Failing leak down testing? Where is it going.
10-20-2017, 11:24 PM
#1
Heel & Toe
Thread Starter
Failing leak down testing? Where is it going.
Ok fellow C4 owners and wizards.
New to me 1985 C4 with 30400 miles. Garage queen.....
No problems starting but had random smell of gas near fuel rail.
So first testing of original fuel pressure found 37 psi with key on position.
But when turned car off it dropped down to O psi within seconds. Wow.
Then did a pinch off test of supply and return lines where I found I can hold pressure. Also checked FPR while running and holding 37 psi and when vacuum removed psi rises to 45 psi. No leaks?
So I pull original fuel sending unit since car sat more then been driven and replaced pump with Walbro and new bulkhead wire setup. Lots of corrosion on sending unit. New pump with key turned to on position now rises to 40 psi then drops to 34 to 36 psi while idling. When FPR vacuum is removed psi jumps to 45. When shut off the pressure drops fast to 25 psi and holds for over 30 mins
With age of car I decided to pull plenum/runners and rebuilt the fuel rail with new orings, new FPR diaphragm and Fel Pro gaskets. And installed set of Bosch III injectors. Old rubber being replaced all over car.
Start up is now great. After letting pressure build up to 40 psi with key to the on position. When started it holds at 34 psi at idle. Pulling FPR vacuum now rises to 45 psi.
Did new pinch off test and holding pressure with both supply and return lines pinched off. When supply released pressure drops when return released pressure drops. No smell of gas at rail. Great!
FPR working at idle with vacuum and when vacuum pulled off it rises.
All good right?
What it at question is the drop of pressure when car is shut off.
From 34 to 25 psi in seconds?
Is that normal?
Where is pressure going so fast? Back to tank through supply line?
Is this a problem?
Do the Walbro pumps have check valve? Cannot find any info on this.
Car runs great. Acceleration is awesome. Idle is fine?
I Drive it like I stole it.
Any suggestions for my new Mistress? I am open for all ideas......
New to me 1985 C4 with 30400 miles. Garage queen.....
No problems starting but had random smell of gas near fuel rail.
So first testing of original fuel pressure found 37 psi with key on position.
But when turned car off it dropped down to O psi within seconds. Wow.
Then did a pinch off test of supply and return lines where I found I can hold pressure. Also checked FPR while running and holding 37 psi and when vacuum removed psi rises to 45 psi. No leaks?
So I pull original fuel sending unit since car sat more then been driven and replaced pump with Walbro and new bulkhead wire setup. Lots of corrosion on sending unit. New pump with key turned to on position now rises to 40 psi then drops to 34 to 36 psi while idling. When FPR vacuum is removed psi jumps to 45. When shut off the pressure drops fast to 25 psi and holds for over 30 mins
With age of car I decided to pull plenum/runners and rebuilt the fuel rail with new orings, new FPR diaphragm and Fel Pro gaskets. And installed set of Bosch III injectors. Old rubber being replaced all over car.
Start up is now great. After letting pressure build up to 40 psi with key to the on position. When started it holds at 34 psi at idle. Pulling FPR vacuum now rises to 45 psi.
Did new pinch off test and holding pressure with both supply and return lines pinched off. When supply released pressure drops when return released pressure drops. No smell of gas at rail. Great!
FPR working at idle with vacuum and when vacuum pulled off it rises.
All good right?
What it at question is the drop of pressure when car is shut off.
From 34 to 25 psi in seconds?
Is that normal?
Where is pressure going so fast? Back to tank through supply line?
Is this a problem?
Do the Walbro pumps have check valve? Cannot find any info on this.
Car runs great. Acceleration is awesome. Idle is fine?
I Drive it like I stole it.
Any suggestions for my new Mistress? I am open for all ideas......
10-21-2017, 11:40 AM
#3
Race Director
New definition of a leak down test.
10-22-2017, 08:46 AM
#4
Le Mans Master
..... Keep in mind that the engine doesn't really care what the fuel pressure gauge says when its not running ... That said , How does it run and drive ? .....
10-24-2017, 02:36 PM
#5
Drifting
Member Since: Nov 2015
Posts: 1,529
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Finalist 2020 C4 of the Year - Unmodified
Test pressure without actually starting. Ignition on, wait 10 seconds or so and then off.
Actual fuel pressure should hold for about 20 minutes, maybe losing a couple psi by then.
If not, there is still a small leak.
Actual fuel pressure should hold for about 20 minutes, maybe losing a couple psi by then.
If not, there is still a small leak.
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abw121 (10-24-2017)
10-24-2017, 06:07 PM
#6
Team Owner
Pro Mechanic
Technically, there is a test, and technically, there is a spec...but it really doesn't matter if it leaks down when not running. If the pressure meets spec when running, that is what matters the most. You can isolate WHERE it's leaking down by clamping delivery and return hoses to isolate them as possibilities. But if it runs fine, I wouldn't care.