monabaker

So I recently had an oppertunity to really push my car to the limit. Well, not so much the limit of the car, but my limits I suppose.

I was able to sustain what I would estimate some where around 160-170mph for around 30 seconds. Which was exciting, yet, probably not the best of ideas.

The car now likes to pop/backfire out the exhaust quite frequently now. It doesen't do this all the time, just at random, sometimes while coasting, or idiling, or boosting, it's not speicific to one type of scenario. Also...when the car is accelarating and like 1/2 boost, it kind of "bucks" and runs rough. However, at full boost, it still pulls just as strong as always.

Any Ideas? It will be going in for service November 4th and I'd like to tell the tech's what to look for and what I think.

Who's got any ideas?

Thanks.

Danno

"The car now likes to pop/backfire out the exhaust quite frequently now. "

Are you saying that before your top-speed run, it didn't backfire as much? Most likely the extended time at boost caused some vacuum leaks, perhaps in the hoses that connect the FPR, fuel-dampener and bypass-valve. The result is too much fuel under non-boost conditions, leading to the pops you're hearing and the bucking at partial-throttle.

hosrom_951

My car does the usual poping after/during some hard drives, yet i have a 2.5" test pipe........

evil 944t

"My car does the usual poping after/during some hard drives, yet i have a 2.5" test pipe"

I don't think thats usual. I have a 4" turbo back and don't make popping noises on or off boost.

I think Danno is right though, has to be some vacuum leaks

hosrom_951

Like i said, only after/during hard driving, also no CAT and it doesn't ALWAYS happen, i even posted a while back on this and got same results.

badass951

Well all I know is it definitely means you are running too rich. Which, as a result would probably be caused by a vacuum leak. If it bucks under acceleration it is probably wayyyyyyy too rich. A bad TPS causes some problems with non-full boost acceleration.

Duke

Without a cat there's not unusual to have popping from time to time. Evil are you saying you have a full 4" exhaust without a cat and you never have popping? Sounds very strange...
And are you really meaning from turbo back? There's a real pain to make a 3" downpipe so a 4" downpipe would be very unusual.

Duke

Just a note, I'm talking about popping when letting off the throttle, not during throttle.

evil 944t

Coming to a slow with no throttle, I get some rumblings very few but NOT under power.

I have a custome exhaust, short 3" to firewall then 4" all the way out

hosrom_951

I get the popping off throttle as well, i should have been clear regarding popping while on throttle, as Duke said, from time to time.........

badass951

Have any sound clips of this exhaust? Never heard a 4" exhaust on a 951 before.

MachSchnell

Um - might just be a beer addled misconception, but wouldn't a vacuum leak cause a LEAN condition rather than a rich condition (which would lead to backfiring from unburnt fuel combusting in the hot exhaust)? Under vacuum (i.e. as he describes, off the throttle) a vacuum leak allows unmetered air to enter the motor in addition to the metered air for which there is no appropriate fuel compensation, creating a LEAN condition, no? I can see under boost, where some metered and compressed air would actually escape after metering, causing a rich condition, but without boost, isn't it exactly the opposite?

Danno

"might just be a beer addled misconception, but wouldn't a vacuum leak cause a LEAN condition rather than a rich condition (which would lead to backfiring from unburnt fuel combusting in the hot exhaust)?"

yes, you're correct, but it depends upon where the leak was. Typically I'll say intake leak for the large hoses like the J-boot, intercooler-pipes and throttle-body if there's a leak there. Under non-boosted, vacuum conditions, yes it would result in extra air getting in and leaner mixtures. Definitely no popping in this case.

However, vacuum leaks are what I refer to as those thin 1/8" hoses that carry vacuum to the FPR, dampener, CBV, heater-valve, evaporative controls, etc. Leaks in these lines would result in too little fuel under boost and too much fuel under vacuum. Usually this is more likely than an intake leak because of the multiple T's and number of hose-sections involved. Leaks here is more likely to cause the pops he's developed.

Laust Pedersen

You should have no popping when fully off the throttle. In that case the injectors should be shut off (if the micro-switch in the TPS is adjusted properly).