My turbo car backfires on decel ! Help !
 

I just got my 82`SC all together with the 1992 c2 turbo engine(CIS injection) and it backfires massively when you lift the throttle . I believe that maybe I have an airleak(but cant find one) in the intake for 2 reasons. First reason is that the sensor plate doesnt move enough to give it fuel to start up once the engine is warm. The other reason is that under boost I hear lots of high psi air leaking going on(constant wooshing sound). Also when I lift the throttle after boosting it I hear what sounds like a rice car blow off valve along with a huge backfire followed by rapid machine gun fire. Im currently have a stock plastic type BOV in place. Ive checked everything for leaks and cant find anything. Of course I cant simulate boost conditions while standing behind the car.
Note that I am using all the correct injection and ignition components from the 92` C2T . This includes an acceleration enrichment circuit and computor controlled timing that has a boost sensor on the module.
Lastly I noticed that at 1200rpm while idling my heat exhanger/headers were glowing red and show just over 800 deg f . Is this normal ? I unplugged the O2 sensor and set the C0 to 3.00 ppm . The engine runs beautiful aside from some breaking up while full throttle .
Im at the end of the rope here and need some suggestions.

Kurt Williams

Congrats on an awsome car! (when it runs well)

Please Help guys. I want to get some suggestions so I can try to fix this at work tomorrow. An answer by 7:30 am would be very helpful. Dont have internet at work.

Kurt Williams

I am no expert, and I am not a mechanic, but my 930 also backfires on decelleration. In fact, it shot flames at the track last week (which I think is pretty darn cool).

First, I looked for air leaks. I never found them.

So now, I am under the understanding that the CIS (when running rich - as is best for a turbo) will allow fuel into the red-hot muffler on decel - hence the explosion.

Perfectly normal, I guess.

If I run lean again (and risk burning pistons) my backfire (and flames) subside.

My backfires are block buster intensity. The muffler will not take no many more of those. Im suprized knowone here stepped up with answers.
Thanks emission . Btw how do you run lean if you choose to. With CIS its really not an easy task to adjust loaded fuel ratios . I know you can mod the WUR but ...

Hmm...exchangers shouldn't be glowing red on idle, have you checked your ignition timing?

There is a big brass deccell valve on OEM 930 engine that prevents abrupt throttle shutdown and backfire. If removed, car will pop more. That being said, most 930's equipped with free-flowing system will backfire on deccel due to absence of fuel-cut-on-decell system available on most EFI cars.

My 88 930 backfires on decel and I love it. If I am lucky, it will shoot flames too. Your problem is perfectly normal for a CIS.

Mine backfires big time too. On track lifting from full throttle, even drivers in front of me thought there's something broken on their car :D

But, glowing headers at idle definitely does not seem right.

Oh, my headers never glow either.

Mine crackles a little in decell but rarely bangs hard.
I would double check for an exhaust leak, then hook up an A/F meter to make sure it isn't swinging too lean when you let off.

Kurt, mine also backfires on decel. It did with the factory exhaust/cat but was muffled to the point of sounding more like a burble. Without a cat and running a fabspeed it is much louder. I continuously monitor A/F with a permanently installed LM1 and my ratios are typical for 930. I do notice though that I get less backfiring when the motor warms up. I think this may be due to the higher control pressure that dampens the airflow plates movement and prevents less dumping of fuel on decel. You mentioned testing for leaks but have you actually tested the BOV to make sure it is opening on decel. It would seem to me that would created an extra rich condition that could exacerbate your problem.

subscribe..mine does the same thing and im sure something is wrong. It is not just the ocasional pop. When you let off the gas it pop pop pop the whole time. I have looked for exaust leaks......I guess i need to go thru the intake side now.

Mine is far from normal. If the loudness could be shown in a video I would post it. Im not exaggerating when I say its a bang equal to a block buster. Im suprized the muffler hasnt split at the seams yet.
Im going to check the running fuel psi`s next. As I also noted above , the engine breaks up at full boost. I think I better check the boost level too. I did put a 1 bar spring in the wastegate. And it was second hand so it may be overboosting. The part throttle acceleration is insanely violent and breaks the tires loose in 2nd gear at 40mph !
Oh as for the intensity of the backfire ... Picture this. Today I took it out for another ride and past by a local strip mall . I boosted it up and released in front of the stores. The bacfire was so loud that about 10 people came out the front doors of the stores to see what just blew up. Is this still normal ? If I cant figure this out myself im going to bring it over to Bill Rudtner . Maybe he can sort this out for me.

Kurt Williams

No that's definitely not normal Kurt. The normal 930 backfire is more a sound rather than a violent explosion and it is rhythmic under compresssion braking, not one big pop. However, for those new to 930's it certainly comes as a surprise and concern nonetheless.

Yours sounds much different, a real backfire. Seems like you've got a really rich mixture and and perhaps some exhaust restriction that exacerbates the intensity?

The break up at full boost may certainly be the overboost switch and that is never a good thing unless you have an accurate way a knowing actual boost.

Sounds a bit louder than mine. My "backfire" sounds pretty cool - not obnoxious. :cool:

Re: My turbo car backfires on decel ! Help !
 

One thing I'm curious about is your BOV. Is it venting to atmosphere (outside the intake track), or is it being recirculated? I'm not sure you should be hearing that ricer sound. On CIS the BOV should never vent to atmosphere. It causes the fuel metering plate to stall which can cause an overly lean condition.

As others mentioned, the headers should not glow at idle...sounds like the ignition timing needs to be checked.

As for backfires, mine has the short 934 style exhaust, which means there is no muffler. I also have all smog equipment removed. My backfires are a constant stream of small pops on decel. No huge explosion as you've described.

I wonder about the condition of your waste gate, too???

Mine backfires under deceleration from day one and I love it = normal. However, it is a rithmic, steady poping nothing like a big bang once in a while. And the headers don't glow as somebody mentioned. Flames from the tail pipe once in while YESSSSS.

I got kicked off the race track Sunday morning for being too loud on throttle lifts during "quite hours" - the is a Church nearby.

Re: Re: My turbo car backfires on decel ! Help !
 

Nope. It causes overly rich condition as metered air is dumped overboard but fuel is still injected, which might explain bangs.

Make sure that your BOV/DV (what you got is not clear to me as the "stock plastic thing" is a 930 type DV) is not staying open under engine braking (high vacuum). It should open only on the pressure differential between boost and vacuum across the throttle plate when you lift off and shift. If you lift and don't shift (engine braking) it should open momentarily and then close tight!
That's where I would look first.
Those cheap diaphragm type stock DV ones often leak even at idle. Now go to big vacuum under engine braking and a small leak is now an Open BOV/DV.