dump valve issue
23 April 2008, 08:50 PM
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dump valve issue
Please can you help with my expensive dump valve problem.
My car 2005 impreza started stalling and back firing, it was towed to subaru who after a week said it was the Forge motorsports dump valve which the diaphram had gone too soft allowing the valve to dump too easy and caused the car to over fuel flooding the engine. They then said as it was a non genuine part the warranty was void and i owed £1,600 for labour and testing after further discussion this was reduced to £578.
Can anyone tell me:
1. If this could happen to the engine because of a dump valve?
2. If any one else has had this problem?
3. Any Ideas on what to do?
Please help.
My car 2005 impreza started stalling and back firing, it was towed to subaru who after a week said it was the Forge motorsports dump valve which the diaphram had gone too soft allowing the valve to dump too easy and caused the car to over fuel flooding the engine. They then said as it was a non genuine part the warranty was void and i owed £1,600 for labour and testing after further discussion this was reduced to £578.
Can anyone tell me:
1. If this could happen to the engine because of a dump valve?
2. If any one else has had this problem?
3. Any Ideas on what to do?
Please help.
23 April 2008, 09:11 PM
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here are some wise words from Andy F 
There are a number of issues to consider when using a VTA or blanking off the OE recirc valve.
VTA (and recirc) DV's come in a range of opening pressure settings. Whether high or low pressure, with a VTA there can be issues.
1 - Overfueling due to excessive venting to atmosphere
2 - Stalling the turbo
3 - MAF revertions and System volume
4 - Repressurisation time
5 - High pressure damage
Looking at the issues one at a time,
1 - Overfueling due to excessive venting to atmosphere
If you run a standard pressure vent to atmosphere dump valve (BOV, VTA, Atmo DV) with a mass air flow monitoring ECU such as the OE Subaru, then you can have problems when the DV opens under cruise conditions. The DV allows air to escape from the system during the high vacuum conditions during cruise, The reason for this is the signal side of the DV is subject to vacuum and the flow side is subject to slight boost pressure (the difference across the throttle valve) If this exceeds the spring pressure then the valve starts to vent.
This airflow has been monitored by the ECU and fuel is added accordingly, the result is too much fuel for the air being consumed by the engine.
Best case - You just waste fuel but who cares when you get that 'pschhhht' you've always wanted
Worst case - You cause bore wash, increase oil consumption and reduce engine life.
2 - Stalling the turbo
The term turbo stall refers to stalling the airflow across the compressor blades, not physically stalling the rotor (common misconception)
This is a case of what happens if you don't use any DV or use a valve with too strong a spring.
When you lift off the throttle on full boost, the compressed air has no where to go and blasts back across the blades giving that (lovely ) sharp 'cacho' or WRC chirp as some call it. Very taxing for the turbo, all this backwards flow.
Best case - Runs forever (TD series generally tougher than VF series)
Worst case - Shatters compressor wheel or thrust bearing fails
3 - MAF revertions and system volume
This is not such an issue with a TMIC but when FMIC's are used the system volume is so large that when the throttle is brought back from power (boost) to cruise, if a strong spring DV is used (strong enough to eliminate issue 1 ) then you can get a sudden pulse of pressurised air back out of the MAF, this is measured as an airflow (the meter can't differentiate) and the engine overfuels giving a hicup/jolt
Best case - Slightly jerky drive as you come off throttle
Worst case - Very jerky !
4 - Repressurisation time
This is where the stiff spring DV or running without DV scores a point. The system holds more pressure between gearshifts as the DV isn't venting it. Boost recovery in the next gear is improved (slightly)
Best case - Improved response after a gearshift
Worst case - Issues 2 and 3 above.
5 - High pressure damage
If you use no DV or a high pressure DV spring then when the turbo is spinning at high speed and the throttle valve is snapped shut, there is a pressure pulse in the system
which can be high enough to damage the intercooler etc.
Best case - At boost below 1.5 bar you will probably be ok
Worst case - Bigger turbo's/ higher boost may pop hoses off or 'balloon' the intercooler core.
Options ?
A light pressure recirc valve (just like std) overcomes all of the above issues but sounds a bit boring
Sequential dump valve. At light pressure it recircs, at high pressure lift off it still goes Pssshhht
You pays your money, you takes your chances
There are a number of issues to consider when using a VTA or blanking off the OE recirc valve.
VTA (and recirc) DV's come in a range of opening pressure settings. Whether high or low pressure, with a VTA there can be issues.
1 - Overfueling due to excessive venting to atmosphere
2 - Stalling the turbo
3 - MAF revertions and System volume
4 - Repressurisation time
5 - High pressure damage
Looking at the issues one at a time,
1 - Overfueling due to excessive venting to atmosphere
If you run a standard pressure vent to atmosphere dump valve (BOV, VTA, Atmo DV) with a mass air flow monitoring ECU such as the OE Subaru, then you can have problems when the DV opens under cruise conditions. The DV allows air to escape from the system during the high vacuum conditions during cruise, The reason for this is the signal side of the DV is subject to vacuum and the flow side is subject to slight boost pressure (the difference across the throttle valve) If this exceeds the spring pressure then the valve starts to vent.
This airflow has been monitored by the ECU and fuel is added accordingly, the result is too much fuel for the air being consumed by the engine.
Best case - You just waste fuel but who cares when you get that 'pschhhht' you've always wanted
Worst case - You cause bore wash, increase oil consumption and reduce engine life.
2 - Stalling the turbo
The term turbo stall refers to stalling the airflow across the compressor blades, not physically stalling the rotor (common misconception)
This is a case of what happens if you don't use any DV or use a valve with too strong a spring.
When you lift off the throttle on full boost, the compressed air has no where to go and blasts back across the blades giving that (lovely ) sharp 'cacho' or WRC chirp as some call it. Very taxing for the turbo, all this backwards flow.
Best case - Runs forever (TD series generally tougher than VF series)
Worst case - Shatters compressor wheel or thrust bearing fails
3 - MAF revertions and system volume
This is not such an issue with a TMIC but when FMIC's are used the system volume is so large that when the throttle is brought back from power (boost) to cruise, if a strong spring DV is used (strong enough to eliminate issue 1 ) then you can get a sudden pulse of pressurised air back out of the MAF, this is measured as an airflow (the meter can't differentiate) and the engine overfuels giving a hicup/jolt
Best case - Slightly jerky drive as you come off throttle
Worst case - Very jerky !
4 - Repressurisation time
This is where the stiff spring DV or running without DV scores a point. The system holds more pressure between gearshifts as the DV isn't venting it. Boost recovery in the next gear is improved (slightly)
Best case - Improved response after a gearshift
Worst case - Issues 2 and 3 above.
5 - High pressure damage
If you use no DV or a high pressure DV spring then when the turbo is spinning at high speed and the throttle valve is snapped shut, there is a pressure pulse in the system
which can be high enough to damage the intercooler etc.
Best case - At boost below 1.5 bar you will probably be ok
Worst case - Bigger turbo's/ higher boost may pop hoses off or 'balloon' the intercooler core.
Options ?
A light pressure recirc valve (just like std) overcomes all of the above issues but sounds a bit boring
Sequential dump valve. At light pressure it recircs, at high pressure lift off it still goes Pssshhht
You pays your money, you takes your chances
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