my 3.2 backfires after installing the test pipe and K & N

[nikramos]

I installed a test pipe and K & N's and it backfires.I had the plugs,cap & rotors and a valve adjustments done too. I am waiting for my Nology wires to complete the tuneup. What did we do wrong to cause the backfire. thanks for any input.

[movin]

I have the same set up, '87 with cat elliminator pipe and K&N filter. Worked fine for the last 2 years. I have a problem now but it's not related.
I don't think the pipe or filter are the source. Did you have all of this work done at the same time?

[twin plugged targa]

I have had a long winded nad painfull ignition problem with my 86 3.2.
It turned out to be the Distributor which was eating rotors and Caps.
The symptoms included severe backfiring!.
Maybe check the condition of the cap and rotor even though it was just changed (I ignored my new ones and it cost me $1,000 diagnostics bill with a wrench!!!!)- why was it changed?
If it was badly pitted or cracked or unusual wear then inspect the distributor- bearings top and bottom etc-not to difficult to do!(I posted a thread recently enquiring how to remove it!)Also check the cap was positioned fixed in place correctly I believe the little lugs which hold it sometimes wear on the distributor body!
I am sure it has little to do with the cat pipe or K&N, unless you have damaged the O2 sensor/connections?.
Hope this helps
rgds Ben

[This message has been edited by 86ragtop (edited 07-01-2001).]

[ljonespwc]

Before I bought my '88 Club Sport, I had 2 pre-purchase inspections. Both mechanics (both reputable, long-standing Porsche fanatics) said the same thing about my car's deceleration backfires. Borla muffler + test pipe = occasional backfires. The backfires are not terribly loud--more aking to popping and burbling sounds. They get louder if I throttle-down quickly from high RPMs (not very often!) or if I match the engines revs on a downshift at 3000+ RPMs. That said, if there are any leaks along the exhaust path, and cold air gets in, you're gonna hear about it.

Lance.

[nikramos]

thanks for the reply, all the work was done at the same time. It was not backfiring before.There are no exhaust leaks and sensors seem to be fine. The wrench who did the work at my home said that he will adjust the position of the distributor when I get the wires.Does he mean the timing? It backfires shifting from 3rd to 4th and it is loud.

[Kurt B]

That backfire is from CAT pipe I guess. I went through the same thing. Now in my case, I had to put my CAT back on to pass emissions and it does not backfire.
The conditions under which my car backfired were when I would sometimes shift at very high RPM with the cat pipe on.
BAAAAAAAAAAAAAM. It would do. Now, I often have my music loud, so I noticed it sporadically, but that doesn't mean it didn't happen every time.
When I put my CAT pipe on, I also changed a few things, and it's possible the CAT link is just coincidence and we both had similar problems with plugs or something, but since you just now reminded me of that, I realize now it has not backfired at high RPM since I put my cat at the end of April.
Just a thought.

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Kurt B
1984 Carrera Cabriolet
911 Page
carrera_cabriolet@yahoo.com

[makaio]

The CAT Test Pipe is causing your backfiring. If you read Bruce Anderson's book, it states that he has experienced this same problem with 3.0 and 3.2 motors with a test pipe. I believe this is why the euro cars came with a pre-muffler, since they didn't have Cats. I put one on my car and it caused it to backfire as well, put the Cat back on and it quit back firing. I don't believe it is your timing, because the 3.2 motors only have one position for the distributor, and the timing is set by the computer.

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Matt Chamblin
78 911 SC

[nikramos]

thanks kurt,Is it safe to say that it is normal and I don't have to worry of hurting the engine. I just read your page and since owning the car 1-3-2001, I've had the tranny rebuilt,replaced the sensor,air flow meter,installed the carrera tensioners since my other wrench said someone replaced it w/ the non hydraulic ones.Forgeline wheels added all in the amount of $7200. I have a 77 slantnose conversion w/ a 3.2 and it needs an external cooler. Bought a used oil hardline for $720 and waiting for the cooler kit for $450. I wonder how much the labor will be. The love for the car never ends..thanks.

I have a Euro 3.2 with stock exhaust including the pre muffler (no cat) and a cone shaped K&N. Sometimes it will go BAM at high rpm gearshifts just like KurtB said. It also did so BEFORE I put the cone shaped K&N on it. So even a stock car can sometimes go BAM from the exhaust pipe.

I'm curious to know if 3.2s equipped with SSI heat exchangers will be prone to backfires??? Anyone knows about that?

[andyu911]

I have the same with the replacement of the test pipe, so I guess it's a common problem. I'm about to install the SSI. I'll post the result as soon as I put it on.
Andy
'87 Carrera

[Thomas Owen]

88' Carrera with stock filter, Bursch exhaust and test pipe. NEVER had a backfire.

[movin]

I wonder about this, there is a fuel cut-off switch just above the throttle full-enrichment switch that is supposed to cut off fuel supply when vacuum is high enough. It helps cut down emissions during deceleration. Maybe there is a problem with it since you experience the backfire between shifts when engine vacuum would be building and fuel supply should be cut off momentarily. Just a thought.

[Leland Pate]

I want to know why someone removed the hydraulic tensioners and retrofitted them with the old mechanical types.

Sounds fishy...

[makaio]

No kidding! Are you sure this is a 3.2 Carrera motor? KNow that I think of it, maybe that's why your mechanic wanted to set the timing. What's the serial number on your motor?

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Matt Chamblin
78 911 SC

[Superman]

Exhaust popping is a sign of an exhaust leak, according to longstanding tradition.

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'83 SC

[Doug Zielke]

Backfire in the exhaust can *also* be caused by: Burning fuel in the exhaust pipe (due to radical cam overlap), advanced ignition timing, exhaust reversion (again, a factor of cam timing and design), fuel mixture, certain conditions of manifold vacuum.
Next time you are at the race track, listen to the various exhausts. Lots of popping, burbling and backfiring. They can't all have leaking pipes.


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'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website

[Clark Griswald]

Nik,

Describe the circumstances under which you experience the backfire in some more detail. What throttle condition, rpm, etc?

As mentioned above, test pipes sometimes cause a backfire like sound. It is unburnt fuel that is igniting in the exhaust, and would normally have been burnt in the cat. This usually happens after a heavy throttle/high rpm blast followed by backing off.

This won't hurt your car (if that is what is happening).

However the description you gave about your experience with the mechanic sounds very suspect. Why would anyone backdate the tensioners?

I don't know that I trust your mechanic. Did you confirm the old tensioners or did you take the mechanics word for it? then he charged you how much to do the job? And $750 for USED hard oil lines! USED? too much.

If someone did backdate the tensioners, there is no telling what else they may have changed on the engine - cams, compression, displacemnet... who knows? This may be a potential backfire source.

[nikramos]

thanks guys for the replies. to answer your concerns, it is a 3.2,I checked the serial no. I switched to this wrench since my previous is the one who did tranny and the questionable tensioners. He is well known here in the Bay Area. I had doubts but I wanted my car fixed and if he duped me then let nature take care of him.I will have the exhaust checked at a muffler shop. I'll probably just put the cat back because I do not notice any power increase.

[makaio]

Nick,
If you are ever looking for another opinion, or wrench in the San francisco area, try Charles @ the Stable. They are off Market strret. I took my car there for my PPI, and they did a very good job. And no I have no affiliation with them.

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Matt Chamblin
78 911 SC

[nikramos]

thanks Matt