Intermittent Engine or Brake Issue?

[monkeyhanger]

I3rady: These GPFs ultimately convert soot (carbon) to carbon dioxide, so when the car isn't regenning, it is storing carbon (soot) to convert to carbon dioxide. Under regen, carbon dioxide output is on the up.

Petrol particulates are worse than diesel particulates. They are generally 10 to 20 times smaller in diameter, so they hang in the air longer and are inhaled deeper into the lungs. Petrol particulates were never an issue before direct injection.

NOx isn't really a petrol problem. Although the exhaust is hotter than a diesel's, the combustion temperature is cooler, and that's where NOx is formed, where Nitrogen and Oxygen are present to excess in a diesel, but not a petrol engine. The amount of NOx a petrol engine produces is much lower than for diesel.

I still can't fathom why mpg suffers so much under GPF regen. The conditions required for regen don't seem to promote massive inefficiency. Power doesn't seem to suffer when it's happening either.

[Dark_cze]

Car itself is very dirty right now but lower part is black (not wet) and in summer this part was visibly black while rest of that lip was nice and shiny.

[monkeyhanger]

My 2018 GTI+ without GPF has sooty tailpipes, my wife's 2019 GTI+ with GOF has spotlessly clean tailpipes.

The tailpipes on my 2015 Golf R were filthy too.

I have gti made in 11/2018 spain. With GPFand my tailpipes are black like my old diesel Audi A4 B6 no fluids are missing and consumption is fine so I guess no technical problems there.

That is interesting, I'd expect that you would find the same as monkeyhanger finds with his wife's GTI+ with GPF, so what exactly is your GPF doing when it is passing the soot that appears on the tips?

For South African Built Polo GTIs, they ceased MY18 production in July 18. They picked up production again for MY19 in Sep 18, and i'm pretty sure that everything built after that point was WLTP compliant and had GPF, whether built in South Africa or Spain (unless some markets have exemption perhaps?).

If you have a GPF and sooty tips, I'd assume it wasn't doing its job effectively. Are you sure you have a GPF and it hasn't been doctored for performance reasons?

[Dark_cze]

If you have a GPF and sooty tips, I'd assume it wasn't doing its job effectively. Are you sure you have a GPF and it hasn't been doctored for performance reasons?

everything made after september 2018 need to be WLTP compliant. Mine was build 11/2018 and registered 12/2018. Engine is code is DKZC, while pre WLTP cars are CZPC. No performance issue, no fluids leaks or missing and nobody touched GPF from day VW delivered it to me. Don't really think there is anything wrong (except noisy suspension and noisy breaks while reversing).

// and if there is something wrong I have warranty till 12/2022

[monkeyhanger]

If you have a GPF and sooty tips, I'd assume it wasn't doing its job effectively. Are you sure you have a GPF and it hasn't been doctored for performance reasons?

everything made after september 2018 need to be WLTP compliant. Mine was build 11/2018 and registered 12/2018. Engine is code is DKZC, while pre WLTP cars are CZPC. No performance issue, no fluids leaks or missing and nobody touched GPF from day VW delivered it to me. Don't really think there is anything wrong (except noisy suspension and noisy breaks while reversing).

// and if there is something wrong I have warranty till 12/2022

I'll take pics of the tailpipes on mine and the wife's GTI+ to show difference between the 2 cars.

[Dark_cze]

All right, thanks I never really cleaned tail pipe on my gti so it was collected over 25 000km and 13 months (not sure if collected is right word). I touched it with finger somewhere in october and it was black dust but just a light deposite. To be honest Iam not even sure if I ever had GPF regeneration (Iam sure it should be done many times already) but probably I didn't notice it. My photo was prety bad, my car is dirty like pig and tailpipe too. I will clean it and will see how it will look after week or two.

[monkeyhanger]

GPF fitted 2019 GTI+ tailpipes, approx 4k miles.



2018 GTI+ with no GPF, approx 12k miles.



Excuse the filthy exterior on the non-GPF pic, that'll be general grime (pipes never been cleaned). Compare the interior of both pics to see how good a job the GPF is doing.

[Dark_cze]

oh nothing like that on my GTI

I edited your photo to show how was mine looking few months back definitly nothing like your non gpf polo

[monkeyhanger]

VW missed a chance to give Polo GTI a bit of a tailpipe identity, a central pair rather than - a pair to the left (like the Golf GTD), or split pipes (Golf GTI) or 2 split pairs (R).

[CageyJ0nnY]

Thanks for all the info and discussion guys! Some interesting stuff here.

What's really getting me though is 2 days before my last regen I drove a 45 minute motorway journey ( 60 mile round trip). Im now wondering why the car NEEDS to regen, especially when considering I do roughly 30 miles per day of mixed driving. It would be fair enough if I only drove the shops a couple of times per week.

[OomStu_ZA]

I have 20000KM's on the clock and have yet to see this regen feature... hopefully never though and suspect its due to my daily travels avg. 100KM's with some freeway some city driving.

