850 T5R momentary cut out under hard acceleration
 

Any help will be greatly appreciated!
Much already done recently when she wouldn't start at all or ran very badly including:
- New MAF sensor
- All new vacuum hoses
- New distributor cap and rotor arm
- New plugs and leads
- New O2 sensor (Bosch) – there’s only one pre-cat
- New fuel pump
- Checked fuel pressure now and c. 40 psi – ok
- Cleaned and serviced injectors (new seals and filters)
- Changed oil, filter and air filter
- new crankshaft position sensor
- new camshaft position sensor
- new radiator
She runs really well now and the problem manifests only under hard acceleration in sport mode when I kick down, revs are above, say 3500 to 4000. It's like I have hit the brakes - it does it 2 or 3 times, the gearbox then shifts up and revs drop and all is good again with strong turbo boost.
Checked the codes and I have got:
A3 (ABS)
142 Faulty brake pedal switch, open or short
424 Pressure switch for TRACS, faulty or short circuit

A7 (Combined Instrument (Trip Computer))
122 Temp. sensor interval too long

The A3 codes may be historic from when I took off and cleaned the brake pedal switch during re-commissioning as it was faulty. Could there be a link or are these faults probable 'red herrings'?

As I said, any help will be greatly appreciated (and apologies for the long post!

Pete

try a new fuel filter.

Thanks Simmy, already done - I should have mentioned that!

Are you seeing a warning lamp flash up in the instrument panel when it happens? Could a dodgy pressure switch be triggering the TRACS and cutting fuel supply?

Had this with my 850 and it was the turbo control valve. The car was cutting the fuel to prevent overboost due to the faulty valve. Fitted a new one and all was well again.

Good call, Dancake, was going to suggest the TCV once the TRACS issue is eliminated.

Thanks Baffler.
No, I don't see any warning light. I must admit I didn't know that the TRACS system cuts the fuel but if the brake light switch was playing up originally then maybe it's still flaky and worth pursuing.

Thanks Dancake. I had hoped it wasn't going to be the TCV! I considered replacing it when I was trying to diagnose the starting, rough running issues but it was going to be one of the last things I changed due to price - £152 from PFS https://www.partsforvolvosonline.com...b7f3c152f76652.

I'm nearly sure I've got an original one sitting about somewhere that you could try. I never tested it so can't comment on whether or not it works properly, but it could be worth a shot. Let me know if you're interested and I'll try to find it if so.

Dancake, you're a star for that thought. Thanks. now that I have cleared the fault codes I am going to give it another test run also then with the TRACS turned off. If there is no improvement I may well come back and take you up on your kind offer.