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 and filter
- 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

 

I'd suggest checking the turbocharger, wastegate, and hoses for any leaks or blockages. Make sure the wastegate is working right and not sticking.
Also, a faulty TPS or throttle body could potentially cause power loss.
Plus, check for any restrictions in the fuel lines or delivery system.

 

Thanks Niklas.
All the vacuum hoses are new and the throttle body was thoroughly cleaned when the inlet manifold was off to change the PCV.
I am hoping that the fuel delivery system is good too after all the work I did on that. It certainly improved the pressure and I checked the injectors were working correctly too.
That does still leave your suggestions that it might be turbo, wastegate or TCP though. The wastegate actuator doesn't appear to be sticking but that does not mean it isn't!
I will investigate and thanks again for the reply.

 

A quick update. I have just taken her out again and with the fault codes cleared and TRACS off, there is no change, the quite violent hesitation occurs around 4500-5000 rpm in sport mode and seemingly just as it wants to change up a gear. No issue when stationary - she winds up to 6k without an issue. When cooled down I will check the wastegate actuator - it does seem a little stiff.

 

I forgot to mention that immediately after switching on the engine the ABS warning light came on and when I got back after the test drive and checked the codes the 424 ('Pressure switch for TRACS, faulty or short circuit') was back!

 

Well, if the code came back and your ABS warning light came on, it sounds like you may have a failing ABS control unit. This has been known to occur on 850's and early S70's. Do a forum search on it and it should point you in the right direction. I remember we had a similar issue years ago on our old 95 850 Turbo, and we got the ABS unit rebuilt and the problem went away.

 

Thank you. So the failing ABS module caused the severe hesitation / jerking on hard acceleration at 3500-4000+ RPM? Interesting. Another forum member is letting me have a TCV to try so I hope to fit that this week but I will look at that if I don't solve it with the TCV. Thanks again.

 

Not a chance the ABS is related. I'm not sure why anybody responded to that. It's not related.

Sounds to me like you're hitting fuel cutoff. The 850's don't have a pressure measurement so I have always assumed that overboost fuel cutoff is really a too-much-air-flow measurement. How much is too much depends on the engine speed. You can decide whether or not you are truly overboosting. Fuel cutoff will lower the boost, and then when it changes gears then that solves the immediate problem.

On my car I have experienced that you might get a hole out there and the leak contributes to the air flow. In that case you'll hit cutoff while the pressure is actually okay. A hole in the idle hoses will open and close based on pressure and create a range of behaviors. Like it'll close of when you're not boosting.

 

also if you suspect the ABS controller, you can google Midwest ABS and Xemodex for some content on typical symptoms etc. You should also be able to remove the ABS controller without opening the system and try a used replacement part (as they are not paired to the ECU). Xemodex will also rebuild your unit as well.

 

I renewed all of my vacuum hoses but that doesn't mean that it isn't a vacuum issue as I couldn't 100% guarantee that very join and every fitting is absolutely sealed. Not least because the diameters of the replacement hoses were close to the originals but not absolutely the same! That said, I would have thought that it wouldn't pull like a train up to the cutoff moment.
I'll check the hoses again and fit the TCV.
One thing you have prompted me to think about by mentioning air flow issues and a nagging doubt I have had is that I put on a new, but non-Bosch, MAF sensor in amongst all the work I did previously. Maybe that was never the problem and I should try the original back on again!
You've been a great help and many thanks.
Pete

 

I will do that mt6127, many thanks.