Topic: what actually causes exhaust 'popping'

[darthekai]

Recently, with zero change in hardware, my car's been popping a LOT more on very light throttle (just enough to keep it off gear overrun fuel cut but not enough to maintain speed unless on a downhill). What exactly causes this? I was at first thinking exhaust leaks, but I got the system re sealed up. It had some pin-holes in the exhaust manifold under the heat shield.

Still happens. I used to think it was a lean condition, but I tried richening up the low end of the map to no avail.

Is it always caused by one thing? I was thinking unburned fuel getting into the exhaust. I am now considering maybe my spark plugs as its been maybe 4 years since I put em in. I am also considering checking my valve clearances. Could this be caused by that?

[diamondedge]

I'm not an engine tuner or engine guy so this is all horseshit, guess work.

Do we have little bit of valve overlap for good exhaust scavenge? Okay, so in your fuel tables, would there be any point in the table that on light, light throttle %, it's going to put fuel that will
not get burned and just passed thru into the exhaust?

Whereas on higher throttle %  you have lots of air and lots of fuel and things go boost like normal. VROO.

okay that's all i have for guesses

paging any engine tuner

[RockThePylon]

Cylinder pressures too low for consistent combustion, dumping the unburned mixture into the exhaust to be ignited by the other cylinder that did fire...?

Also, you have decel fuel cut enabled? Weak.

[darthekai]

I have gear overrun fuel cut enabled.. Is decel cut different?

I'll do a comp check and I'll plan on setting lash as soon as i can get a set of gaskets.

Could be weak or no combustion for some reason at that high vac, and since my engine has a single exhaust port per head... But it doesn't really explain why it would just start happening unless I've lost mad compression for some reason.

[RockThePylon]

It's because you're hanging to the right today, instead of the left.

[diamondedge]

*slowly backs out of thread

[darthekai]

Cylinder pressures too low for consistent combustion, dumping the unburned mixture into the exhaust to be ignited by the other cylinder that did fire...?

Also, you have decel fuel cut enabled? Weak.

oh i thought you said disabled.
I like it cuz it gives better engine braking when coasting up to lights.

[WAGOWN]

It's almost guaranteed afterfire - unburnt fuel burning when it hits your hot exhaust.

Adding a VTA BOV can easily cause this, maybe yours is stuck or has developed a leak? Or something else causing you to run extra rich.

[darthekai]

can a boost leak cause a MAP car to run rich?

[silent]

How large is your exhaust?  If it's a large one, the smaller pressure drop from exhaust -> atmosphere may allow the unburnt fuel to stay in the exhaust system longer due to lower flow rates.   That combined with any incomplete combustion could significantly increase popping.  Just spitballing here, the first thing to look at should be your plugs.

[RockThePylon]

can a boost leak cause a MAP car to run rich?

I doubt it. Just makes your turbo work harder to try and meet targets. That's the beauty of SD, even if it's leaking all over the place, MAP is MAP, and your VE shouldn't really change.

[darthekai]

I doubt it. Just makes your turbo work harder to try and meet targets. That's the beauty of SD, even if it's leaking all over the place, MAP is MAP, and your VE shouldn't really change.

Yeah that was my thought but maybe if the IM is leaking thru the phenolic spacer just into a single runner the reference isn't picking up the extra pressure.