Here's the update.
Just before the weekend I got the car up on (modified) ramps and wrapped the trubo to exhaust nuts in bits of old rags, which I soaked in WD-40 overnight. Glad I did this as it meant removing them the following day was reasonably easy. I don't have a dedicated 1/4" socket wrench and my 3/8" wrench isn't great so removing the nuts (especially the top, forward one) took a bit of time. Thankfully, they all came off in good order and the gasket was easy to remove in good condition. With some old wire I tied the exhaust end out of the way, hopefully without putting too much strain on the flex part. I also used an old plastic bag with some PVC tape to wrap around the driveshaft bellows and protect this as best as I could.
Following advice I made a cover to seal in the oven cleaner as best as possible (see picture). The template for this was the turbo to exhaust gasket (cleaned with a wire wheel) that I traced onto some spare, clear acrylic sheet (which is valuable stuff these days). I drilled and tapped a hole for a mate bolt into this (near the top of the void) so that I could block off the hole with the oven cleaner inside, but in practice I wish I'd made this smaller however, being a foam not a lot of the oven cleaner leaked out.
Before fitting the cover I checked free rotation of the turbo and could not detect any slop or side play. The cover was fixed in place with just two nuts (the easier two to get to).
Once PPE'd up (gloves, long sleeves and a face mask) I put an old WD-40 'smart straw' onto the oven cleaner can and gave it a shake then squirted into the turbo. After letting it soak I tried the turbo actuator and gradually, with adding more oven cleaner and more wiggling I started to make progress. In hindsight I think the whole thing was completely gummed up. I found the best way to wiggle the actuator was with a length of cane, vertically from above.
It took quite a few hours and re-applications to get anywhere, but I felt I'd got as much movement as there would be towards the end of the day and so put the car back together, connected up my code reader/logger and took the car for a blast.
First impressions were that there was just so much more low-down oomph, and the car picked up so much better, but very annoyingly it was actually easier to trigger the car into limp mode. Looking through the fault code data I still got:
17965 - Charge Pressure Control. P1557 - 35-10 - Positive Deviation – Intermittent.
And looking at the data there was still underboost when little boost was called for and significant overboost when a lot of boost was asked for. At extreme, I was able to get 2983.5 mBar when 2346 mBar was asked for. To drill into these data more fully I’d need to chunk the data into smaller bits, but I decided to spend the time taking the car apart again as it definitely was not fixed.
During the second go, I used a bit more squirty and more wiggling, but the setup was the same. On the second go I was soon able to ‘feel’ a much more definite end-stop for the sprung-loaded actuator, when previously it had felt rather mushy and the foam was much darker after a bit of time.
In hindsight I might have been able to avoid the second clean if I’d taken the opportunity to wash out the gunk every so often as advised above and this is probably a valuable take-away message for the future. I can’t see this being the only time I have to do this, although this is my first time in a lot of miles with this car.
Hopefully getting the positive end-stop is a positive result and I again took the car for a blast. This time I got even more low-down oomph and could not (did not) trigger limp mode. The max observed boost wasn’t much lower at 2913.3 mBar when 2346 mBar (max) was requested but this was during a very long acceleration uphill on an ‘on-ramp’. I was overtaken (being stationary) off the line by a car that judged the lights better and didn’t have to stop and my car worked very hard to get past this other car up the hill and so boost was sustained for an extended period (see graph). Sadly, the dual carriageway was closed due to an accident about ~halfway to my planned ‘destination’ very long steep hill and as it took ages to filter off my logging laptop might not have had the battery to get the distance so I cut the journey short and returned. Luckily I tackled the same other places where I was able to trigger limp mode after the first cleaning and could not after the second go.
Due to a change I did not have to make the ‘test run’ journey to the big hill during the day today and so this remains uncertain.
In conclusion, I am hopeful that after two ‘cleans’ the max boost observed may not be too far above the expected values but looking at the comparison graph it’s not 100% clear from these plots. I’ll have to keep an eye on this and see what happens in the coming weeks/months. I’ll also see what happens when towing a trailer, which was another reason for wanting to get to the bottom of the current problem. All this work takes quite a bit of time and effort to prepare so might think about using the Revive product in the future. Will see.
Many thanks to all for your help. In this ‘fix’ I’ve discovered two other issues and a further query and I’ll post these in the right places, hopefully.
Side note: decent socket wrenches have 72 teeth giving a 5 degree throw between clicks. This would make life much more simple to get the difficult exhaust nuts free. I recommend an overnight soak for these nuts and glad I spent the time in making a cover to the exposed turbo.
Further: Part of the prep time was to sort out some car ramps. I was given some decent ones many years ago but these remained unused as they are too steep for the car to get up them without removing the valance and even then they had a habit of skating across the garage floor. To fix the steepness I built some ramp extenders, which are a bit like the Seeley ones, but are longer (at 750mm, but just because I had two pieces of 1500mm angle to hand). Being longer I was also able to build the ramps up with some flat pieces of OSB screwed together from both sides to which the ramps were also screwed. Might sound shonky but take a look at the picture and see what you think. It’s provided 25cm of lift, which is very useful while under the car. I also painted where this all sits onto the garage floor to aid future setup
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