exhaust backfire when i let off the pedal to shift.

mostly stock, SLP lid and SLP dual-dual exhaust. 47xxx miles.

just wondering if its normal for it to pop when it a cold start?

Is it a real afterfire or just a pop? If it is just a pop that could be normal. I know mine pops while decelerating.

**pajeff02** · 06-20-2011, 08:32 AM

A backfire of that nature can be a sign of a lean condition. Any SES codes? Have you done any work to the car lately?

**Homer J** · 06-20-2011, 08:49 AM

Originally Posted by pajeff02

A backfire of that nature can be a sign of a lean condition. Any SES codes? Have you done any work to the car lately?

no SES codes, just a pop when i let off the gas to shift. it used to do it in the mornings but only from 1st to 2nd. now it does it through every gear till i get on the highway, which is less then 1/4 mile away. i leave the house and leave it in 1st through the neighborhood. pull up to the road, make a right hand turn on the road to he highway, then make a right onto the highway. i only drive 1 mile on the highway everyday. when i get off, it doesnt backfire or pop any more.

only thing that has been done to the car lately is about 100 miles ago i replaced my clutch master.

**98maro** · 06-20-2011, 10:18 AM

Mine does it too.
Do you warm the car before you drive it?
If I warm up mine at least 1min or less it doesn't back fire.
And if it does it's only for few seconds

My old z was the same both m6 and same mods.

I'll say it's normal if you run it cold

***SMWS6TA*** · 06-20-2011, 10:43 AM

Originally Posted by Homer J

SLP dual-dual exhaust

here's why. Deleting the CATS, EGR* and Air System* does this.

* for 98-00 yrs

The EGR and Air was suppose to help heat the CATs up in order for them to burn off the excess fuel vapors not burned when the exhaust leaves the engine.

If you let your car warm up for a few it will lessen it.

I seem to have header pop when I'm letting off the peddle and down shifting from hwy speeds. Its all part of having headers.

**Spaz** · 06-20-2011, 12:23 PM

your PCM is in closed loop and running a bit more fuel... not all the fuel gets burnt up and combusts in your exhaust... once the car hits open loop it stops backfireing when it leans out the table...

**98maro** · 06-20-2011, 12:31 PM

Originally Posted by Spaz

your PCM is in closed loop and running a bit more fuel... not all the fuel gets burnt up and combusts in your exhaust... once the car hits open loop it stops backfireing when it leans out the table...

what does this mean in english?lol
so it needs a tune ?

***SMWS6TA*** · 06-20-2011, 12:45 PM

have you tuned out the rear O2's?

**Spaz** · 06-20-2011, 12:51 PM

Originally Posted by 98maro

what does this mean in english?lol
so it needs a tune ?

when you first start your car your PCM is in closed loop... fuel and air tables are closed while the PCM gathers info about the conditions... damn... i don't know how to explain this... i'd ask frost to tell you better... the man knows his stuff!!!

**whitetrashbird** · 06-20-2011, 02:19 PM

Originally Posted by Spaz

when you first start your car your PCM is in closed loop... fuel and air tables are closed while the PCM gathers info about the conditions... damn... i don't know how to explain this... i'd ask frost to tell you better... the man knows his stuff!!!

Spaz posted up exactly what I was thinking. In "closed loop" the PCM basically operates from a base set of parameters until the engine comes up to temp. When it comes up to normal operating temps it goes into open loop, i.e. it tunes itself to the actual sensor input.

Dan

***SMWS6TA*** · 06-20-2011, 04:48 PM

Originally Posted by Spaz

when you first start your car your PCM is in closed loop... fuel and air tables are closed while the PCM gathers info about the conditions... damn... i don't know how to explain this... i'd ask frost to tell you better... the man knows his stuff!!!

Even I learn new stuff everyday with these cars.

**pajeff02** · 06-20-2011, 06:23 PM

Spaz got it right. Until the engine coolant temp reaches about 140 degrees, the engine ignores feedback from the O2 senders and simply dumps in whatever fuel is called for in the tables. This is called "open loop". After 140 degrees, the PCM switches over to "closed loop" and fueling is then a function based in part upon feedback from the O2 senders.

**Homer J** · 06-22-2011, 06:35 AM

thanks guys, all that made sense to me.

i dont let the car warm up in the mornings. i dont know if thats bad or not, but i usually just strap my daughter into her seat, jump in and fire it up. i might put my sunglasses on first, then i begin to back out of the garage.