9 replies to this topic

[Big Man]

Hi all,

Right my 998 goes loke a bomb, standard apart from a K and N replacement filter and peco back box. However after my drive into work at approx 70 - 80 mph and switching off, it over runs for about one second then after three seconds pops! bangs form the exhaust.

Q. Is it too rich or too hot????

Cheers

BM

[mini_dave]

More than lighty running a bit rich, so just the exses fuel burning, get it on a rolling road and they should be able to sort the mixture out.

[dklawson]

after my drive into work at approx 70 - 80 mph and switching off, it over runs for about one second then after three seconds pops! bangs form the exhaust.

So following a high-speed run (and running well) you switch off the engine. Are you describing dieseling or run-on where the engine keeps going with the ignition off? Then after the engine finally stops it backfires out the exhaust? Do I have that correct?

If I've got that all correct it implies one or more of several things could be wrong. How you address this depends on how involved you want the solution to be. The run-on is typically a sign that something is running very hot in the combustion chamber. Since several things can cause this there are several possible solutions. You can try changing the ignition timing a bit, you can try running higher octane fuel, you can try colder spark plugs, and/or you can remove the cylinder head and clean all the carbon and deposits from the combustion chamber. One or more of these may help.

However, there is a tried and true work-around that has been used for years. To use this method you have to change your personal habit/routine when shutting off the car. When you are ready to switch the engine off, depress the clutch and place the car in gear. Simultaneously, as you turn off the key, smoothly release the clutch so the engine stalls. That's it. It's important not to let your car diesel (run-on), it's very bad for the crank and bearings. Stalling the engine every time you switch off is a gentle process that prevents this. Before the detractors weigh in, no... this doesn't put undue stress or wear on the clutch, it's much gentler that accelerating from a stop. If you prevent the dieseling from ever starting (by always stalling the engine) you wont experience the run-on and backfire you've got now.

[Big Man]

Thanks for the above comprehensive reply, I turned the mixture down a quarter turn on the mixture making it very slightly leaner and after the same drive to work at 80 mph did not run on or pop!. However I was stuck in traffic for about two minutes before switching off at work if that makes a diference.

Cheers

BM

[dklawson]

Backfires out the exhaust are typically an indication of a rich fuel condition and/or an exhaust leak. However, what you're describing happens AFTER switching off so the scenario is a bit different. Leaning out the mix a bit may help but you won't know until you recreate the events that caused it to happen the first time.

[Bungle]

is your exhause too big for a 998 ?

[Big Man]

Its a one and half inch peco big bore back box with a reducer onto the standard exhaust system (no centre section)

Cheers

BM

[Matt]

lol so the car isnt standard other than the k+n then..

[CooperTrooper]

i get very slight popping when im cruising in 3rd (downhill) after changing down from 4th nothing major infact i really like the sound

Edited by CooperTrooper, 10 August 2007 - 09:52 AM.

[dklawson]

"slight popping when im cruising in 3rd (downhill) after changing down from 4th"

That's VERY consistent with the symptoms of an exhaust leak. Check all the joints starting at the head and working back. Look for leaks between pipes and possible perforations.