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

[Cookeh]

Hiya,

My T5 is showing signs of low boosting, going building boost until roughly halfway into the boost guage before dropping back to one third and holding there throughout the rev range. This is, of course, lower than it should be.

I have checked wastegate adjustment, all vacuum lines and their routing. All I have left to check is the BCS or the CBV. Unfortunately my symptoms match failure points for both items, so I am wondering if anyone knows of a method of testing either that will help in eliminating one suspect?

I know there is the option of unplugging vac line to wastegate and going for a drive, with the idea being if you have full boost back it means your BCS is bad. Frankly, I think this is a truly stupid method and I will not risk potential overboosting or running max boost (15psi!) throughout the full rev range.

Cheers chaps.

[JohnM 855 T5R]

Wastegate adjustment is almost never the cause of low boost. I think I'm correct in saying that a faulty wastegate normally results in overboost if its stuck shut or underboost if its stuck open. If the wastegate actuating arm is moving fairly freely (there will be spring pressure). it's unlikely to be there.
The TCV (or BCS) could well be the problem, though when they fail, I gather overboost is more common than underboost. A replacement Pierburg unit is about £38 from "Parts for Volvos"
A faulty TRC (or recirculating valve) is a very common problem (and more likely), that gives rise to underboost. This is normally caused by a split or damaged diaphragm. You can get a spring and diaphragm repair kit for about £20 online which can be fitted in situ. It's a bit of a fiddle but just remove the 3 screws holding the cover down, whip out the old spring and diaphragm, make sure all surfaces are clean and pop the new diaphragm and spring in and replace the cover. Robert has this on video in Youtube. If you want to splash the cash, you can get a Forge piston type TRV which is very robust for about £150 from Forge direct or Parts for Volvos.

John

[DSK]

Your issue sounds similar to what I had several years ago, well more like over a decade and yes still got the same car.

Floor it, it starts to build, noise a bit louder but, then as it builds, it lost almost all boost pressure. Volvo diagnosed it down to a TCV at the time but, when I picked the car up, before leaving the car park it didn't feel right so f*ck know what else they ****ed around with.

I had another issue on the same car after fitting an IPD recirc valve trying to fix what ever Volvo f*cked up. The original at 150,000 miles was perfect but, since I had a new IPD one to hand, I dropped it in. However, that split in 2 weeks causing an issue of minimal boost, it never built up much.

I would suggest a TCV anyway as its good to replace these routinely. Secondly I would suggest a smoke test to see if you have any leaks you can't see, such as a faulty MAF>Turbo which was only easily spotted via a smoke test.

[Cookeh]

Thanks both. I think we're on the same page regarding suspects (just under different names!) and I appreciate the insight. I'm not one to load a parts cannon, so I'd rather try and diagnose which is faulty first; to that end, does anyone know of any tests for either part?

I was wondering if applying vacuum and/or pressure to the CBV port would work? The assumption being that if it holds vacuum then it can't be holed?

Smoke test for unseen leaks is an excellent suggestion, will have to have a look into how to rig up a DIY option. I have often wondered about leaks around the PCT and vac tree.

[PNuT]

My money is on the recirculation valve.....

[JohnM 855 T5R]

Originally Posted by PNuT

My money is on the recirculation valve.....

I agree - not the most robust things. age takes it's toll on both the rubber and the spring tension. Easy and cheap to fix with a diaphragm and spring kit

[Cookeh]

Is this the correct P/N, for future use? Kinugawa K5T09678. Perhaps you could share the itme you bought, John, as I can only find Kinugawa replacements!

[JohnM 855 T5R]

I actually bought the Forge piston type unit at £150.

The Kinugawa K5T09678 is correct - this is for all Mitsubishi TD04 turbos (which is what you have).

A whole replacement Kinugawa unit is K5T09671 and surprisingly doesn't cost much more.

Regards
John

[Cookeh]

Thanks for the confirmation, John, much appreciated.

[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.