Intermittent Limp Mode - P0299 - Underboost

[RandomZephyr]

Hey guys,

Wanted to get some advice on my 170hp A4 Avant, BRD engine. Only owned the car a few months but I've been having an issue where the car will go into limp mode on the motorway in 5th/6th gear sometimes.

I can do the same journey, on the same conditions and sometimes the car will go into limp mode there/on the return, and sometimes it will be fine. I'm sure you guys know but limp mode is painful, quite frequently I'm holding up traffic as the car refuses to go above 40 on an incline, seems like the turbo isn't working at all.

Most of the time when the motorway is clear enough I can turn the engine off and restart, and it will be fine for a little bit but go back into limp mode.

During limp mode I get a flashing glow plug on the dash, and a few times an EML. This usually disappears, last time the EML light went away after a day, I started the car in the morning and it was still there, went to the shops later on in the day and it was gone. The flashing glow plug goes away on every restart.

Now, I have taken the car to a mechanic who is a family friend (because it's cheap) but he keeps going on about the DPF which I'm almost certain is not the case.

In my reading, it seems like people generally say to check the boost hoses or the actuator, but I'm not really sure where/what I'm looking for, and I don't have a vacuum pump.

Another mechanic has told me it's almost certainly the turbo veins, so I should look into getting a carbon clean but again this doesn't look like it will give the turbo a proper clean like it seems like it needs, I'd rather fix the issue properly rather than spend however much on a clean only for the issue to return, if it even fixes it.

So, just wanted some advice from you guys on which direction to go, or if any work can be done myself I'm open to it. The little work I've done on the car thus far has been extremely difficult due to very rusted bolts, well over half are either rounded or stripped so doing anything takes a lot longer than it should!

Cheers guys.

 

[NHN]

My 1st advice is to get a full scan of the car after its limped again incase they've cleared the codes, this will help us in guidance.

 

[Pie-eyedpiper]

Are you able to try this, it's actually very simple-
Its on an A3, so you'll have to rotate your screen 90° (only joking!), just remember your engine is not transverse when trying to find the components.
If it works it's a very cheap fix for sticky vanes. I do mine as a matter of course every 6k mls when I service.

 

[NHN]

If it's that clogged, Mr muscle won't do much & is only a temp fix as it'll come back easily imho.

My best advice is, if it's vanes, to remove & clean back to the metal, peace of mind of for a long time I'd say, each to there own.

 

[Pie-eyedpiper]

Its clearly not completely clogged, the vanes must be moving to some extent.
His car is atleast 12 years old, with 12 years build up. Ofcourse it'll come back eventually, but not in the lifetime of his car and not if done regularly.
A £5 can of MM and a bit of spanner work is the cheapest way. But as you say, each to his own.

 

[NHN]

Yes I agree, it's not completely, but unfortunately it only takes one moment for it to get stuck which could be from debris & the turbo goes pop & it's either game over or new turbo & a hell of a mess, given age of car probably not worth the expense.

As we've said, each to there own, but I know which path of choose, however, that's because I can do it myself as per you, so it's not a cost issue so to speak.

With respect, I've encountered quite a few people who did this trick, then short time later, week, month even days said it returned, so lifetime of his car, it's wont last that long from 1 treatment imho, however it's upto him if he wants longevity with no concerns it'll comeback anytime soon & some.of these people I've had to swap turbos out or remove & clean properly.

 

[Pie-eyedpiper]

If it really is bad then he might need more than one clean, plenty from a can anyway.
But sadly that is all a lot of garages will do, say your turbo is goosed and you need a new one. Rather than dismantle and clean. If you do clean them up and rebuild that all respect to you.
And in my book anyway, prevention is better than cure.

 

[mm707]

Vacuum unit on inlet manifold. They have a diaphragm inside which can split.
Cheap fix, about £20. Diy job if you're ok with it.
I did mine a few months back.
Get a few cans of choke cleaner as your inlet manifold and possibly egr will be full of crud.

 

[mm707]

Vacuum unit on inlet manifold. They have a diaphragm inside which can split.
Cheap fix, about £20. Diy job if you're ok with it.
I did mine a few months back.
Get a few cans of choke cleaner as your inlet manifold and possibly egr will be full of crud.

 

[AvantMike]

Vacuum unit on inlet manifold. They have a diaphragm inside which can split.
Cheap fix, about £20. Diy job if you're ok with it.
I did mine a few months back.
Get a few cans of choke cleaner as your inlet manifold and possibly egr will be full of crud.

MM - did you have a set of instructions to follow or just go for it (and if so, do you feel like sharing them as I fancy having a go at this) Thanks

 

[mm707]

There's a thread on here somewhere with good pics of how to remove the egr , it is fairly simple.
once it's off you remove the inlet manifold, easy enough but be aware there's one bolt easy to miss in hole where egr bolts on, usually covered in muck but ok when cleaned off.
once you have manifold on bench you will see how the vac unit goes on, i think just one allen key.

clean everything and put back together.
you could easily get manifold off in an hour , max.
allow another hour though to get it all cleaned up and de crudded.

just need torx socket set for bolts.
all parts inc gasgets available from tps.

 

[RandomZephyr]

My 1st advice is to get a full scan of the car after its limped again incase they've cleared the codes, this will help us in guidance.

Hi, I've done a few scans and the only message that is persistent is the P0299 underboost after limp mode, the car has no fault codes before it goes into limp mode and will keep the P0299 until I clear it.

Are you able to try this, it's actually very simple-
Its on an A3, so you'll have to rotate your screen 90° (only joking!), just remember your engine is not transverse when trying to find the components.
If it works it's a very cheap fix for sticky vanes. I do mine as a matter of course every 6k mls when I service.

Thanks! This defiantly looks like something to try, I'll get onto looking at doing this. The longitudinal engine layout really makes things difficult in my opinion!

Vacuum unit on inlet manifold. They have a diaphragm inside which can split.
Cheap fix, about £20. Diy job if you're ok with it.
I did mine a few months back.
Get a few cans of choke cleaner as your inlet manifold and possibly egr will be full of crud.

Sweet, seen quite a few people say this and it doesn't seem like there's any drawback to trying this, I can't seem to find the part in the UK though, do you know anywhere I can grab one?

Thanks for the help everyone, looks like I've got a few avenues to go down so I'm going to try and see if I can fix this myself.

The only issue is how unreliable the problem is, I won't really know if it's fixed until taking it on the motorway for a few hours, several times.

 

[RandomZephyr]

Nevermind, found S03G129061C on Darkside, cheers!

 

[RandomZephyr]

Hey guys, just to quickly post that I got this fixed by changing the swirl flap on the inlet manifold. No more limp mode!

 

[Pie-eyedpiper]

Good news you're sorted.
Did you manage to get in and clean anything up?