Question for tuning experts

v8hgt

Well-Known Member
Joined
Mar 31, 2016
Threads
52
Messages
1,231
Reaction score
443
Location
Cheshire, UK
First Name
Nick
Vehicle(s)
Race Red GT PP Coupe
Vehicle Showcase
1
How does the digital vacuum gauge calculate the vacuum?

I have a gt350 IM and TB on a GT. My tuner (Shaun at AED) told me I had a vac leak on the remote tune. My digital vac gauge reads 15 in/hg which is low suggesting a leak.
However I recently hooked up an analogue vac gauge to the inlet port pipe behind the throttle body and im getting a lovely 23 in/hg of vacuum on that mechanical gauge. Ive checked all over for leaks using the cigarette smoke and the propane method and there is no leak. The AFR is bang on at 14.2 or so. Hence I would love to know what equation and sensors are used to determine digital vac as one of them isn't right on my car.

Thanks in advance

 

Basspro302

Well-Known Member
Joined
Apr 13, 2021
Threads
6
Messages
244
Reaction score
105
Location
Ohio
Vehicle(s)
2019 mustang gt
Shaun isn’t looking at a vacuum reading to determine a vacuum leak. Its based on the fuel trims.
You either need a real smoke machine or a way to look at the short term fuel trims and look for them to correct when you spray brake cleaner or propane around the leak.

The gauge is calculating it based on throttle blade angle, engine load, MAF probably not 100% sure. But I can tell you it’s useless.
 
OP
OP
v8hgt

v8hgt

Well-Known Member
Joined
Mar 31, 2016
Threads
52
Messages
1,231
Reaction score
443
Location
Cheshire, UK
First Name
Nick
Vehicle(s)
Race Red GT PP Coupe
Vehicle Showcase
1
Thanks, that's hugely helpful. If memory serves, Shaun elevated my fuel trims very slightly to compensate for what he perceived to be a very very slight vac leak by observing my closed loop lambda readings/fuel trims, and now my long term trims read dead on zero most of the time. The fact that my digital dash vac gauge is at least 5 in/hg out is probably because its basing its calculated readings off a stock throttle body blade angle not the gt350 TB blade angle and increased circumference.

When I used the propane method I was listening for a change in engine rpm. Now I realise as you suggested, that the closed loop system will just adjust the trims to keep rpm the same. I need to be spraying and watching short term fuel trims on the laptop at the same time to find any leak.

The fact that my mechanical gauge is showing 23 in/hg at 500ft above sea level and 20 degrees centigrade suggests if there is a leak its bloody small as that's a very strong vacuum being sustained.
 

Basspro302

Well-Known Member
Joined
Apr 13, 2021
Threads
6
Messages
244
Reaction score
105
Location
Ohio
Vehicle(s)
2019 mustang gt
Check your purge valve they are known to leak. Unplug the line on side that goes back to the canister (gas tank area) and feel for vacuum. Make sure you unplug the electrical connector on it. The valve should close and not leak when the electrical connector is removed.
 

GregO

Well-Known Member
Joined
Mar 5, 2018
Threads
38
Messages
1,506
Reaction score
888
Location
Illinois
Vehicle(s)
S550 GTPP
Ive checked all over for leaks using the cigarette smoke and the propane method and there is no leak.
How many miles are on your car and what type of fuel do you use including supplements ?
Change out the injector to manifold O-rings.
 


OP
OP
v8hgt

v8hgt

Well-Known Member
Joined
Mar 31, 2016
Threads
52
Messages
1,231
Reaction score
443
Location
Cheshire, UK
First Name
Nick
Vehicle(s)
Race Red GT PP Coupe
Vehicle Showcase
1
How many miles are on your car and what type of fuel do you use including supplements ?
Change out the injector to manifold O-rings.
It’s done 15,000 miles and it’s been run exclusively on shell v power 99 RON from new.
All the plugs are the same colour suggesting no single cylinder is leaking more than any other. Lambda sensors on both sides of the motor show similar. I will check the o rings on the injectors and double check the torque on the fuel rail bolts. Thanks for the advice.
 
OP
OP
v8hgt

v8hgt

Well-Known Member
Joined
Mar 31, 2016
Threads
52
Messages
1,231
Reaction score
443
Location
Cheshire, UK
First Name
Nick
Vehicle(s)
Race Red GT PP Coupe
Vehicle Showcase
1
Check your purge valve they are known to leak. Unplug the line on side that goes back to the canister (gas tank area) and feel for vacuum. Make sure you unplug the electrical connector on it. The valve should close and not leak when the electrical connector is removed.
Checked the purge valve and it’s working as intended with no leak sadly. That would have been a nice easy fix
 

Plimmer

Well-Known Member
Joined
May 10, 2015
Threads
8
Messages
660
Reaction score
366
Location
San Antonio, TX
First Name
Rob
Vehicle(s)
2015 Mustang GT
I’ve also GT350 swapped my car. You might need to double check how all the manifold hoses are routed. There are more than several little ports before and after the throttle body on this manifold. You might want to double check that the way they’ve been hooked up makes sense.

Looks like your tuner is saying some airflow is bypassing the MAF cuz of fuel trims, so vacuum might still be great after the throttle body, but somehow air might be bypassing the MAF.
 

Mdtaylorjr

Well-Known Member
Joined
Aug 8, 2020
Threads
1
Messages
184
Reaction score
76
Location
Houston tx
First Name
michael
Vehicle(s)
2017 mustang gt
I've also got the gt350 top end. My vac readings are always lower than stock as well when idling but on the scanner fuel trims are pretty close to 0 so I'm sure I don't have a vac. Leak. Car runs pretty solid except in the dead ass heat of day when in bumper to bumper traffic the IAT hits 150-160 sometimes so the car bucks on accel sometimes. Other than that both tunes I run are pretty smooth.
 
OP
OP
v8hgt

v8hgt

Well-Known Member
Joined
Mar 31, 2016
Threads
52
Messages
1,231
Reaction score
443
Location
Cheshire, UK
First Name
Nick
Vehicle(s)
Race Red GT PP Coupe
Vehicle Showcase
1
I got it good and hot today then put the mechanical vac gauge on the big port just behind the throttle body where a real gt350 connects its brake booster. She is reading a steady 23in/hg on the mechanical gauge so very strong! Happy now. I was just a bit concerned when I saw such low readings in the centre dash display but I guess they are wildly inaccurate. I will have to learn to ignore the digital readout and go with my gut and analog tools.
 

 
5 SPURS - 2
Top