Hi, hopefully you won't mind my question here - I don't have an Octavia, but its close relative from a Spanish brand. But mechanically, they are same.

Anyway, I have a 77kW CBZB. It worked flawlessly for the year I own the car, but when this summer started, I noticed occasional loss of power. It only happens in hot weather (around 30 degrees and up) with AC on. In most cases, it happened when I wanted to accelerate after mild climbing at lower gear at ~2000RPM.

It feels almost like a limp mode, but no check light and I can still somewhat accelerate to around 60mph. Observing fuel consumption, it drops from 11-12l full throttle on 5th gear to around 9l/100km, so between 20-30% of the engine power is lost.

Diagnostics have shown code P155A and P0299. That would suggest turbo regulation, but that's usually accompanied with P334A/B/C error. Also, it's weird it only happens in hot weather with AC on. When I tried to replicate the issue, without AC, it didn't happen even in the recent heatwave.

Anyone can point me in the right direction? I don't want to just start swapping parts without proper diagnosis of the issue.

Also, what's the difference between P155A and P0299? Both point to under boost issues, but it seems they are triggered by different problems. If anyone explained me what are conditions for these issues, it would be very helpful.

Thanks in advance!

53 minutes ago, Papez said:

Also, it's weird it only happens in hot weather with AC on. When I tried to replicate the issue, without AC, it didn't happen even in the recent heatwave.

Monitor battery voltage in situations where the problem is occurring. The extra power being used by the radiator fan (plus cabin fan) and the extra heat from A/C condenser may be causing the charging system to struggle.

Something with voltage display plugged into the cigarette lighter is maybe the easiest way of looking at this while driving.

15 minutes ago, Breezy_Pete said:

Monitor battery voltage in situations where the problem is occurring. The extra power being used by the radiator fan (plus cabin fan) and the extra heat from A/C condenser may be causing the charging system to struggle.

Something with voltage display plugged into the cigarette lighter is maybe the easiest way of looking at this while driving.

You reminded me - the battery died shortly after this happened for the first time. But it was still original from 2012, so I didn't think much of it. However, new battery didn't fix the issue.

I'm thinking more about the alternator than the battery.

Thanks, I'll check it

Looks like charging is fine.

After deleting the faults and driving until the issue reappeared, only the P155A error persisted.

Could it be anything else besides the wastegate actuator and turbo itself?

Could be the actuator sticking at some point. My old 1.9tdi used to go into limp mode for that reason but no EML on . If I stopped engine and restarted it was fine till next time. I assume your actuator is electronic. It might be the wiring to it. Check for corosion and breaks. If it happens more in hot weather it may be due to something thats cracked and expanding and causing a fault in wiring.

Alasdair

So the service had some time to take some measurements and was able to replicate the fail.

Apparently the turbo only pushes around 1.3-1.5 bar absolute in 2000-3000RPM when specified pressure is around 1.8. full pressure is only reached beyond 3000RPM.

Wastegate actuator position seems to correspond with driving voltage and changes in specified pressure, but I noticed that at some points, voltage goes above endpoint setting and the actual position goes negative. If I understood it correctly, 0 should be fully closed wastegate, right? Or is there some extra range on these engines and the wastegate never really shuts off?

Also, what is normal voltage range with wastegate shut on these engines? On mine, the endpoint adaptation is at 3.8V, but when it goes negative, it goes up to 4.2V.

I'm asking, because my turbo is the older type, with 2mm spacer fitted between the actuator and its mount. From what I could found on forums, this spacer changes endpoint voltage from ~4.2 to ~3.6V, so this could mean that something wore enough to make the spacer ineffective? But why didn't ECU adapt to this wear then?