Engine 850: B5234T/2.3/20v/T5: Boost issues - Eliminating BCS or CBV

[sdg1970]

Originally Posted by Cookeh

Acquired a new CBV diaphragm (thanks SDG!), just wondering if there are any tips for getting at the bottom 10mm bolt on the casing? The other two seem fairly simplistic with the intercooler pipe off and heatshield removed.

Good weather here is rare currently, so I want to get all the info I can to cover all eventualities before the sun shines its rays for 30 seconds.

Yes that bolt is nasty, I remember it well. I recall when I did mine I removed something that was in the way to make it easier. Can't recall what though?? It was still very difficult, but doable. In the end I fitted the Forge Motorsports Recirc valve with the slightly stronger spring - this was after I bought the Kinugawa unit, hence I had
it leftover.

It turned out there was nowt wrong with it anyway, as it still under-boosted (chronically) after I fitted it and in the end the culprit was a split throttle body hose, DOH!!

[Cookeh]

Thanks. As it turns out the bottom bolt is stupidly easy to get at and remove from under the car. It's also easy to get at the O/S upper bolt from under the car too. Top N/S is easy enough to get to with the intercooler pipe off too.

I got the under the car tip from a local connection, and once the car was in the air swapping the CBVs took all of 30mins.

[JohnM 855 T5R]

Has replacing the TRV diaphragm fixed the low boost ok?

John

[Cookeh]

The car has lower vacuum and is less hesitant on boost, so its certainly addressed something. There were no tears in the old, but is was deteriorated slightly. It wasn't as firm or elastic, which would explain the decreased hesitancy. I'm still boosting up to 8psi and then dropping to 5psi after a second or so, though. BCS is next, I guess.

[JohnM 855 T5R]

It's good the new diaphragm improved response and vacuum - that does suggest the old one had gone hard and was on its way out which is common. It's worth replacing them every 150K miles or 10 years. Did you fit a new spring as well (green one)? The springs do age and go weak - that commonly causes pressure leakage and low boost.
You say the pressure builds to 8 psi (I think 9.5 psi is about right for a T5) then drops back quickly to 5psi. Pressure will always momentarily spike a little and drop back as the TCV "catches up" and does it's job. Assuming there are no leaks from any of the pipework or intercooler, I can only think of 3 things -

1) the turbo can't push enough air - unlikely - as if it were that worn you would also probably have other issues such as oil burn or leakage.
2) the control valve is acting up - most likely and cheap at about £35 to £40 for a Pierburg unit and easy to fix.
3) wastegate faults - possible - Basically only 3 things can really go wrong. The spring that holds the waste gate shut has gone weak so its blowing off at a lower pressure. The wastegate valve isn't seating properly and allowing blow by, or the internal diaphragm is acting up though this I would have thought would cause too much boost. A standard replacement wastegate for a TD04HL can be had through eBay for about £50. You would need to follow the adjustment procedure after installation which you can find on here or all over the net.

John

[Cookeh]

Thanks, John. Excellent information. Yes, I fitted a new spring with the diaphragm. Piping and vac lines have all been inspected - they are in good condition and correctly routed. Regarding the actuator, whilst I can't rule it out I have adjusted fairly recently and it opens at 12.5kPa like I believe it should. This is what led me down the BCS vs CBV in the first instance, but there is no harm in double checking!