Part Throttle surge at 0 PSI?
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Part Throttle surge at 0 PSI?
I've got a strange part throttle surge that only happens at 0 PSI (well, -1 to +1 PSI) at moderate load, part throttle. Any ideas??? I've never had the A/F checked at those conditions, but I'm running fine at WOT.
Vortech kit on a 92 LT1.
Thanks...
Vortech kit on a 92 LT1.
Thanks...
Race Director
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From: Honolulu, HI
Originally Posted by ztrips
I've got a strange part throttle surge that only happens at 0 PSI (well, -1 to +1 PSI) at moderate load, part throttle. Any ideas??? I've never had the A/F checked at those conditions, but I'm running fine at WOT.
Vortech kit on a 92 LT1.
Thanks...
Vortech kit on a 92 LT1.
Thanks...
Race Director
Joined: Feb 2002
Posts: 11,268
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From: Honolulu, HI
Originally Posted by ztrips
Uhhh... 12 years old. Don't tell me it's the opti... Please, please, don't tell me it's the opti.... 
Do you get bucking and surging at low RPM under WOT under load?
Pro
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From: Toronto, ON, Canada
No not necessarily Optispark problem. I have the same problem and was able eliminate the problem by removing my ATI intercooler and using a Vortech hard tube. At best guess the problem is related to the pressure differential caused by the intercooler and the rubber pipes it uses. I just live with the problem and try to stay away from the throttle position that causes it.
Melting Slicks
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From: Rochester NY
Originally Posted by XTwinTurbo
No not necessarily Optispark problem. I have the same problem and was able eliminate the problem by removing my ATI intercooler and using a Vortech hard tube. At best guess the problem is related to the pressure differential caused by the intercooler and the rubber pipes it uses. I just live with the problem and try to stay away from the throttle position that causes it.
Bruce
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From: Toronto, ON, Canada
Bruce how much boost are you running. I think the problem is more evident at lower boost numbers. I use to get the problem worse when I ran 6-8PSI across the I/C. As I moved up to a 10PSI the problem happens less.
Switching back to the Vortech hard tube has eliminated the problem as I have done this twice. The surge feels almost like a disconnect between the volume of air the MAF thinks it's taking in and the amount the engine is seeing.
The problem I face is crusing moderately the car will hit a flat spot, I press on the gas a tad more and after a certain point (very moderate change) I feel a push of power making up for the difference in pedal movement. It's such a moderate problem and controllable I just live with it.
Switching back to the Vortech hard tube has eliminated the problem as I have done this twice. The surge feels almost like a disconnect between the volume of air the MAF thinks it's taking in and the amount the engine is seeing.
The problem I face is crusing moderately the car will hit a flat spot, I press on the gas a tad more and after a certain point (very moderate change) I feel a push of power making up for the difference in pedal movement. It's such a moderate problem and controllable I just live with it.
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Originally Posted by XTwinTurbo
Bruce how much boost are you running. I think the problem is more evident at lower boost numbers. I use to get the problem worse when I ran 6-8PSI across the I/C. As I moved up to a 10PSI the problem happens less.
Switching back to the Vortech hard tube has eliminated the problem as I have done this twice. The surge feels almost like a disconnect between the volume of air the MAF thinks it's taking in and the amount the engine is seeing.
The problem I face is crusing moderately the car will hit a flat spot, I press on the gas a tad more and after a certain point (very moderate change) I feel a push of power making up for the difference in pedal movement. It's such a moderate problem and controllable I just live with it.
Switching back to the Vortech hard tube has eliminated the problem as I have done this twice. The surge feels almost like a disconnect between the volume of air the MAF thinks it's taking in and the amount the engine is seeing.
The problem I face is crusing moderately the car will hit a flat spot, I press on the gas a tad more and after a certain point (very moderate change) I feel a push of power making up for the difference in pedal movement. It's such a moderate problem and controllable I just live with it.
Could this be an issue with the bypass valve?? I'm still trying to figure out exactly how thise things work (I'm used to computer controller blow off valves...). But could the bypass valve be fluctuating at the near boost levels? I have no problems at WOT, and this ONLY happens right around 0 PSI no matter what RPM or throttle position.
There are no codes. Stock ECM.
Le Mans Master
Joined: Jan 2003
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From: Portland Oregon
Originally Posted by ztrips
I've got a strange part throttle surge that only happens at 0 PSI (well, -1 to +1 PSI) at moderate load, part throttle. Any ideas??? I've never had the A/F checked at those conditions, but I'm running fine at WOT.
Vortech kit on a 92 LT1.
Thanks...
Vortech kit on a 92 LT1.
Thanks...
