Disclaimer - I can only tie the following to my own experience over the last 6-12months with my CAYC engine coded 1.6 common rail diesel Mk2 Octavia. Also, I am not a mechanic, nor overly handy with a spanner. I do however have "The Nack" as Dilbert's paediatrian once put it, so tend to be able to see correlation and decide if causation is linked or not so... here goes!

How to tell early on and as symptoms persist whether your EGR is starting to fail and what will happen as it gets worse, broken down into increasing levels of severity, with a quick description of how the egr system works (broad strokes)

What does the Exhaust Gas Recirculation system do?

Nutshell version - it pumps a certain amount of exhaust gases back into the cylinders via the air intake to cool the burn and help reduce overall emissions.

Ladybird book version - (my understanding)

On cold start and running the EGR inlet valve is closed. When the engine is up to normal running temp and the engine is running at less than full throttle, the valve is opened. at this time a certain amount of the Exhaust gas is diverted into the EGR cooler which on the CAYC is buried down the back of the engine, very inaccessible and  the labour time to access is why replacing the system is so expensive. Out of the cooler, the gas is mixed back into the air flow around the inlet manifold, and the volume of gas sent is metered to match the Fuel:Air mix the ECU is expecting. The exhaust gas reduces the volume of combustible mix i. the cylinder which thus results in a slower burn that is meant to be a lower temp and helps burn off more of the exhaust emissions. When you lift off the accelerator and coast in gear or go full throttle, the egr valve is closed - Coasting cuts the fuel flow to the engine, thus no emissions so no gases need to be sent back for reburning. When on full throttle, the ECU doesnt care about emissions, and wants to deliver max power and torque available, so wants the "cleanest" fastest burn, so it doeant want exhaust gases reducing that efficiency.

The above is all great when things are working properly. But, EGRs are notoriously bad at doing their job properly so, below is how detect when yours has had enough.

Level 1 - Early signs

While driving your car along at a consistent speed, such as NSL on a motorway, you have to slow down considerably, or stop very briefly, to pay a toll or similar situation. on accelerating back up to NSL you see the glow plug flashing at you. Cycling the ignition makes this go away and your journey continues uneventful, other than the nagging feeeling in your head.

If you have a fault scanner, there will likely be no stored fault, unless you scan while the GP light is flashing. if you do find a code it will relate to the Mass Air Flow Readings. This is most likely a false error caused by the following - When you coast, the egr should close, but if yours is sticking open, it wont and so after Five Seconds (based on my monitoring over time tracking this) the ecu is going to have unexpected gases flowing into the chamber. When you put your foot down again, the ECU will still be finding unexpected "air volume" in the system as EGR should be closed on full throttle too and so will assume bad readings from the MAF and set the glow plug light flashing.

There will be no limp mode nor will there be any loss or change in power delivery.

Cycling the engine will reset that fault code and it wont come back unless you repeat the above circumstances. If you can't safely cycle the ignition, its absolutely fine, just an annoying little light on the dash.

Level 1 is very intermittent, but will over time (1000s of miles) become more frequent. ignition cycling to clear the code seems to also kick it back to normal operation.

 

NB - an initial check of your brake lights should be done, as a flashing glow plug light on some cars is also an alert that there is a problem with brake light illumination, probably the brake pedal switch. We will assume the brake lights are all working...

 

I'll add Level 2 shortly, as ive tyoed all of this on my phone, while drinking brandy and eating crisps on a Sunday night...

Edited 3 January, 20251 yr by mac11irl

Part 2, Finally got it typed up and ready...

 

Level 2

 

Advancing from Level 1, you have a repeat episode on the same journey of the MAF turning on a glow plug warning.

 

However, you also get an EML light that wont turn off with the ignition Cycle.

 

When scan the ecu, there will be a stored MAF fault.

 

Clearing the code, it wont come back unless you again have a double MAF fault in same journey.

 

I do not recall seeing any EGR Fault Codes during the level 2 period.

 

 

 

Level 3

 

Things are getting worse.. the Valve has stuck open and isn’t resetting properly. What do you experience?

 

Cold starting will be normal, but when driving away any revving over 2500rpm or a full throttle under 2500 will induce a limp mode, EML and flashing glow plug.

 

Cycling the ignition will clear the glow plug and the limp mode, unless you go over 2500 rpm again.

 

This will continue until oil temp indicator on the dash (assuming a maxidot display) shows over 70c. Once the engine is warmed up to this temp, you can rev / throttle as normal.

 

Long driving symptoms revert to level 2.

 

Fault codes will show up as – MAF, EGR Sensor A & EGR Sensor B

 

Additional observations on my experience at Level 3 – Air Temperature affect.

 

When Cold starting, the ambient air temp seems to have an effect on the symptoms.

 

During warmer weather, when the air temp (maxidot) shows between 14.5 – 20 degrees, limp mode risk from revving over 2500 before reaching the oil temp vanishes, but full throttle will induce it.

 

Above 20deg the chance of a limp mode seems to increase again, I don’t know why.

 

 

 

Level 4

 

Cold start will lead to an immediate flashing glow plug and driving in limp mode.

 

Cycling the ignition will do nothing until the engine oil is up to 70deg or higher, after which you once again are back to Level 2.

 

Fault codes will show up as – MAF, EGR Sensor A & EGR Sensor B

 

 

 

Level 5

 

If you get here, its as bad as you can really expect short of the car being completely undrivable.

 

As per Level 4, Cold starting issues go away once over 70 deg oil temp.

 

However, even after clearing codes and continuing on you are not 100% in the clear.

 

Revving over 2500 rpm is okay, however, any full throttle acceleration, regardless of engine speed / gear selection, carries about a 20% risk of popping another limp mode.

 

 

 

Solutions –

 

Obviously a new EGR valve, and the earlier in the symptoms the better, however, having looked into this, due to the EGR system’s position on the CAYC, off down the back of the engine, replacement is time consuming due to how much stuff needs to come off to get access to it from underneath on a ram, thus its gets expensive. Saw quotes of 1200 to 1600 quid on similar engine Golfs etc.

 

 

 

Blank off and code out your EGR. This may cause an increase in the number dpf regens the car requires, likely has a net negative effect on total emissions etc.

 

 

 

Other Notes –

 

Please be aware that while I did let things get bad on mine prior to getting it resolved via option B above, there was a simple reason for this –

 

I don’t drive the car most days currently as I have a work van I now use for all my work stuff. So, poor donkey 2 is sitting on the driveway for the vast majority of the week, other than school run once a week, and the shopping collection once a week. All low speed, short journeys that diesels hate. One or 2 long drives per month that I take it on, mostly to try and keep the dpf sorted and stretch its legs.

 

I also saw an opportunity to learn as much as I could about how this system operates and how the failure develops over time so that icould do this write up on it to hopefully help others with similar issues.

 

 

 

And finally, I’ve been thinking about changing it (don’t tell it though, it’ll get cross and get the hump), and so weighing the options on whether it was going to be worth my while putting the money into it for the trade in value. Ultimately, I got it blanked while it was getting a full service and mild TLC before its NCT (MOT, which passed with only an advisory for tyre tread under 3mm), as I didn’t want to risk it failing by throwing any of the faults during the test, and saw the blanking out as worth the cost for when I do change it, hopefully this year for a petrol… something… hopefully a bit interesting, yet to be decided on.

 

 

 

I hope the above does prove useful for somebody, somewhere, banging their head off the steering wheel trying to figure out why it seems random, especially if you go down the early doors option and needlessly change the MAF sensor , until you really look at everything that happening together. The air temp influence in particular I found very interesting.