82 sc pops and "cracks" on deceleration

[miketheautopartsguy]

Ok new injectors (still running cis) Pre 74 non-cat and non-thermal SSI heat exchangers and a very free flowing M&K stainless muffler. Car REALY "cracks and pops" under deceleration, Is it normal with these cars? do I have air coming up thru an exhaust leak? To much unburnt fuel? Its realy kinda embarrasing to gear down and sound like a popcorn maker on steroids!
HELP PLEASE!!!!!
PS I am a member but for some reason I cannot log in fully? so sorry.

Mike

[Staylo]

aside from any possible exhaust leaks, are all of your head studs intact?

[miketheautopartsguy]

Yes all studs in place just had the engine out......... Replaced a snapped rocker on number five and decided to upgrade exhaust, reseal some leaks and chase down a "heat sink hard start issue"". Thats why the new injectors. Could I have done something wrong with the rocker replacement? Car runs great and realy cooks for an otherwise stock 3.0L

Thanks for any info guys!
Mike

[d.a.autry]

Mike,

I don't know if this will help you, but read through my thread regarding ignition timing.

There were some great ideas in there (not just timing related). . . however deleting the vacuum retard and resetting the timing has gone a long way towards fixing my problem. . . same thing, early exhaust, etc. but just on a 2.7 . .

Hope it helps.

Here's a ? for early model (2.7) tuners

[jason2guy]

sounds like an exhaust leak to me. i was under the car today for an unrelated matter, i look up and see 1 heat exchanger nut MISSING. i just had them off less than 6 mos ago. point being- check ur HE to head fasteners.

[Gordo2]

If its not an exhaust leak, here's another one to investigate.

I had some high idle issues and found that my Decel Valve was sticking. I have disconnected the vacuum line to the valve which has fixed the high idle problem but have found that my car pops on deceleration (while in gear).

BTW, I've had it disconnected like this for ~ 2 years and have grown used to it - it only happens on hard deceleration, especially going down hill in a low gear.

Good luck, Gordo

[ninelevenick]

I had this issue while back, and it turned out to be the vacuum hose that runs from the distributor to...well, I'm not exactly sure to what it ultimately connects, but the hose was disconnected. However, your '82 may not have a vacuum hose connection.

[BGCarrera32]

And, if you have a loose or disconnected vacuum hose you'll be running lean, (sucking in more air) which is exactly what this sounds like. Start by finding someone with a wideband o2 sensor and getting it in the exhaust stream to see where the mixture is at.

Your "free flowing" M&K will likely magnify the sound, but not cause it.

[miketheautopartsguy]

Ok guys,
I will climb under this week end and re-torque the nuts/studs from exchanger to heads and check that I didn't miss anything else there.
Also I "smoke" tested the engine while on the stand and chased down two small vacum leaks. BUT.... while dropping the motor in place it is possible that I snagged a vacum line so will do a good full search of the vacum lines and realy focus on the vac advance line to dist.

Thanks again guys!

Mike

[woland]

Hi

I had a similar problem (frighten to have my airbox crashed soon - planned to install the pop off valve) until... I changed the alternator brushes.

Now I have no more pops in deceleration and my five years old battery charges wonderfully and engine fires in no time

[Hotwatermusic]

Originally Posted by miketheautopartsguy

Ok new injectors (still running cis) Pre 74 non-cat and non-thermal SSI heat exchangers and a very free flowing M&K stainless muffler. Car REALY "cracks and pops" under deceleration, Is it normal with these cars? do I have air coming up thru an exhaust leak? To much unburnt fuel? Its realy kinda embarrasing to gear down and sound like a popcorn maker on steroids!
HELP PLEASE!!!!!
PS I am a member but for some reason I cannot log in fully? so sorry.

Mike

It's not just normal back-rap? People pay extra for that! When I changed my stock muffler to a flowmaster, having changed nothing else, on deceleration my exhaust would sound like black cats going off. I've always been told it's normal and nothing to worry about. I think going to the freer-flowing, much louder, flowmaster just made much more audible what was there all along. Like I said, A LOT of muffler set-ups actually advertise their appealing back-rap. Usually if you ease down off the gas instead of a sudden pull-off, the back-rap is minimal. But I for one love it.

