850 cuts out on hard acceleration

I have an 850 AWD estate, which drives fine in all cicumstances ..except under hard acceleration.

Things go well until about 4000rpm when the engine just dies, but restarts as the rpm falls again.
Any suggestions on cause are wellcome.

I would check the ignition and fueling on your car. It could be a fuel surge problem (but unlikely with an injected engine) or more probably ignition. When were the dizzy cap, rotor arm and leads last changed?

Thanks Dave,
Dizzy cap and rotor have been changed, that was my first thought too. Not changed the leads but they are probably original 86,000 miles and 6yrs old though they look ok.

[Simon Linton]

Try running gently up to 4K revs - it could be that the rev limiter is set wrong and check the handbook to see if AWD vehicles have a low rev limit to preserve their transmissions.

You're I'm sure aware that tyres are a very sensitive point on AWD cars - search the 850 and Performance forums for the many posts on this subject.

Thanks Simon,
Checked handbook but no restriction on rpm mentioned.
Tyre size difference problems I'm aware of, but the roads around here are so bad the tyres don't get a chance to wear, damage much more likely cause for replacement . Tyres all within 2mm.
Running slowly up to 4000rpm and beyond can be done. Even when accelerating hard this can be achieved on occaision, after the "cutting out" has been experienced several times the extent of the fault seems to lessen.

Gmurr.

The fact that the car actually dies like you mentioned means thatyou don't have a fuel problem. If it was a fuel/air restriction you would lose power and the cut-off wouldn't be instant. My guess would be electrical. Possibly a AFM, TPS, etc.

Thanks JG,
Will investigate, presumably onboard diagnostics should come up with something if it's electrical.
Cut off, when it happens, is instant. Full power ..nothing..power..nothing on..off.... and so on as the rpm rises then falls. Alarming when overtaking!

Gmurr.

My T5 Estate does the same when its 'over-boosting'.

Back off the throttle and feed the revs up gently and it does not do it. Has anybody been playing with the waistegate ??

Just a thought.

[Chris_Rogers]

I agree it seems that ECU has detected overboost. DTC should be logged, look for air leaks,bleed valve or wrongly adjusted wastegate actuator.