EGR system questions

[muddyboots]

This is geared around the engine in my Passat but the theory should apply equally to the PD TDIs in Ibizas/Leon etc.

I'm trying to understand the operation of the EGR system, to see if the hissing noise I'm hearing coincides with EGR operation.

First, I assume the EGR valve opens under zero or very light throttle conditions to allow some exhaust gases to recirculate into the intake.
Then under any acceleration or boost, the valve closes off.

Second, I'm assuming the EGR would be designed to only open when vacuum is applied - so if the vac pipe was damaged it would fail "safe" and stay closed. In other words it needs constant vacuum to keep it open at idle/low throttle conditions.

So - there must be a constant source of vacuum supplied to the EGR, with a control valve somewhere in the middle to restrict the vacuum applied to the EGR valve as necessary.

Does anyone know how the control valve works - does it just "block" the vacuum from reaching the EGR valve under acceleration and that's that ?

I hear my hissing noise start (after a very short delay) whenever I accelerate, even when acceleration is gentle and at low revs (before the turbo spins up).
Then when I lift off it goes away.
In my mind this seems to coincide exactly with the EGR operation, does my theory sound correct ?

In which case the hissing sound could be related to the control valve letting air in, or a leak in the pipe leading from the vacuum source to the control valve.
I'm wondering whether this could all be related to the poor mpg I'm getting too.

I doubt if it's related to the VNT actuator, as I've sat in the car at idle and cycled the VNT actuator with vag-com in engine bock 11 - and heard no noise.

Thoughts ?

Thanks for reading, hope I haven't bored you to sleep

 

[sssstew]

your correct in the operation lee.

My understanding is the N18 blocks the flow of vacuum under acceleration, you wont have aN18 but a combined N75 and N18 black box but the principle is the same.

does the hissing go away after a certain rpm?

 

[muddyboots]

your correct in the operation lee.

My understanding is the N18 blocks the flow of vacuum under acceleration, you wont have aN18 but a combined N75 and N18 black box but the principle is the same.

does the hissing go away after a certain rpm?

This is the Passat - I think it does have all the separate valves (not like the Ibiza).

I can't tell if it goes away because engine noise take over and drowns it out...

 

[craig-pd130]

I think of the N18 as a solenoid switch on a main vacuum line. The vac runs continuously THROUGH the valve until the ECU flicks the switch to divert the vacuum to suck open the EGR valve (on very light / zero throttle).

On mine the main vacuum "line" which loops through the N18 also connects via a Y-branch to a stub on the airbox lid (to draw filtered air into the vac system). If this is loose / not connected it will hiss, and will lower the vac level a bit.

 

[muddyboots]

So do you think that the N18 vents the main vacuum line to air, or is just a dead end, when the EGR is not active (ie the conditions where I hear a hiss)?

 

[craig-pd130]

When the EGR is not active, the vac **should** just flow through the N18 and maintain vac levels in the overall system. The hissing could indicate a leak in one of the vac hoses ... the tricky thing is that the entire vac system is interconnected so it could be at any one of the joints.

I'll take a guess and say it's most likely to be a joint somewhere, unless there's a bit of hose that's badly routed and has chafed.

 

[muddyboots]

It's all pointing that way isn't it....

Do you know of any way you can switch the N18 with vag-com while the engine is running ? (IE like you can with the VNT actuator ?)

 

[craig-pd130]

No idea, sorry .... never used vag-com

Just another thought -- are the joints for the intercooler / inlet pipes dry, especially the top hose from the intercooler?

 

[Deleted member 10725]

Im also getting this hissing noise, and i also suspect the egr or some leak.
I get this noise at very little throtle, some say its the turbo spining but i think it not...
I get this sound from 1100 to 1600 rpm then as Muddyboots says the engine noise covers it...
Im getting really poor mpg....

 

[muddyboots]

Im also getting this hissing noise, and i also suspect the egr or some leak.
I get this noise at very little throtle, some say its the turbo spining but i think it not...
I get this sound from 1100 to 1600 rpm then as Muddyboots says the engine noise covers it...
Im getting really poor mpg....

That sounds exactly right. Comforting to know, in a bad way !
Judging by your username, you drive a Bora ? Presumably a PD TDI of some sort.

 

[Deleted member 10725]

Yep im driving a pd 115.
I also was getting some kind of hesitation but a New Maf eliminated 80% of the problem.
I changed the N75, cleanned and rebuild the turbo (But the sound stays the same), cleaned the egr and the intake and still get poor mpg...

The only thing left is the n18 , i tried to block the egr blocking the tube above it but the car strangely didnt start or had much more trouble starting and was much less responsive ???

 

[sssstew]

how about the old technique of putting slightly watered down washing up liquid on the joints (for the vah hosing this time rather than intercooler pipes) and then go for a short drive then have a look for bubbles/foaming?

 

[muddyboots]

'cos it'll suck it in, rather than bubble outwards

 

[craig-pd130]

Muddy -- over on Scoobynet there's a bloke with a Passat Sport estate, he reports getting 40 around town and 45 / 46 on a run at 80-odd mph, says he hasn't hit 50mpg yet.

Worth checking all is as it should be, but maybe mid-40s is par for the estates?

 

[Deleted member 10725]

My father has a passat 130 and gets 50 mpg, he may not be a gasser but he sure doesnt do Grany driving.

Its must be something, because in my case a 2 years ago i did 50 mpg and now it sits around 40mpg somedays even less....

Frustating and no errors on vag com.

 

[OLDOILER]

Muddy -- over on Scoobynet there's a bloke with a Passat Sport estate, he reports getting 40 around town and 45 / 46 on a run at 80-odd mph, says he hasn't hit 50mpg yet.

I have a saloon Passat 130 + remap and have never got more than 44mpg av 40mpg, doesnt seem to change much if Im doing 60 or 80 - 130+ gives me around 36mpg

 

[sssstew]

'cos it'll suck it in, rather than bubble outwards

ah chuff.

 

[stoneyfordNI]

on this egr thing , when u log boost etc on vagcom u get an egr % readout in one of the blocks ( 3 i think) i take it this how open the valve is expressed as a %age , now is this value a specified value or an actual one ,


IE , could it read 4.8% ( thats wat i see on full throttle) but stilll be open , or is the 4.8 actually its true state

just wondering

 

[cheshire cat]

I guess that value is taken from the requested signal as unlike the later ones it 's a dumb system, don't forget at tickover it shuts after two minutes, how long for I can't remember ps thanks again Stoney'

 

[Deleted member 10725]

As i said i was getting hesitation that the Maf solved in most part.

But if e let the car iddle for 2 minutes and give it full throtlle i get a better response from de pedal and better aceleration.

Thats one more reason that i think my problem is egr/n18 related....