What does a vacuum leak look like on the scanner?

[razzer]

I have a 2016 GT that I installed a Roush s/c on. It has an intercooler with pump with dual fans, a GT500 fuel pump, aftermarket fuel rails with larger injectors. I have been struggling to get my tune finished at the local tuner. I picked my car up and they told me that it has a vacuum leak. Old school, spraying carb cleaner doesn't work to help find a leak. I never did find a leak, but I installed new intake gaskets anyway, and had the manifold checked for flatness, and it was fine (less than .002 out). The tuner says that they see the vacuum leak by watching the VCM scanner. What am I looking for in my data logs to try to identify if there truly is a vacuum leaks? As it searches idle between 750 and 900, I can see the stft bobble above and below zero, but is this a cause or an effect? The main issue I see is sometimes while driving the idle hangs at 1500 rpm, and won't come down, even with a kick of the throttle. I doesn't do it all the time, which makes me think it has a leak (only at certain temps), or is the computer reacting to something else?

[TheMechanic]

Vac leak on the scanner will show +numbers on STFT and LTFT at an idle. This will progressively correct as load and or engine speed increases. You may also see a corresponding low MAF number at Idle and also a more closed throttle at idle as ECM/PCM attempts to correct for high idle.
A hanging idle is rarely a simple vac leak. Do you have any codes in the computer? Just did a "vac" leak due to a broken evap line to the motor. Purge solenoid would activate and engine would expect gas vapor only to have fresh air but I got a large evap leak code so it was an easy diag. Best bet is find someone with a smoke machine and test it that way.
Funny story I had a misfire on an Audi with corresponding lean codes. Turned out to be a rear main seal after a smoke test. Crazy stuff. Replaced the seal and it fixed the lean codes and the misfires. Didn't surge any more either.

[smokeshow]

Sounds a bit more like your car doesn't know how to control the throttle with the extra vacuum behind it. Same effect on on all blower swapped NA vehicles, regardless of manufacturer. Search SCIAP install on the forum. That's what I'd put my money on.

[razzer]

After more checking last night, I find that the pump included with my s/c kit (GT350 pump, but not sure why, as when I look up the specs on it, it has same psi as stock GT pump), is putting out 100 psi. I'm guessing its forcing too much fuel thru/past the injectors. I will remedy this before I do anything else. We also couldn't find a way to release pressure in the fuel system before opening the fuel lines to add the test gauge. Wouldn't I expect to see a loss of pressure when I pull the evap line from the purge valve? Also I noted that after priming the system, it started and dies out like no fuel. This was caused by the clip on the fuel line at the pump hat broke, and the line was off, spraying fuel under the car. The odd thing was, the gauge showed it was still holding pressure at about 40 psi??

To answer "the mechanics" question about codes, I just see random misfire codes. I smoked my engine cold, and then hot last week. Only sign of leak was when it came out the air filter. I thought it odd that my tuner told me a smoke test really wont tell me anything.

[TheMechanic]

EVAP and fuel supply system are two different animals.
One is for limiting raw fuel vapor into the atmosphere and will typically be pressures/vacuum measure in inches of water (very low).
Of course you know what the fuel supply system is and does.
Yeah 100 psi is not good. That would have to be addressed first before any additional diagnostics would be performed as it could potentially cure all concerns you have.

[razzer]

EVAP and fuel supply system are two different animals.
One is for limiting raw fuel vapor into the atmosphere and will typically be pressures/vacuum measure in inches of water (very low).
Of course you know what the fuel supply system is and does.
Yeah 100 psi is not good. That would have to be addressed first before any additional diagnostics would be performed as it could potentially cure all concerns you have.

What would an exceptable pressure be for my application?

[TheMechanic]

I will have to check my Pro-Demand when I get home but I believe stock pressure is 63 psi for fuel pressure.
Pressure from the tanks EVAP system would typically never exceed a pound or two and often runs in a very slight vacuum when there is a purge/vent seal command.

[razzer]

I have corrected the high fuel pressure issue. I put the GT350 pump ( 100 psi, not sure why, since the stock GT350 pump spec is 53-75psi) back in, and added a vacuum controlled regulator. Vacuum pulled from intake where the dash gauge draws from. I re-installed the return line. Routing goes from the secondary fuel rail to the regulator, then to the tank. Since the ID1050x injectors are rated to be 43.5 psi injectors, I started at 43.5 psi on the regulator, then tried 50, 55, 60, 65, and 70. It doesn't seem to matter greatly on the pressure, it still isn't right. I am still getting a random idle, I see 2k, 1k, and some in between. I have logged and studied, but I don't really know what to look for. I did notice my injector DC varies from .7 up to 6.? at times. I thought DC should be more consistent around .8 to 1. something?
I scared to take it back to the tuner, because I think he will just say its not the tune again, and its a vacuum leak, or now something else. I can't afford to pay tuner rates for diagnostics. I got e return email from Roush, stating that their s/c kit (422001) does not require an inflated fuel pressure, and that is a stock "Ford spec'd" 53-63 psi. Is that maybe why their canned tuned are so mild?

[TheMechanic]

I have corrected the high fuel pressure issue. I put the GT350 pump ( 100 psi, not sure why, since the stock GT350 pump spec is 53-75psi) back in, and added a vacuum controlled regulator. Vacuum pulled from intake where the dash gauge draws from. I re-installed the return line. Routing goes from the secondary fuel rail to the regulator, then to the tank. Since the ID1050x injectors are rated to be 43.5 psi injectors, I started at 43.5 psi on the regulator, then tried 50, 55, 60, 65, and 70. It doesn't seem to matter greatly on the pressure, it still isn't right. I am still getting a random idle, I see 2k, 1k, and some in between. I have logged and studied, but I don't really know what to look for. I did notice my injector DC varies from .7 up to 6.? at times. I thought DC should be more consistent around .8 to 1. something?
I scared to take it back to the tuner, because I think he will just say its not the tune again, and its a vacuum leak, or now something else. I can't afford to pay tuner rates for diagnostics. I got e return email from Roush, stating that their s/c kit (422001) does not require an inflated fuel pressure, and that is a stock "Ford spec'd" 53-63 psi. Is that maybe why their canned tuned are so mild?

The rated at injector pressure does not mean it has to run at XXpsi. You can run a different pressure you just need to adjust the lbs for coincide with the new pressure. You need to post your tune

[razzer]

thank you again for the response. I was away for a few days, and back with it now. I was able to find a vacuum leak around the bearing cap, that holds the end of the butterfly rod for the boost relief at the bottom of the blower. Once I got that sealed up, the leak is gone. I do still have a ghost high idle, but I can make it go away when I unplug the MAF sensor. I believe this tells me that it must be a tune issue. I am taking it back to the tuner today, and I'll see if I am correct. I'd like to still post my tune, and with any chance maybe I could learn what is causing the idle issue. Just because the tuner can correct it, doesn't mean I don't want to know what's happening. The idle hangs when I depress the clutch, and it will slowly rise while sitting at stop lights. sometimes it will come down to 750 if I kick the gas pedal, but not always.