Smithydxb

Hi All,

I’ve trawled the web for this answer, but it seems it doesn’t exist, not for the V8TT anyway.

My CTT drives absolutely fine, 90% of the time, but I’m having a boost issue, whereby the turbos work with no issues most of the time, but sometimes mid boost, they cut of, it drops from 15psi back to 0, then will only hit 2 max (basically no turbo) and it drives on the V8 power only. It showed no dash warnings (see below) until after i took it back to Porsche and stores no codes (Porsche confirms this).

I can turn the ignition off and straight back on again and the boost is back to normal. The issue is completely sporadic and doesn’t seem to matter whether i am hammering the throttle or just accelerating normally on the highway, I can’t do any one thing to replicate it.

Services have all been normal and I’ve just changed the Spark Plugs (due anyway).

Porsche took a look, hooked it up to their computer and updated some software (even after telling me there was no update for the car). I had initially pointed out this thread below to the service centre about some V6 models having what sounds like the closest thing to this issue as i’m having, so whether the update they done was the same or not i dont know (they can’t seem to tell me). https://rennlist.com/forums/cayenne-...rbo-issue.html

Either way, it hasn’t fixed the issue, it happened 20 mins after picking it back up and again today. Turned the car on and off and made some full throttle pulls and it boosts back to 15 and pulls hard. The only thing that now happens is i get the Reduced Engine Power, Drive On Possible, Speak to Service on the dashboard (this wasn’t coming up prior to Porsche updating whatever software they did).

Anyone have any other ideas that i may be missing? I’ve thought of diverted valves, MAF/MAP sensor, actuators sticking etc…

Porsche now want to take the car in and hook it up to the computer for Germany (I’m in Dubai) to take a look because they have said they can’t see what the issue is and they dont know how long they will need it for and cant actually get it booked in till end of Jan. I’m not keen on pulling the engine out to take a look at the turbos if there is a simpler explanation for this issue. I’ve not heard of many turbos going bad on these 2nd gen 958s

Thanks All

BrendonSF

How many miles on it? My next thought would be sloppy diverter valves, but it's tough to say at this point. You're correct that turbos going bad are not a common occurence. Curious what octane fuel you are running over in Dubai?

Smithydxb

Its done 90k km (so 56k miles)

it’s been run on 98 (RON), highest pump fuel available here. The lesser is 95.

Diverted valves were one of my items next up on the list.

mafpolo

The turbo on mine had a failing boost pump, so they replaced the turbo. My hesitation problem ended after that.

BrendonSF

56k miles is a bit early for diverter valves, but you're also operating in a very warm climate, so it's conceivable that it would shorten their lifespan.

Smithydxb

Update:

The Cayenne was back in the garage getting a leaking washer nozzle replaced. While the bumper was off, i got them to check the Diverter Valves, which seemed fine, but had a little oil in them (not loads, so I’m not too concerned about that). I said to replace them as they are relatively cheap anyway, but there is a 15 day back order. So they were cleaned a bit and put back on. Car has run great for about a week, no issues, even when trying to trigger the issue. Low and behold, driving this morning, in Sport mode (which may be part of the issue?), it came back. The car was c.4k+ revs, sustained (so in the higher rev ranges for a sustained period) and the boost just dropped to 0 from around 10 and we are back to needing to turn the car off and on again to get the turbo back.

Diverter valves are coming in soon so will be replaced anyway. I was thinking about replacing the MAP Sensor too as its cheap to do, but I’m not getting any sort of warnings or dash lights that would indicate that this needs replaced, i may just be running out of ideas.

The only other thing that has been suggested is that there is an actual issue with the turbos and the actuators are sticking, but i cannot find anything online about these going bad and the car has only done 56k miles and has a full service history.

I’m keen to avoid looking into this scenario till i know I’ve covered everything else as its an engine out job, just to even see if that is the problem, so could be a costly exercise to find out there’s nothing wrong with them.

I may try another Porsche Specialist here as its getting beyond my knowledge and understanding. I’ve spent hours and hours trying to find info on this happening elsewhere but i cant seem to find anything with a similar problem.

Any other suggestions welcome, I’ll keep searching and update as i go.

Versace

What ist the normal nominal boost? 15psi/1bar? I only get 0,6 bar maximum. Diverter valves probably?

PFDGSB

On the 958.2 GTS and S there's a known issue where overly sensitive knock sensors prevent the turbos from boosting after the engine is hot. There is a software fix for this.

I don't believe it's a known issue on your car, however it sounds similar to the problem on the GTS. If Porsche could replicate the issue, maybe they could see what the knock sensors are reading when the issue happens.

Also, does it seem to happen in your car only after it has been driven for an hour on the highway? That's when the issue happens on the smaller turbo cars.



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Radi

Is the car have a software tuning, you better check that. I think that is too many boost from turbo that supercharge the engine and the computer is turn off turbo. Maybe check exhaust system, is catalyst good, bad fuel my damage catalyst. Cheers

Smithydxb

15psi it will boost to normally with no issues.

Smithydxb

I found a software issue online, possibly the same one you speak of as it was captured in a bulletin and I had the reference for it. Ran it past Porsche dealer here. When I spoke to them they clearly told me there was no update for my car but took the car in for a day to do a quick check as the appointment availability was over a month. Low and behold, when they plugged it in there was an update. They wouldn’t or couldn’t tell me what the update was but hoped it would fix it (it clearly didn’t), but something tells me I showed them the same update that was relevant to the S (and GTS I suppose).

before they done this update I wouldn’t even get a dash warning and there were no codes as I had it scanned at my Indy after I managed to replicate the issue (I got Porsche to scan with the issue live too and their system didn’t pick up any codes). After they done the update I now get a yellow warning when it happens saying loss of engine power, drive in possible, contact service centre. Turn the car off and on and all goes back to normal, including the boost.

im currently contemplating replacing MAP sensor and maybe the N75 sensor (cheap options to try). Going to have a chat with my Indy tomorrow.

in terms of when it happens, completely random. I drove 10 mins to get a coffee this morning and on way back, it happened in sport mode. Then this evening i was driving in normal mode (after about 5 mins) and when the revs were sustained at c. 3k in 3rd gear, when it goes to change to 4th and boost would naturally drop it just loses all boost and throws the dash warning now.

hard acceleration with full throttle doesn’t seem to cause it which is odd.

who knows 🤦🏼‍♂️

Smithydxb

Car is 100% standard. No tune or any other performance parts. Only thing it didn’t come with was the sports exhaust but it’s the standard Porsche exhaust, not an aftermarket one. Fuel is 98 Octane and what every other high performance cars run on here. I’ve had multiple high performance cars here and never had an issue with fuel quality.

catalyst I’m not sure but it’s not had any issues with emissions testing

PFDGSB

Wouldn't the dash warning you're now getting throw a code to reference? Do you have an OBDC scanner?

Smithydxb

You would have thought so. I’ve had it checked at 2 separate places as well as the actual Porsche Service Centre and it hasn’t stored any info.

Porsche actually said, if this happened again after they had done the update, they’d need to take the car back in, plug it into their computers and let Germany look at it remotely as they dont actually know what the issue is.

don16

Coincidence that it seemed okay after they cleaned up the Dvs or maybe they are faulty/weak? Be interesting to know how it goes once replaced. Not seen much about the N75( Boost controller) being an issue on any newer Porsche's.