[monkeyhanger]

Thanks for all the info and discussion guys! Some interesting stuff here.

What's really getting me though is 2 days before my last regen I drove a 45 minute motorway journey ( 60 mile round trip). Im now wondering why the car NEEDS to regen, especially when considering I do roughly 30 miles per day of mixed driving. It would be fair enough if I only drove the shops a couple of times per week.

I really do think that VW has scheduled these regens to happen at fixed intervals, regardless of the GPF loading as a "just in case". It seems hugely coincidental that people seem to be getting their first one at about 4000 miles/6000Km, regardless of their average journey length.

[stuartrendall]

My guess would be the car is doing an active GPF regeneration. How many miles has your car done, and roughly when (number of miles) did it happen last time? Is your exhaust a bit roomier than normal? When you say labouring at 50mph, what's the engine speed (revs) and what gear are you in?

My wife's 2019 GTI+ had 2 concurrent 8 mile journeys happen with horrendous mpg, then revert back to normal.

Is yours DSG or manual? I'm assuming 1.0TSI variant (which output?) due to your quoted "normal" mpg.

How long are your commute journeys typically? If you do 30 more miles and the car hasn't rectified itself, I'd get it to the garage.

My car has just ticked over 6000 miles and first did it a few month ago, i'd guess around 4500. The exhaust is boomier at 50 mph in 6th at around 1500 ish rpm. At those conditions it was doing 25ish mpg.

Mine is a DSG and my commute is 13 miles doing duel carriage way and country road driving.

From reading the thread linked above it does sound like a regen thing. It's frustrating though as I used a quarter of a tank today getting to work.

13 miles each way should be ample for the GPF to regen passively (it is for a diesel with a DPF), considering how hot the exhaust gets on a petrol car.

There was a big discussion on this within the Golf GTI forum recently.

There are 3 things needed for combustion - fuel (in this case, soot), heat (in this case, exhaust heat, 550-600C to burn the soot) and oxygen (in the air).

For a DPF, diesel combusts under an excess of air, so oxygen in the exhaust is plentiful, and you have the soot, so limiting factor is how hot the exhaust gets - an issue on shorter journeys.

For a GPF, we have the fuel (soot), we have the heat (so much energy kicked out through the exhaust as heat for a petrol car, so exhaust warms up quickly), but there's a lack of oxygen because petrol engines bring in only as much air as they need for combustion with the fuel injected (stoichiometric mixture).

The only time a petrol car puts a lot of air (containing oxygen) through the exhaust is when it is overrunning - when you take your foot off the accelerator and the engine is still motoring the car with no fuel input and you get engine braking.

My theory as to why GPF regenning doesn't seem to be a doddle compared to DPF is that DSGs aren't set up for allowing overrun, if you take your foot off the accelerator, the drop off in speed is minimal, compared to engine braking on a manual. The DSG is still fuelling the engine to a degree, when you take your foot off the accelerator to avoid heavy engine braking. If it is, then it's depriving the exhaust of oxygen containing air too, as it's taking in just enough air to burn the fuel it's still putting in under deceleration.

GPF regenning passively needs engine braking to provide the oxygen to convert soot (carbon) into CO2. Seems this happens far more readily with a manual box where you can do effective engine braking.

The puzzling thing is, during an active GPF regen, I haven't a plausible reason why the mpg suffers so much. It needs oxygen, not more fuel for the GPF reegen to happen. There must be some other factor at play too which makes the engine run ridiculously insufficiently with the fuel it's getting, but I can't find any explanation for it. My wife's mpg dropped from a normal 30mpg on her commute to 19mpg during the GPF regen. That's a far bigger penalty than for a diesel engined car with a DPF that is actively regenerating.

When an active regen is taking place and the car is not decelerating to allow more oxygen to pass through to the GPF the engines are tuned to see this and so the timing of the engines valves changes allowing more air to enter the engine thus causing the vehicle to in-turn run leaner and that is why MPG is reduced. In my opinion you would be better off accelerating to 80mph then letting the car slow naturally down to 65mph and back up to 80, repeated over a few miles to see if that does the trick.

[monkeyhanger]

The missus' car is regenning.again this morning - her 31mpg commute was 21mpg this morning. I've told the missus to do a bit of motoring/engine braking on her way home in it.

[monkeyhanger]

After taking a little bit of offence at my suggestion that the missus is always on the accelerator or on the brake, and not doing any engine braking, she did a little engine braking and on the way home from work she managed 25mpg (so that's 21mpg there and 25mpg back vs 19mpg there and 21mpg back first time it happened).

Hopefully she keeps up the engine braking.

I persevered with full manual control all the way home from work in mine. It still felt like hard work vs a manual box. It seems more difficult to "feel" the engine speed than with the manual box VWs I've previously had - or maybe I've just gotten lazy with DSG.