RAV4.2 D4D 2l - Losing power at higher revs/speeds - EGR? SCV?

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Hi, I've had my 2005 Rav4 XT5 diesel for a couple of years now and it seems to have developed a fault. It started when I got an engine light on and limp mode kicked in over a year ago, but my mechanic told me just to put better fuel in and it had only ever reared it's head a couple more times when I'd skanked it for a while and put supermarket fuel in the engine. 

More recently it's been accompanied by hesitation when accellerating at higher speeds. Basically it seems to act normally up to about 4th gear, by which point if I hit over 2.5k revs I lose power, with the occasional intermittent burst if I hold my foot down. If I'm going uphill the issue is exasperated and occurs at slightly lower revs, and downhill I'm generally able to get it up to speed with little issue. Overtaking on motorways however feels like a gamble and overtaking on A roads is neither safe nor feasible! The issue started a while back and wasn't that noticeable but in recent weeks it's become much worse.

There doesn't appear to be any smoke at the back (not that I've seen anyway), and the engine faults (also lights up TRK VSC when it happens) are clearing themselves after a couple of days and apparently leaving no fault codes behind. 

I'm aware similar issues have been well documented on here but I also see many people put a lot of money into their vehicles and not necessarily getting results. I have changed the fuel and air filter since this was due anyway, but since the issue persists and there's no codes being thrown out by the engine faults the mechanic says the best bet is to get a diesel specialist in to look at it - and this dude costs £60 for the first half hour and another £60 for every hour after that - don't really fancy dropping a lot of money I don't have just to confirm it's the thing I thought it was anyway! 

I'm seeing a lot of people recommending cleaning EGR valve as a quick potential fix, and also replacing the SCV valves appears to have been met with more success than not. Any guides out there on cleaning the EGR correctly, and any easy way to test the SCVs to see if one or both of them is the problem? 

One other thing to note is the engine faults appear to occur at high altitudes - not sure if that's a coincidence but 3 of them occured on the same stretch of road near Bradford airport at the top of a large hill, and the final time occurred driving up near Tan Hill (highest pub in britain). Probably nothing that, just something I observed is all.

[Devon Aygo]

What is the fault code stored in the Engine ECU after the light comes on ?

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As I said, no fault codes present. Light comes on but both times the mechanic checked there was no fault codes.

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As an update finally got more faults and the car started having the same hesitation issues at lower revs too - fault code was P0627 though so this indicated SCV.

Got mechanic to put some new Denso ones in place and the car was great.. for about an hour when halfway up the A1 just lost all acceleration. Had my foot to the floor and could barely coax it past 60mph. Unlike the previous issue it now accellerates extremely slowly all the time in all gears. It pretty much won't exceed 2000rpm. No engine light. No fault codes. If anyone has any ideas would be appreciated, stuck up north now so need to find a good mechanic to diagnose it.. anyone with the same issue might tell us where to look first though!

EDIT - Turns out the SCVs were indeed fine after replacement but when I looked under the bonnet the large pipe from the intercooler had come loose at the bottom! Has to be removed to get to the SCVs so guessing the mechanic hadn't put it back on tight enough, also there was some resudiue which I'm guessing was from the carb cleaner I sprayed into the airflow and EGR systems so cleaned the edges of the pipes up and affixed them nice and solidly, drives like a dream now.