[ninelevenick]

Your "free flowing" M&K will likely magnify the sound, but not cause it.

You are dead-on.

It's not just normal back-rap? People pay extra for that! When I changed my stock muffler to a flowmaster, having changed nothing else, on deceleration my exhaust would sound like black cats going off. I've always been told it's normal and nothing to worry about. I think going to the freer-flowing, much louder, flowmaster just made much more audible what was there all along. Like I said, A LOT of muffler set-ups actually advertise their appealing back-rap. Usually if you ease down off the gas instead of a sudden pull-off, the back-rap is minimal. But I for one love it.

A free-flowing exhaust will fart a bit especially while driving the car during the cold start enrichment process, but fire crackers during deceleration is not normal.

[mickey356]

I had a little of it when I went from stock to a cat bypass and after I installed SSI's and an M&K the crackle on decel got more prevelent, I love it! It's not backfire or exhaust leak, its back-rap. There is a thread here somewhere that discusses this issue. I'd say don't worry about it unless you don't find it sounds bad-ass as I do, then maybe some solution is available to stop it. I think there is a pretty big difference in sound between this and an exhaust leak.

[mlangbauer]

I have the same exact thing. I put a Bursch cat bypass pipe and the Dynomax 1 in 1 out muffler on it and it does all that on decel. Too much for me in fact, so I put the cat back on and it pretty much stopped. So, now I am going to have the exhaust shop put in a 12" resonator and see how that works. I love the way the free flowing system sounds when it idles but on full acceleration it is just too much......

[JFairman]

It's caused by the fact that CIS (constant injection system) keeps injecting fuel on deceleration which is totally unneccesary and wrong but there is no way to stop that with ancient crappy mechanical K jetronic fuel injection, and also the excessive vacuum retard during deceleration that is there to raise exhaust temperature and heat up the catalytic converter during decel to help burn off the unburnt hydrocarbons, carbon monoxide, and nitrogen dioxide emissions.

If you remove or disable the vacuum limiter deceleration valve it gets worse if everything else is left stock.. and just holding the throttle open a tiny bit while decelerating will let in the same amount of air as the decel valve is bypassing around the throttle body into the intake manifold.
That wil reduce the popping in the exhaust during deceleration without removing or modifying the vacuum retard.

[fred cook]

I get a small amount of popping in the exhaust from my nearly stock '80 911SC w/SSI exhausts. I can't remember any car that I have owned (many) with free flowing exhausts that did not pop and burble at least some. I had my engine out about 2 years ago and replaced all of the rubber hoses, etc on the top of the engine so I don't believe that there are any vacuum leaks. I believe some of this type exhaust noise is normal.

[OttoB]

I recently purchased a 1991 911 turbo with B&B stainless exhaust set up, along with a few other modifications, and the car back-rap quite a bit. However, I love the sound and was told its not affecting performance, nor causing any damage to the engine, so... people know when I'm approaching, contrary to the gf's preference...

[ruf-porsche]

Originally Posted by miketheautopartsguy

Yes all studs in place just had the engine out......... Replaced a snapped rocker on number five and decided to upgrade exhaust, reseal some leaks and chase down a "heat sink hard start issue"". Thats why the new injectors. Could I have done something wrong with the rocker replacement? Car runs great and realy cooks for an otherwise stock 3.0L

Thanks for any info guys!
Mike

Other than visually looking at the rocker, how will you know if you snapped a rocker?

[miketheautopartsguy]

ruf-porsche
When you snap a rocker the spring will go as well and the rattle you hear sounds like a death rattle! I feel like I was luckly to escape any major damages. I will try to post video with audio so you guys can hear my car.

[miketheautopartsguy]

JF,
Are you saying that you think this will vanish if I proceed with the webber carb upgrade next summer? Please say yes! Cause the girlfriend is already upset that I am thinking about dropping ANOTHER $4000.00 on my "mistress!" But, she hates the decel noise even more than me so this might be a great way to justify the expense!!