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700/900 Series General Forum for the Volvo 740, 760, 780, 940, 960 & S/V90 cars |
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Boost gaugeViews : 595 Replies : 6Users Viewing This Thread : |
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Oct 31st, 2023, 10:06 | #1 |
Rogerthechorister
Last Online: Dec 16th, 2023 02:15
Join Date: May 2006
Location: Rochester
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Boost gauge
Suddenly the boost gauge on my Sport edition with turbo+ is under-reading. Plenty of boost judging by power but hardly showing on gauge. Suggestions please.
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B234F estate, H reg 100k miles. RIP melted B280E. 760T estate, F reg 133k miles. 940 Sport M90 estate, N reg 170k miles. 940 Sport M90 estate, N reg 100k miles, ex Lovejoy. 960 Estate, N reg, 56k miles, blown up and sold. |
Oct 31st, 2023, 10:21 | #2 |
VOC Member
Last Online: Today 01:48
Join Date: Feb 2008
Location: Gloucester
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Leak or blockage in the hose that goes to it? Probably easiest to start at the manifold end and see how the gauge responds to pulling a vacuum or applying pressure to the tube. Stick something suitable in the brass nipple to check it’s not blocked too.
It’s not all that hard on a 940 to pull the cluster and check that end if necessary. You might be able to see what’s going on just by removing the driver’s kick panel. If it is a faulty gauge it’s fairly easy to swap instruments between clusters if you can find or have got a spare. |
Oct 31st, 2023, 15:42 | #3 |
Grumpy Old Git
Last Online: Apr 19th, 2024 09:16
Join Date: Dec 2004
Location: South Yorkshire
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IIRC the rigid vacuum line that comes off the manifold goes into a rubber type pipe at the back of the cluster - maybe there is a split in that? Otherwise just check for a vacuum leak in the usual places.
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Nov 2nd, 2023, 09:21 | #4 |
Rogerthechorister
Last Online: Dec 16th, 2023 02:15
Join Date: May 2006
Location: Rochester
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Thinks
Surely the oddity is that the gauge shows little boost (works OK off-boost) but the power in the engine (on my butt-dyno) says plenty boost.
__________________
B234F estate, H reg 100k miles. RIP melted B280E. 760T estate, F reg 133k miles. 940 Sport M90 estate, N reg 170k miles. 940 Sport M90 estate, N reg 100k miles, ex Lovejoy. 960 Estate, N reg, 56k miles, blown up and sold. |
Nov 2nd, 2023, 12:04 | #5 | |
Senior Member
Last Online: Yesterday 23:09
Join Date: Aug 2006
Location: Cannington
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Quote:
Under vacuum the split closes up. Under low boost the split opens a touch but still shows some pressure. Under more boost the split opens up further and acts a bit like a bleed valve for the gauge. |
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Nov 3rd, 2023, 15:32 | #6 |
Rogerthechorister
Last Online: Dec 16th, 2023 02:15
Join Date: May 2006
Location: Rochester
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But
But surely that would also reduce boost to or from the turbo+ so also reducing actual boost from the turbo.
__________________
B234F estate, H reg 100k miles. RIP melted B280E. 760T estate, F reg 133k miles. 940 Sport M90 estate, N reg 170k miles. 940 Sport M90 estate, N reg 100k miles, ex Lovejoy. 960 Estate, N reg, 56k miles, blown up and sold. |
Nov 3rd, 2023, 22:16 | #7 |
Senior Member
Last Online: Yesterday 23:09
Join Date: Aug 2006
Location: Cannington
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If anything, dependent on how the pipes are routed to the ECU, the opposite could be true and you could even be overboosting but not hitting the ignition/fuel cutout because the ECU isn't seeing it.
Fun, until something melts. I've just skimmed over the installation manual for the Turbo+ and it looks to me like its just an electronically controlled bleed valve installed between the compressor and the wastegate that is activated at certain times. That leaks off air deliberately in order to increase boost over stock. If you put a gauge either side of the Turbo+ valve, the the one after the valve would show a lower pressure when the Turbo+ is engaged but the actual boost is higher than without the 'leak'. You've got three options: 1) Your gauge is faulty. 2) You have a leak in the line to the guage (and possibly the ECU too). 3) You are really only making a tiny bit of boost. Since you say the car can still move iout of its own light, my money is on 1 or 2. |
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