Diag Help Needed
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Joined: Sep 2012
Posts: 531
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From: Central East Florida
Diag Help Needed
After a recent super charger installation the car now has two issues,
1. at 1500 RPM while not in gear the engine surges. The attached video shows the condition. I am just holding the accelerator, the engine is surging on its own. This only happens in neutral and I cannot reproduce this condition at any other RPM or when the car is in gear or under a load.
2. When coming to a stop or at idle the transmission seems to disengage. On an incline (lift ramp, driveway) the car will actually roll backwards. Once the accelerator is pressed the car will re-engage and perform normal. The data log is indicating the car is trying to stall.
The car performs normal under load. No codes shifts fine. Installer is telling me its transmission. They have replaced TCM with no change. I have a hard time believing it is trans since it performs perfect under acceleration and normal driving. Seems like a vacuum issue to me. Car was perfect prior to install of SC.
Attached is the surge video and the results of the data log. Any help would be greatly appreciated. Car as been at installer 2 weeks. I am starting to get concerned that they are not going to resolve the issue. I am not beating up the installer, just trying to get some help from the knowledge of the experienced people on this group.
Video link
1. at 1500 RPM while not in gear the engine surges. The attached video shows the condition. I am just holding the accelerator, the engine is surging on its own. This only happens in neutral and I cannot reproduce this condition at any other RPM or when the car is in gear or under a load.
2. When coming to a stop or at idle the transmission seems to disengage. On an incline (lift ramp, driveway) the car will actually roll backwards. Once the accelerator is pressed the car will re-engage and perform normal. The data log is indicating the car is trying to stall.
The car performs normal under load. No codes shifts fine. Installer is telling me its transmission. They have replaced TCM with no change. I have a hard time believing it is trans since it performs perfect under acceleration and normal driving. Seems like a vacuum issue to me. Car was perfect prior to install of SC.
Attached is the surge video and the results of the data log. Any help would be greatly appreciated. Car as been at installer 2 weeks. I am starting to get concerned that they are not going to resolve the issue. I am not beating up the installer, just trying to get some help from the knowledge of the experienced people on this group.
Video link
Last edited by randyfl; Jun 8, 2025 at 12:19 PM.
I'll hang out here a while. 
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Joined: May 2005
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