Need help diagnosing/finding a vacuum leak

Mr.Redline · 06-19-2025, 09:45 PM

Hello all this is my first post and I’m in need of help. I just got my car back from the shop after installing a cam, long tube headers, a 95mm throttle body, a lower pulley, a Katech oil pump, and forged internals. The car already had an LT4 blower, LT4 fueling components (including injectors, a high-pressure fuel pump, and an in-tank fuel pump), a Rotofab intake, a Mighty Mouse mild catch can, and a flex fuel kit.

While doing some logs my tuner noticed that my long-term fuel trims (LTFT) were about +20% on both banks at idle, which decreased as the RPMs increased. I was getting a P0171 code. The idle also sounds rough which sounded like a vacuum leak. I've replaced two EVAP lines, the EVAP purge solenoid, the vent valve, header gaskets, MAF and MAP sensors. Despite these changes, the car still runs lean.

I have smoked the system multiple times to check for leaks, and none were found. I also tried using starter fluid and sprayed around hoses, the throttle body and all around the supercharger. I even took apart the catch can and put the stock pcv back in, but there was no change in the logs. I have disassembled the supercharger and plan to replace the gaskets and the PCV seal on the blower. After replacing the gaskets and ensuring everything is torqued to spec, I’m uncertain about what to do next. I tried uploading a log but it wouldn’t let me so I did a screenshot of an idle log. I figured it’s better than nothing lol. I'm open to any suggestions on what this could be or what to check next.

Megahurtz · 06-20-2025, 06:08 AM

Do you have a Rotofab standard or Big Gulp? Did the tuner know which one?

I've seen this before and it turned out the shop tuned for a rotofab standard on the dyno, didn't do any real road tests so only WOT was tuned. Then the car drives terribly and fuel trims max out trying to make up for the bad tune.

The Big Gulp takes substantial changes compared to the Standard.

Mr.Redline · 06-20-2025, 07:34 AM

I have the standard Rotofab. During the tuning process, my tuner asked if I had switched to a Big Gulp, which is why we suspect there might be a vacuum leak. I also returned to the shop that installed everything, and they performed a continuity test on the fueling and TMAP harnesses. Everything checked out fine.

Yesterday, I removed the supercharger and noticed some blow-by on top of my LT4 adapter plates. I will be installing new gaskets this weekend, hoping that will resolve the issue.

06-19-2025, 09:45 PM	#1
Mr.Redline Drives: Camaro 2SS 2021 Join Date: Jun 2025 Location: Chicago Posts: 2	Need help diagnosing/finding a vacuum leak Hello all this is my first post and I’m in need of help. I just got my car back from the shop after installing a cam, long tube headers, a 95mm throttle body, a lower pulley, a Katech oil pump, and forged internals. The car already had an LT4 blower, LT4 fueling components (including injectors, a high-pressure fuel pump, and an in-tank fuel pump), a Rotofab intake, a Mighty Mouse mild catch can, and a flex fuel kit. While doing some logs my tuner noticed that my long-term fuel trims (LTFT) were about +20% on both banks at idle, which decreased as the RPMs increased. I was getting a P0171 code. The idle also sounds rough which sounded like a vacuum leak. I've replaced two EVAP lines, the EVAP purge solenoid, the vent valve, header gaskets, MAF and MAP sensors. Despite these changes, the car still runs lean. I have smoked the system multiple times to check for leaks, and none were found. I also tried using starter fluid and sprayed around hoses, the throttle body and all around the supercharger. I even took apart the catch can and put the stock pcv back in, but there was no change in the logs. I have disassembled the supercharger and plan to replace the gaskets and the PCV seal on the blower. After replacing the gaskets and ensuring everything is torqued to spec, I’m uncertain about what to do next. I tried uploading a log but it wouldn’t let me so I did a screenshot of an idle log. I figured it’s better than nothing lol. I'm open to any suggestions on what this could be or what to check next. Attached Images

06-20-2025, 06:08 AM	#2
Megahurtz Snackbar Tuning Drives: 2023 SGM ZL1 Join Date: Oct 2016 Location: Florida Posts: 1,522	Do you have a Rotofab standard or Big Gulp? Did the tuner know which one? I've seen this before and it turned out the shop tuned for a rotofab standard on the dyno, didn't do any real road tests so only WOT was tuned. Then the car drives terribly and fuel trims max out trying to make up for the bad tune. The Big Gulp takes substantial changes compared to the Standard. __________________ GM Gen V Calibrator

06-20-2025, 07:34 AM	#3
Mr.Redline Drives: Camaro 2SS 2021 Join Date: Jun 2025 Location: Chicago Posts: 2	I have the standard Rotofab. During the tuning process, my tuner asked if I had switched to a Big Gulp, which is why we suspect there might be a vacuum leak. I also returned to the shop that installed everything, and they performed a continuity test on the fueling and TMAP harnesses. Everything checked out fine. Yesterday, I removed the supercharger and noticed some blow-by on top of my LT4 adapter plates. I will be installing new gaskets this weekend, hoping that will resolve the issue.

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