decel question

[Belgian1979]

Is it possible for an engine while doing a high rpm decel, to draw fresh air through a zero restriction exhaust all the way from the back to the engine ?

I have been battling with some serious decel popping for a long time already and not matter what I do, cutting fuel on decel, making the fuel richer on decel, seems to make and difference. The problem is bigger the more I rpm I turn and when getting off the throttle again.

Thanks

[JoePorting]

Maybe your power valves are blown which is causing excess fuel to be sucked out of the pv vacuum port.

[Belgian1979]

no carb. An efi system.

[peejay]

Do you have any exhaust leaks?

Usually when I get popping in the exhaust, it's because of a leak.

How much extra fuel did you add on decel?

[Belgian1979]

Not that I know of. I recently changed exhaust header gaskets. No sign on any of the individual exhaust O2's that there is a leak.

I don't add anything on decel as for fuel.

[peejay]

Try adding some. My decel fuel bins on my Mazda are in the 30-40 range. Absolutely no exhaust popping.

[roc]

Try adding some. My decel fuel bins on my Mazda are in the 30-40 range. Absolutely no exhaust popping.

Does that result in perceptible slower decal?

[novadude]

Just curious... How does timing impact the tendency to pop on decel (if at all)?

[Belgian1979]

I have decem bins with 30-40 % ve

I recently upped my timing in those decel areas. No diff though

I also closed my iac completly(actually kept it closed longer) and this did seem to cause it to want to pop more ( not that opening it makes it go away

[Newold1]

On your EFI system using a IAC? I've seen IAC programming or a slow or sticking IAC input a lot of extra air on a high rpm decel.

[Belgian1979]

On your EFI system using a IAC? I've seen IAC programming or a slow or sticking IAC input a lot of extra air on a high rpm decel.

No, I can see the IAC steps commanded in the logs and the each of the cyls AFR. Not a hint of this sticking open or something.

[MadBill]

If the decel cutoff is working, where's the fuel coming from to cause the pops? Perhaps some slight injector leaks?

[Belgian1979]

Not sure, I assume with fuel cutoff, when you go back on the throttle or open it some again, it starts to inject. The air is maybe to diluted to burn the first charges that then go into the exhaust where they burn.

I tested these injectors for dead time before. At that time they were not leaking. This was maximum 1000 miles ago. Injectors are only 1500 miles old.

[user-23911]

You've got lots of problems.

Like uneven AFRs.


Maybe when you finally fix one of the problems, the others will just go away.


EFI is supposed to have the fuel cut off with decel, so you won't get popping.
No doubt it's got lots of leaks too.
Maybe you should test then fix them?

[RednGold86Z]

The first few cycles after fuel cut will still have the wall film stripping off into the cylinders, and eventually be too lean to burn completely (if at all), but then goes boom after it leaves.

The return fuel can be programmed to not have any lean or rich cycles - if it re-builds the wall films in the first injections.

If slamming the throttle shut from WOT, the wall film is huge, and tons of fuel comes off - even rich misfire can occur (you can't un-inject fuel, but you can compensate up to the point of turning off the injectors during a decel transient, but wall films can be quite thick). If it's a gentle easing, the internal EGR can cause some bad cycles just before cut off. It only takes one good miss to have a pop in the exhaust.

With ITBs, there is more imbalance at closed throttle than at any other point - and the injectors are at their worst, and cylinder pressure sucks, and everything is working against you. Jacking up the spark a bunch might help reduce the chance of a slow burn going out the exhaust that ignites the misfired cylinders. Making the IAC open a bunch might help (keeps more cylinder pressure, less internal EGR, and reduces the vacuum on the wall film to strip it slower).

There's also the more difficult option of re-aiming your injectors to hit the intake valves, if they're not already. That reduces the wall film, and vaporizes the fuel better.