Or if the bypass valve isn't working properly on light throttle and closed throttle if the valve isn't open it could cause a phenomenom called "compressor surge" this happens when the blower is creating boost against a closed throttle and basically it is dead heading against the throttle plates which overheats the compressed air in the blower. It is just like running a water pump against a closed valve.
Another thing to check is the fuel pressure regulator or the FMU that Vortech gives you. It may not be adjusted correctly and when MAP goes positive it maybe raising the fuel pressure too high which can also cause excessive enrichment and the surging problem.
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Originally Posted by tjwong
Is the bypass valve functional? It seems that there is a transition issue with the bypass valve. Seeing how your car is a Speed Density strategy going from vacuum to boost at X RPM may cause a surge due to it changing rapidly from vacuum to positive pressure. This can cause a rich condition which can cause the surge. Being a Speed Density strategy the ECM fuels the engine based on MAP and Engine Speed, if at a given engine speed and light partial load, and if the ECM sees a high (Low Vacuum) MAP value its going to throw fuel at the engine where it may not really need it.
Or if the bypass valve isn't working properly on light throttle and closed throttle if the valve isn't open it could cause a phenomenom called "compressor surge" this happens when the blower is creating boost against a closed throttle and basically it is dead heading against the throttle plates which overheats the compressed air in the blower. It is just like running a water pump against a closed valve.
Another thing to check is the fuel pressure regulator or the FMU that Vortech gives you. It may not be adjusted correctly and when MAP goes positive it maybe raising the fuel pressure too high which can also cause excessive enrichment and the surging problem.
Or if the bypass valve isn't working properly on light throttle and closed throttle if the valve isn't open it could cause a phenomenom called "compressor surge" this happens when the blower is creating boost against a closed throttle and basically it is dead heading against the throttle plates which overheats the compressed air in the blower. It is just like running a water pump against a closed valve.
Another thing to check is the fuel pressure regulator or the FMU that Vortech gives you. It may not be adjusted correctly and when MAP goes positive it maybe raising the fuel pressure too high which can also cause excessive enrichment and the surging problem.
Is there an easy test for the bypass valve? I've pulled the valve outlet tube off and at idle there is air blowing out.
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From: Honolulu, HI
I would think just by watching the vacuum/boost gauge (I have a combo gauge) that will tell you. When I rev it up at idle I get no boost. When I give it part throttle under load I still see vacuum. When I give WOT I see boost.
If you're seeing boost at part throttle under load I would guess your bypass isn't working.
If you're seeing boost at part throttle under load I would guess your bypass isn't working.
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Originally Posted by vvv90
I would think just by watching the vacuum/boost gauge (I have a combo gauge) that will tell you. When I rev it up at idle I get no boost. When I give it part throttle under load I still see vacuum. When I give WOT I see boost.
If you're seeing boost at part throttle under load I would guess your bypass isn't working.
If you're seeing boost at part throttle under load I would guess your bypass isn't working.
Le Mans Master
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From: Redmond WA
vvv90, the bypass triggers on manifold vacuum/boost signal, not throttle position, so its certainly possible and normal to get boost at mid throttle.
Back to the original problem:
I had this on my car when I switched from the Vortech discharge tube to the setup for the ATI intercooler. I think its definately a bypass valve issue or perhaps MAF turbulence.
I would try unplugging your MAF to force the car into speed density. If that fixes it, I'd consider relocating where your bypass valve dumps the air back in to a location further back from the MAF.
If that doesn't cure it, with the MAF still disconnected, try just venting the bypass to the atmosphere ( you can only do this with the MAF disconnected because in MAF mode it will have already metered and fueled for all that dir you're dumping ).
Back to the original problem:
I had this on my car when I switched from the Vortech discharge tube to the setup for the ATI intercooler. I think its definately a bypass valve issue or perhaps MAF turbulence.
I would try unplugging your MAF to force the car into speed density. If that fixes it, I'd consider relocating where your bypass valve dumps the air back in to a location further back from the MAF.
If that doesn't cure it, with the MAF still disconnected, try just venting the bypass to the atmosphere ( you can only do this with the MAF disconnected because in MAF mode it will have already metered and fueled for all that dir you're dumping ).
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From: Honolulu, HI
Originally Posted by DAVE396LT1
vvv90, the bypass triggers on manifold vacuum/boost signal, not throttle position, so its certainly possible and normal to get boost at mid throttle.
During WOT the engine loses vacuum which causes the bypass to close giving boost. At part throttle under load when the engine is PULLING VACUUM the bypass should remain open. That's the point I was trying to make.
If he's NOT seeing vaccum at part throttle under load and he IS seeing boost there is a bypass valve problem.
