Backfiring when decelerating

Turbo92PGT · Jul 28, 2004

Parts Replaced or serviced
- Purge flow sensor
- Purge flow valve
- Fuel filter
- Cleaned MAF
- Seafoam intake cleaner
- Spark plugs Autolite 104's
- Ford Racing plug wires

Symptoms - when decelerating my car will pop and backfire. Sounds kind of cool sometimes, but sometimes it is a definite backfire "boom" type of thing. It only does it when I'm decelerating in gear or when I free rev to 5K. (love the exhaust sound, i know its bad... i dont do it often)

Anyone have any ideas? My gas cap does not hiss when I open it. It hasn't done it in a long time. Could this be causing anything?

As far as mods - I have a zex dry kit, a metal intake tube and a filter on the MAF (replacing with FIPK), and a mac o/r H pipe. I would really like to get this taken care of. I have a feeling this cure for this problem will fix my rough idle

twogts4us · Jul 28, 2004

Have you cleaned the IAC? Check the EGR diaphram for vacuum? Sounds like a vacuum leak somewhere.

Turbo92PGT · Jul 28, 2004

Oh ya, forgot about that. The IAC was replaced maybe 5K ago!

When you say check for vacuum on the egr diaphram. Are you just talking about that one tube thats on the egr valve which is on the intake plenum? Just make sure it has vacuum to the valve?

twogts4us · Jul 28, 2004

Correct! Check all the vacuum lines on the manifold/plenum, but definately the EGR. Check the hose for signs of cracking, leaks, etc - if in doubt, replace it...they sell it by the foot cheap at the auto parts store.
Use a stethoscope or a length of fuel hose held against your ear to listen for vacuum leaks while the engine is running. A hissing sound will be heard. A soapy water solution will also detect leaks.
Also check you PCV valve (pass side cam cover) - clean it and reinstall or just replace (take the old one with you if you do replace because I've heard of some people getting the wrong part according to the book at the store).
When did this start? Did it start right after you installed something or did something?
Does the intake tube go into the fender or is it straight?
Have you replaced the TPS? (since I see you said you have a bit of a problem with the idle as well)

Turbo92PGT · Jul 28, 2004

The car has not idled very smoothly for as long as I can remember. I've had the car for about 7 months. Its idled OK, but never smooth like it should. Sometimes it will idle just perfect, cant feel any vibrations or anything. and then other times its vibrating and making different exhaust tones and such.

I pulled the vacuum line off the EGR tonight, and there is very very little vacuum going to the EGR. My dad said there should be none at idle. The egr does open and close because i put a piece of vacuum tubing on the egr and sucked on it and the car almost died.
The TPS has not been replaced, nor has the voltage been adjusted. Do you know what its supposed to be set at? and is it adjustable?
Theres a little black canister thing that looks like it has a knob ontop of it, that the EGR vacuum lines go into. I took them off and sprayed carb cleaner through that black plastic thing. Made no difference really.

Please feel free to IM me if you want. I have some sound clips and pics of my car and stuff that maybe you can narrow stuff down. BoostedStang302 is my AIM

Turbo92PGT · Jul 28, 2004

The intake is just a cone filter right on the end of the MAF. Theres no bend or anything.

twogts4us · Jul 28, 2004

Right - the EGR only opens at WOT (or near WOT, with enough vacuum). And with the suck-test it sounds like it's working - cool.
How about the PCV valve. Pull it out and shake. The valve inside should move freely. If not, replace it.
Is the MAF the OEM one?
Any leaks around the air filter or anywhere along the air intake tract?

Turbo92PGT · Jul 28, 2004

PCV valve is fairly new and about a week ago when I took the plugs out and replaced em it was rattling around and working fine. No leaks that I can find man. Does your car idle like perfectly smooth? Maybe I'm just expecting too much from it. I'm buying new coil packs next payday though.

twogts4us · Jul 28, 2004

Honestly, no mine doesn't idle perfectly smoothly either - neither of our GTs do...it's not an embarassing or cam-lopey kinda thing, it's just not a perfect, quiet idle like a Grand Marquis, where you can barely tell the car is even running. And this condition is more noticable as you open up the exhaust. So this may be more or less normal.
OK, so no vacuum leaks, no intake leaks, plugs are good, PCV valve good. When did this start? Recently, or has it been backfiring for a while? What kind of mufflers do you have and how are they? Any evidence on the outside to suggest they are shot? Popping is somewhat normal when a lack of back pressure is present, but backfires can be dangerous. Have you dynoed the car and gotten an A/F ratio reading? Backfiring is often caused by running too lean (a VERY dangerous thing with nitrous oxide!).

Backfiring when decelerating

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