Backfiring on Decel

rgdtoo · 10-30-2008, 01:22 PM

This is my track only 85' Carrera.

I want to know if I should worry about my car backfiring.
It happens every time I let up on gas at high speed, around 100mph +.
It's always one very loud pop.

Your input would be greatly appreciated.

Thanks,
rgd

Bart_dood · 10-30-2008, 01:30 PM

I'm not an expert but I believe whats happening is this: during full throttle the DME injection runs on the rich side of things as it guesstimates the air flow etc, then when the throttle is snapped shut the DME shuts off injector flow, so now you are getting air flow with no fuel, the air hits the hot exhaust with some fuel vapor build up (from the full throttle) in there and its very hot so it pops due to ignition.

rgdtoo · 10-30-2008, 08:37 PM

From what you are saying it doesn't sound like anything to worry about.

Thanks for your input!

azasadny · azasadny's Garage

I had some backfiring on deceleration and I put a can of SeaFoam in the gas tank and after about 100 miles of driving, no more backfires... worked for me.

hcoles · 10-31-2008, 03:37 AM

this happened to me on the track....
very likely the idle control switch is not staying "off"
this is bear to adjust but straight forward

ruf-porsche · ruf-porsche's Garage

Exhaust Leak?

84_Carrera · 10-31-2008, 05:28 AM

My '84 does it with the muffler bypass. Mine's set to run rich anyway (+6% fuel), and with the muffpass it's very noticeable. I've replaced all of the gaskets at the heat exchangers in the past year, and I'm running coppers on the cat & muffler flanges.

I only have very light popping & only occasionally with Ben's muffler.

masraum · 10-31-2008, 06:33 AM

There are two main causes for backfire. The first is an exhaust leak. The second is unburned fuel off throttle.

The unburnt fuel should be taken care of in an '85 because the ECU should kill fuel into the cyl off throttle. Unless you keep your foot partially on the throttle or you have a leaky injector, this shouldn't be the problem.

Superman · 10-31-2008, 07:36 AM

Quote:

Originally Posted by masraum

There are two main causes for backfire. The first is an exhaust leak. The second is unburned fuel off throttle.

The unburnt fuel should be taken care of in an '85 because the ECU should kill fuel into the cyl off throttle. Unless you keep your foot partially on the throttle or you have a leaky injector, this shouldn't be the problem.

Exhaust leak is the usual culprit. However....this generally causes simple popping, not big explosions. In the olden days, we would sometimes turn the ignition off for a moment while coasting down a hill in gear. When we turned the ignition back on, there was a nice BANG.

WIL84911 · WIL84911's Garage

Had a similar loud pop before adding 911chips.

Joe Bob · Joe Bob's Garage

Ditto on the exhaust leak.....

gtc · gtc's Garage

Ditto on the chip... mine would shoot flames sometimes on decel after I installed a cat bypass and open muffler. I bought a chip from Steve Wong in conjunction with an upgrade to headers, and haven't had so much as a pop since.

richde · 11-01-2008, 03:49 AM

Quote:

Originally Posted by gtc

Ditto on the chip... mine would shoot flames sometimes on decel after I installed a cat bypass and open muffler. I bought a chip from Steve Wong in conjunction with an upgrade to headers, and haven't had so much as a pop since.

It seems like I've gotten more popping with a SW chip. But only below 1500 or so RPM,
(stock exchangers -w- test pipe and Fabspeed muffler)