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Intake reversion at idle speed = soot and extra regens?

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Intake reversion at idle speed = soot and extra regens?

Postby marcrbarker » Fri Jun 02, 2023 8:30 pm

I've got an early mk8 that had a previous history of too frequent dpf regens. This could be a more commonplace thing. The dpf was deleted in the end and then there was a 'sooty idle' from the exhaust that got worse and worse. Hold the throttle at just above idle and the soot completely dissapeared. Then the idle smoke got so bad you had to every time pull up the last few yards each stop with the parking brake instead, keeping right foot on throttle revs just over idle and all is fine. Let sooty idle too long chokes up and a pig to restart hot. I got this van because previous owners had replaced everything but didn't fix smoke. Every time the identical problem. I mean everything, everything replaced even the entire engine and wiring loom, then finally gave up when an entire ECU set was tried but it didn't even crank engine. Every avenue of faultfinding had been tried nothing else to replace.

This gets interesting because since everything feasable has been replaced more than once it gives a blank canvas for experiment (where I came in). What I think as a theory may be going on is there could be a tendency (because of tight advanced emissions control) for this model to suffer from intake reversion at idle under some circumstances (to find out). If so that causes extra soot generation that trigger extra regens.

During every other TDC there's the scavenging period when both intake and exhaust are open, what's supposed to happen in that moment is some intake air sweeps out the exhaust before the intake valve closes. My theory is at idle there's a bounceback somehow that's blowing back during scavenging way beyond EGR amount. The combustion gets starved of burnable oxygen and causes the sooting up of DPF.
With the intercooler hose off at idle I can see puffs coming back out the intake. At idle the turbo isn't blowing them back in. I've got a suspisicon there's something going on with the exhaust somehow in previous history. Resonance or something that's not properly damped.
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Re: Intake reversion at idle speed = soot and extra regens?

Postby Sam123 » Fri Jun 02, 2023 10:45 pm

Going back to first principles, black smoke means excess fuel, or conversely insufficient airflow. So I’d suspect the air supply. If it were me I’d check/replace MAF sensor, turbo and EGR in that order, and check all boost hoses and the inter cooler for leaks.
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Re: Intake reversion at idle speed = soot and extra regens?

Postby marcrbarker » Fri Jun 02, 2023 11:56 pm

Already done all that and multiple times before me. A regular repeating pattern, starts off exactly like this (EGR/MAF/hoses/turbo/airflow/intercooler), no improvement and the mechanic either chooses something else to replace or says it's just a crap engine design on these. and always every time distancing themselves :). So I'm trying my best not to let anyone else here fall down the same hole by going over the same ground. Everything has been replaced there's very little left of the original van.

Yes it's insufficient airflow intake. There's something I've noticed about this MK8 is emissions control design is pushed to the very extreme.
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Re: Intake reversion at idle speed = soot and extra regens?

Postby marcrbarker » Sat Jun 03, 2023 12:22 am

Just to tie up loose ends (these had all been done) Correct me if I'm wrong...

MAF sensor

(problem is smoke at idle speed and no other time). If I'm not mistaken there's always excess air on diesels especialy at idle. Not a gasoline engine 14:1 ratio.. and the MAF doesn't need to tell ECU to reduce fuel at idle and if it did the engine would stop

turbo

at idle there's little or zero boost so fail turbo not much effect. Is atmospheric presure.

and EGR

That however does affect things a lot at idle and is why I tried blanking it off and did not much effect.

all boost hoses and the inter cooler for leaks
.
Turbo not doing much at idle speed, intake is almost atmospheric pressure and there's supposed to be excess air pulling in. So at idle leaks on hoses wouldn't make difference as there's no significant boost pressure to leak out.
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Re: Intake reversion at idle speed = soot and extra regens?

Postby dumper » Sat Jun 03, 2023 7:30 am

You say that the DPF has been deleted is it on the van and still full of soot before the delete was the vaporiser working properly they do tend to surfer from wiring faults when my van had DPF problems and not doing a regeneration it was as simple as a faulty thermostat.
MK 8 L4 H3 Motorsport campervan
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1974 mk1 v4 with 2.0 pinto fitted
1986 mk3 2.5 di swb
1990 190 lwb 2.5 di
1998 100 lwb 2.5 di
2006 350 jumbo 135 tdci
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Re: Intake reversion at idle speed = soot and extra regens?

Postby marcrbarker » Sat Jun 03, 2023 10:20 am

Just to rule out a new thermostat included with replacement engine, and old engine had several thermostats along with all its replacement MAFs and turbos etc.
I'm suspecting the exhaust too, but I'm also trying my best not allow "blaming the victim" which misses the underlying cause and only deals with a visible effect.
Exhaust may well now be too clean because when the previous owner washed it out also physically removed anything that could be causing an obstruction not just poking a hole through the meshes, removed them com0letely.
If the exhaust is choking up then there would be visible signs of overheating upstream of the restriction, none, this van is regularly thrashed to obliviion (runs really well) in ongoing effort to "clear out the soot". But just as parking up after a no-smoke motorway thrash the smoke comes back because the only time there's ever smoke is only at idle speed.

I think there's some kind of intake reversion going on while at very low rate of airflow through unrestricted paths, when the turbo's idle and inlet manifold is almost atmospheric pressure. Can see gas blowing back out the (new) butterfly valve intake on the (new) engine with the (new) intercooler/pipes/MAF/airbox disconnected.
Something very much like this happens after performance-tuning. for example when gas-flowing tuning a cylinder head if the head's exhaust port is by mistake opened out too large to match the exhaust manifold diameter without the needed sudden step size change it deletes the backflow blocking effect. Race engines don't idle.
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Re: Intake reversion at idle speed = soot and extra regens?

Postby marcrbarker » Sat Jun 03, 2023 5:29 pm

dumper wrote: the DPF has been deleted is it on the van and still full of soot before the delete
I'm trying best not to blame the victim! :) was all removed.

was the vaporiser working properly
Too frequently, that's earlier on in the history when the first round of replacement thermostats/maf/turbo etc. It regenned OK but too often. The reason why the DPF got "cleaned" and deleted. All soot now goes straight out the tailpipe without get caught up anywhere.
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