T4 Boost cut off!

[Alex121]

Evening all,

I've had this problem for a while now on/off (yes, har har, my car is poo)

It will randomly go into some sort of 'limp' mode and limit boost to 0.5 bar, only I've got no idea what causes it or how to stop it from happening.

It does it every few weeks but it did it once yesterday and twice this morning. A quick restart of the car and it's all fine and reaches full 1.35/1.4bar boost again. There are no boost spikes, it peaks and holds exactly as it should when it's working, temps are fine..

Things that have recently been replaced (turbo wise):

- Replaced my TCV - Problem occurred on the old TCV too.
- Checked all boost pipes
- Turbo
- Replaced spark plugs (gen Volvo)
- Replaced both coil packs for new parts
- Replaced HT leads
- Full service inc filters - Air, oil and fuel
- Had codes read from Volvo every time it happens - Nothing shows up other than 'faulty xenon self leveling unit' which I'm aware of
- No engine management lights come up

So it can be fine for days/weeks boosting well, then randomly limit to 0.5 bar - restart the car and it's fine again, without logging any codes.

Any ideas?

Thanks

[mitchyboy01]

Hi Alex,

It may be intermittently overshooting on boost. Once the MAF senses airflow over a certain limit the ECU will pull back and go into limp mode. My car has done the same. Could be actuator adjustment, exhaust leak, Vac/boost leak and probably several other sensor related issues.

I still need to hunt down a vac leak on mine somewhere causing a rough idle sporadically. Although not putting it into limp mode, I get the odd lambda code. But..... Yesterday I found I had a very slight exhaust manifold leak so sorting that first to see if it cures it (fingers crossed). I hate Vac leaks sometimes they are a complete pain to find. I simply can't be bothered ATM!!

[Alex121]

Hi Mitch,

The actuator is setup properly, was calibrated by Volvo a few months back when I had the new turbo put on, obviously waste gate is new so I doubt that is playing up. I had my exhaust manifold off last year, refurbed properly and put back on, no leaks there - Pipes I've checked and there are no issues there either.

MAF is an interesting idea. That's one of the few things left of the car since I bought it that I've not replaced. I also have a K&N panel filter so I'm wondering if maybe some of the oil has seeped through onto the MAF - I would have thought I would have had more issues if that was the case though.

Maybe I should look into that next.

I've spoken with Ashok and he agrees it's an odd one, I've tried all the things he's suggested so far to no avail. Volvo are just as stuck and they know the car pretty well lol.

[Matt86]

MAF are interesting things though Alex - Mine has a fault within the wiring yet the car runs perfectly!

[Paul t4]

Coolant temp sensor fault?
My t4 limits boost pressure to 6-7psi when the coolant temp is lower than it should be.

[Alex121]

Replaced thermostat last week, wasn't getting hot before but was still hitting full boost so don't think that's the problem.

Thanks though

[Paul t4]

Yes but are you sure the ecu is not losing the coolant sensor for a short amount of time, just enough to trip it into limp mode but not record a fault.

[Alex121]

Hmm, interesting. I'll have a look into that. Thanks

[Alex121]

Apparently if it's the coolant sensor it'll throw up a code...

Given I've never had any codes I doubt that's the issue.

Thanks

[tt82]

Sounds very much like the problem my brother had with his Audi 1.9Tdi. That would go into limp mode for no reason. A VAG specialist reckoned a new turbo as the old one was over boosting. Me, my dad and my brother all reasoned that if the turbo was overboosting, then it was certainly fine. If I remember correctly we narrowed it down to 3 things.

1 Intake pressure sensor
2 Wastegate sticking
3 A control valve that was operated by vacuum pressure