Marco's 2003 V70 T5 journey Topic is solved

[Marcobrick]

Well folks, it’s been a while since I’ve posted any progress so I guess it’s time for a catch up. I have a good excuse though. We’ve been very busy with garden and landscaping projects with Spring being upon us.

I took a punt on the boost pressure sensor not giving the correct information and it looks like I was on to something. I fitted a new one, went for a drive and... WOOOOOAAAHHHH! the boost is back in town!

There is still slight jerkiness at low throttle though, but in BREAKING NEWS it appears that I have a leaking brake booster! Sigh. This could be contributing to the jerky herky? Thoughts?

[Marcobrick]

I have done some more investigating.

I ordered a mechanical boost gauge some time ago and this morning I have fitted it (temporarily)
Idle vacuum (hot) 16in hg
Removed rubber tail from brake booster and capped hard line, idle vacuum 18in. We have a leak (booster is leaking) The joint between the rubber hose and the plastic plug from the booster was quite loose. I tightened the oetiker clamp as much as I could and pressure tested it under water with my Mityvac (everyone should have one) and no more leak. I also tested the hard lines and the check valve and all is well, no leaks, check valve works perfectly.
Reconnected the booster, and idle vacuum still 18 in. Win! ( booster still leaks down after 2 minutes though, so sad face)

I am confident the booster vacuum leak is not significant enough from an engine vacuum point of view to cause any running issues.

Right, now let’s see how much boost we’re actually making.. turns out quite a LOT!
Under moderate load (careful throttle application) it builds to and holds at 11-12psi
Floor it and it goes to 15 psi and sloooooowly comes back to 11
Nail the throttle and pin it and it goes over 20 and doesn’t come back!!

Am I getting enough wastegate travel? Let’s find out. Bridge the yellow and red TCV lines and you have full boost straight to the wastegate actuator. Test drive again and maximum boost tops out at 9 psi, so no problem there me thinks. That leaves the TCV. It’s brand new. But wait, I have a spare. Nope same massive overboost.

Help me out here please... In the past, I have experienced failing TCV on a first gen V70 and older BCS on one of my 850’s. This tended to result in a boost deficit, not a surplus, and that seems to be what others report also.
What is going on?

[Marcobrick]

Today I thought, what if the wastegate initial pressure is wrong? I have quadruple checked this but it’s easy to do. It’s still set at 4.5 psi exactly. What if I do something rash, like lengthen the rod 3 full turns? Well, 3 turns takes the initial pressure to exactly zero.
Predictably that made it a bit more sluggish off the line but it still overboosts the same as before.

I didi a few more test runs and it’s really strange. Pulling in third gear it holds 11psi holding, holding, holding and then suddenly as if the car’s brain says “oh, I give up!” there’s a great rush as the boost runs away.

[abscate]

Yikes. The only thing stopping you from blowing a rod is the CBV. I hope that is working well.

[Marcobrick]

Yikes. The only thing stopping you from blowing a rod is the CBV. I hope that is working well.

Brand new, fitted recently, but you never know. Actually, maybe it’s not working, because I don’t think it’s meant to hold 20+ psi (stock part)

[RickHaleParker]

I don’t think it’s meant to hold 20+ psi (stock part)

B5234T3 Maximum boost pressure: 184 ± 10 kPa @ 5100 RPM. (26.687 ± 1.450 PSI)

[Marcobrick]

B5234T3 Maximum boost pressure: 184 ± 10 kPa @ 5100 RPM. (26.687 ± 1.450 PSI)

Is that absolute manifold pressure? It would make sense because that would equal about 12psi of boost

[abscate]

I think that is absolute pressure as you note. Good catch.

[RickHaleParker]

Is that absolute manifold pressure? It would make sense because that would equal about 12psi of boost

It does not say atmospheric or absolute. However 27 absolute would correlate with observed 10 - 12 atmospheric.

[abscate]

Its a quirk of Volvo that everything is measure in absolute pressure in software and diagnostics, isnt it?