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MK4 Rough idle/running


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So a mere week into owning my MK4 Invincible, it’s not living up to its name. Despite covering almost 1,000 trouble free miles in 2 days, it appears I have angered the motoring gods.

Coming back with the kids in the car and coming off a roundabout down a hill, it almost felt like the TC kicked in for a moment, since that point it’s been gutless. No smoke, no change in noise/note, this is followed by rough idle, a reluctance to accelerate, when it does (and it will run higher RPM’s) it doesn’t like pulling away from a standing start or going over 2,000 rpm. It’s as if it’s running in safe mode, but no EML etc. I can feel it try to spin up the turbo, but nothing happens but a stutter. The only code logged (my VCI lead is AWOL, so no TIS for me just yet) is P0101 Mass or Volume Air Flow ‘A’ Circuit Range/Performance which initially showed pending when read and I have managed to provoke the TC/EML at one point. I’ve cleaned the MAF, EGR and pipe work and no joy, a replacement MAF has been ordered, but have I missed something obvious? I realise it could be turbo related (83k, FSH), and suppose it could also be a hose leak etc. but any suggestions are welcome.

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So, quick update, MAF wasn’t the issue, or at least it wasn’t the main issue. I remember a slight smell while sat at idle which combined with the TC/EML light has me wondering about the DPF side of things, though given the car did 2,000 trouble free miles in a little over 10 days, I would hope the DPF was pretty clean. Still need to check pipe work for vac/boost leaks and waiting in a new VCI lead to arrive.

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So after thinking the worst, it looks like it’s a lot simpler than I suspected, lower boost hose on FMIC had come loose just enough to cause issues. You can’t tell because it’s partially obscured by the coolant hose above it and it still sits where it should. Now I just need the block to cool down enough to get the old clip off and replace it with something more substantial and ideally stainless.

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Before I start unbolting the front sub frame to access the hose and it’s rubber insert (this is something of a novelty coming from the 4.2 with everything on top), anyone have any experience doing this? It looks the easier route vs pulling the entire front end apart to take the radiator and intercooler out and have to do the coolant as well. Probably should have ordered that Haynes manual and paid for the ToyotaTech access, but where’s the fun in that?

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For anyone else faced with this, the process is as follows:

Undo under tray push pins, remove, or at least let hang down so you can get easy access.

Remove 2 longer and 2 smaller nuts that bolt the sub frame to the crash bar and the support legs on each side, and one of the bolts on the support leg of one side - this is enough to allow it to pivot so the sub frame can be removed more easily.

Undo the centre bolt on the sub frame and the two hidden bolts that secure the engine mount - always a good time to see if it’s OK and replace if required. Sub frame should now be free, but when sliding forward will catch on the crash bar.

Slide support leg towards wheel, this should allow enough room for access - if you have small hands - or the sub frame to drop down... obviously don’t be under it.

You now have access to the intercooler pipe work. It goes without saying, but make sure both sides are right while you are at it. The pipe itself is two parts, a lipped rubber insert that sits inside an outer plastic tube, in my case the outer lip of the hard plastic tube had deformed slightly and I needed to cut it back slightly, if you need to do this try and avoid cutting as much as possible and don’t cut the ridge the lip sits against if possible. I removed the rear sensor plug and twisted it round to get better access, but didn’t mess with the sensor line. When I have future issues with boost, that vac line T is going to be the second place I look as it screams weak point. Once you have reinstated the IC, replaced sensors and snapped the small black plastic cable guide as it refuses to come out and tightened the jubilees to a point you’re happy with, in true Haynes style, refitting is the reverse of removal. Given the bolts are alloy, I wouldn’t be copper slipping them and the sub frame bolts were loc-tite red from the factory by the look of it.

Road test to confirm normal operation is restored.

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