Violent bucking and loss of power on boost PLEASE HELP

[Dobbojr98]

So I have a b7 avant s line 1.8t BFB, my mods are decat 3” with a full custom 3” exhaust, big front mount cooler, pod filter and a stage 2 map. My issue I’m having is when I bought the car a couple of months ago it had a stage 1 with no other mods what so ever. I changed the air filter and instantly got a p0441 code for evap. Ever since then as soon as it builds boost over 5lbs it starts bucking violently and start what I can only describe as misfiring or backfiring. I have tried the following,

Brand new parts;

Fuel pump ( in tank )
Turbo ( standard k03 kkk )
Intercooler and hoses
Vac lines simplified with n249, evap, sai, pcv and brake booster deleted/simplified
Manual boost controller ( in case of overboost )
Air filter
Timing chain kit
Plugs
Coils
Stage 2 custom map
Full turbo back exhaust
N75 (OEM AND N75j)
DV (OEM and removal)
MAF ( OEM )


Nothing has worked. I’m now pulling my hair out wondering what is wrong with this car. There’s no leaks as I’ve had multiple smoke tests and boost leak tests, but everyone I speak to has said it’s not even trying to boost. My gauge flutters badly when it gets over 5lbs and then as soon as it hits 15lbs+ it starts with the bucking and missing. But it doesn’t pull at all. Pulls like an NA fiesta in my opinion. Could anyone help as it’s costing me a lot of money. Cheers

[Dobbojr98]

Just to add, I do have a small video of the gauge to show what happens whilst the issue occurs. Not sure on how to show this though

[Dobbojr98]

It also really struggles to start. Hot or cold, it doesn’t matter.

[Jay-Bee]

Inspect your wastegate flap and actuator and post a full VCDS scan of codes.

My guess is leaks in the (deleted) evap and PCV system or something not routed properly.

[Dobbojr98]

Inspect your wastegate flap and actuator and post a full VCDS scan of codes.

My guess is leaks in the (deleted) evap and PCV system or something not routed properly.

It’s a brand new turbo with wastegate, all fully checked over before installation due to me not wanting to throw a bad unit on the car, I don’t have access to vcds lol. That’s the only thing I haven’t done. I haven’t had it plugged into vagcom or vcds for logs as I don’t have the cable, software or knowledge of the required blocks to log

[Dobbojr98]

There is a plastic line that runs under my IM to the passenger side (uk) wing but unsure of what it’s for as I can’t follow the line fully

[67Twisty]

Could be unrelated to your mods/parts. A bad/failing crank sensor can cause those symptoms.

[Dobbojr98]

Could be unrelated to your mods/parts. A bad/failing crank sensor can cause those symptoms.

There’s no CEL and no codes ever come up for it could it still be a bad crank sensor? It’s like as soon as it boosts the ecu thinks it’s getting that massive amount of air too soon and like the pressure I’m seeing through the gauge is back pressure. I know the TB works fine as I tested it earlier? It’s hard to explain but hopefully you get the idea

[oVeRdOsE]

maf? unplug it.

Had what you've described once or twice. Mix of small leak because of the water meth, and probably bad fuel .

But never get the finger on it, it was impossible to reproduce.

[Dobbojr98]

its a brand new maf, already tried all the tricks with it and it never made a difference

[67Twisty]

There’s no CEL and no codes ever come up for it could it still be a bad crank sensor? It’s like as soon as it boosts the ecu thinks it’s getting that massive amount of air too soon and like the pressure I’m seeing through the gauge is back pressure. I know the TB works fine as I tested it earlier? It’s hard to explain but hopefully you get the idea

When my CPS went bad it never threw a code, but it did cause a bucking and misfire situation that took me a long time to diagnose. I kept thinking it was ignition related. Faulty CPS could also cause starting issues as the car can’t accurately determine TDC. Sounds like you need to buy/borrow a VCDS cable in order to see if the ECU is logging errors that could help diagnose yourissue.

[Dobbojr98]

maf? unplug it.

Had what you've described once or twice. Mix of small leak because of the water meth, and probably bad fuel .

But never get the finger on it, it was impossible to reproduce.


When my CPS went bad it never threw a code, but it did cause a bucking and misfire situation that took me a long time to diagnose. I kept thinking it was ignition related. Faulty CPS could also cause starting issues as the car can’t accurately determine TDC. Sounds like you need to buy/borrow a VCDS cable in order to see if the ECU is logging errors that could help diagnose yourissue.

thank you, ill buy a new crank sensor tomorrow and throw it in, its on 150k so probably past its time anyway :)

[67Twisty]

I probably wouldn’t throw any more parts at the car until you can scan it. A CPS isn’t particularly expensive, but it’s hard to isolate cause/symptoms given you’ve made a number of changes to the car since your purchased it.

[Dobbojr98]

I probably wouldn’t throw any more parts at the car until you can scan it. A CPS isn’t particularly expensive, but it’s hard to isolate cause/symptoms given you’ve made a number of changes to the car since your purchased it.

Yeah I understand that and so does my bank haha the issue started before I made any changes to the car and at this point the cps is the only thing I haven’t changed. Because it started before I modified it I’m thinking it’s got to be something original that’s come to the end of its life

[80sGuy]

Could be your transmission or TCM.

[Dobbojr98]

TCM? im assuming thats the trans control module? its a manual so didnt think it had one? also, wouldnt it throw a code?

[80sGuy]

That was my last resort since you've listed just about everything. Didn't know you have a manual unless I'm not familiar with something here. Not every 'intermittent' problem will throw a code (yet).

[Dobbojr98]

Just changed the crank sensor and it’s made it worse. Any form of positive pressure and it holds it back, as if it’s stalling out. When I lift off it sounds like a metal gasket slapping against something accompanied by a pop. When off boost or above 3k with 5lbs it runs fine. This is really starting to anger me now

[oVeRdOsE]

about the timing? skip a tooth ?

I do remember a member changed his belt and align arrows before put the tension , so he was out of 1 tooth. The car was running, but poorly

[Dobbojr98]

about the timing? skip a tooth ?

I do remember a member changed his belt and align arrows before put the tension , so he was out of 1 tooth. The car was running, but poorly

All the timing is correct as nothing like that has been touched. I just can’t work out for the life of me what it could be

[AbstraktA4]

Sounds like a massive evap leak, I had a very similar issue when once trying to boost would buck very aggressive and not idle the best afterward and have rough starts. Ended being my intake manifold leaking.


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[Dobbojr98]

It’s been boost leak and vac leak tested and there’s nothing what so ever. It is down on compression by 1 cylinder though so suspected head or piston issues. I’ve been offered 1of 2 heads but unsure if they fit my bfb engine. One is a bam and one is an agu

[Theiceman]

It’s been boost leak and vac leak tested and there’s nothing what so ever. It is down on compression by 1 cylinder though so suspected head or piston issues. I’ve been offered 1of 2 heads but unsure if they fit my bfb engine. One is a bam and one is an agu

you are telling people this NOW ??

what is the compression in that one cylinder ?
also you say nothing in the timing has been touched but list a brand new timing kit in your very first post.