Moreover, you stated that it's possible to see boost at part throttle under vacuum.....Well, I guess that would depend on how much air the compressor is moving and how efficient the bypass valve is. I would find it unlikely that a street blower at 6-8psi (normally coming on at higher RPM's also) could produce boost at part throttle.
Last edited by vvv90; Sep 14, 2004 at 12:50 PM.
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Originally Posted by vvv90
I never said Throttle position directly triggers a bypass. However, throttle position IS directly related to how much vacuum an engine will pull, which in turn affect bypass valve operation like you stated.
During WOT the engine loses vacuum which causes the bypass to close giving boost. At part throttle under load when the engine is PULLING VACUUM the bypass should remain open. That's the point I was trying to make.
If he's NOT seeing vaccum at part throttle under load and he IS seeing boost there is a bypass valve problem.
Moreover, you stated that it's possible to see boost at part throttle under vacuum.....Well, I guess that would depend on how much air the compressor is moving and how efficient the bypass valve is. I would find it unlikely that a street blower at 6-8psi (normally coming on at higher RPM's also) could produce boost at part throttle.
During WOT the engine loses vacuum which causes the bypass to close giving boost. At part throttle under load when the engine is PULLING VACUUM the bypass should remain open. That's the point I was trying to make.
If he's NOT seeing vaccum at part throttle under load and he IS seeing boost there is a bypass valve problem.
Moreover, you stated that it's possible to see boost at part throttle under vacuum.....Well, I guess that would depend on how much air the compressor is moving and how efficient the bypass valve is. I would find it unlikely that a street blower at 6-8psi (normally coming on at higher RPM's also) could produce boost at part throttle.
That brings up an interesting question... All of this talk about bypassing to atmosphere for air that is already metered by the MAF. It seems to me that if your MAF is before your bypass there is no way you can get an accurate reading on airflow into the engine, with or without recirculation. If you are recirculating to the intake side of the blower, you are effectively venting to atmosphere. The area prior to the blower is unpressurized and only regulated by the speed at which the blower is sucking in air. In theory, if you are venting from high pressure (during shifting) you could in fact be blowing air backwards, out of your air filter! If you are venting to an area after the blower, you're not accomplishing anything.
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From: Honolulu, HI
Originally Posted by ztrips
I've got a 92, so no MAF. I'm going to try and vent the bypass instead of recirculating.
That brings up an interesting question... All of this talk about bypassing to atmosphere for air that is already metered by the MAF. It seems to me that if your MAF is before your bypass there is no way you can get an accurate reading on airflow into the engine, with or without recirculation. If you are recirculating to the intake side of the blower, you are effectively venting to atmosphere. The area prior to the blower is unpressurized and only regulated by the speed at which the blower is sucking in air. In theory, if you are venting from high pressure (during shifting) you could in fact be blowing air backwards, out of your air filter! If you are venting to an area after the blower, you're not accomplishing anything.
That brings up an interesting question... All of this talk about bypassing to atmosphere for air that is already metered by the MAF. It seems to me that if your MAF is before your bypass there is no way you can get an accurate reading on airflow into the engine, with or without recirculation. If you are recirculating to the intake side of the blower, you are effectively venting to atmosphere. The area prior to the blower is unpressurized and only regulated by the speed at which the blower is sucking in air. In theory, if you are venting from high pressure (during shifting) you could in fact be blowing air backwards, out of your air filter! If you are venting to an area after the blower, you're not accomplishing anything.
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Originally Posted by vvv90
Yes, basically you are correct, but you have to understand how a MAF works to understand the symptom that DAVE is explaining. Basically a MAF is a heated wire and when air flows across it the wire is cooled and this is measured as mass air. If your bypass valve is venting across the MAF, thus cooling it, it will signal that it is flowing more air than it is. If the bypass is far enough away from the MAF it will not give this false reading and theoretically vent OUT the air filter. this probably only happens if the bypass outlet is VERY close to the MAF, but I haven't seen a MAF setup. I have a '92 SD car myself.
MAF arguement aside, is the bypass valve supposed to be prgressive (variable) or just on/off? Could a weak spring be causing it to "flutter" instead of operating in a smooth transition?
Last edited by ztrips; Sep 14, 2004 at 01:56 PM.
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From: Redmond WA
Originally Posted by vvv90
I would find it unlikely that a street blower at 6-8psi (normally coming on at higher RPM's also) could produce boost at part throttle.
I see boost as the rpms come up at part throttle. That's to be expected: the blower achieves greater than 100% VE even with the "restriction" of half-throttle, and it does it easily on my car... and my bypass works fine, as you can clearly hear when you vent it to the atmosphere.