Lift pump pressure spikes after shutdown
#1
Registered User
Thread Starter
Lift pump pressure spikes after shutdown
This adventure started out with the engine not starting after setting for a few days. It would start after priming with the lift pump but without bleeding. I temporally installed a 0-15 psi analog gauge at the banjo bolt to the filter to check the pressure.
It has good pressure at cold idle (8 psi) and a small bit of throttle it would climb to 12 psi and just after shut down the psi would climb past 15 psi (18psi?), wrap the needle all the way around and then slowly drop to zero psi. This can't be normal!
My guess is the IP (VE) if feeding the pressure back to the top of the lift pump. But why, how?
Ideas, suggestions?
The lift was replaced July of 2017.
It has good pressure at cold idle (8 psi) and a small bit of throttle it would climb to 12 psi and just after shut down the psi would climb past 15 psi (18psi?), wrap the needle all the way around and then slowly drop to zero psi. This can't be normal!
My guess is the IP (VE) if feeding the pressure back to the top of the lift pump. But why, how?
Ideas, suggestions?
The lift was replaced July of 2017.
#2
Registered User
I bet when the FSS closes it is causing a back pressure for a quick moment while the LP is still producing pressure.
Resulting in a slight spike in pressure.
This is just a guess, as I don't run a fuel pump on my 1st gen trucks all the time. I just install one for diagnostic purposes when needed in the shop.
Resulting in a slight spike in pressure.
This is just a guess, as I don't run a fuel pump on my 1st gen trucks all the time. I just install one for diagnostic purposes when needed in the shop.
#3
Registered User
Thread Starter
Yes could be that the IP still rotating is causing the spike. Never thought of looking at the gauge during shutdown until now, could have been happening all the time.
I'm not using the FSS just the emergency lever if that makes any difference.
I'm not using the FSS just the emergency lever if that makes any difference.
#4
Administrator
I think that this is probably normal, especially because your gauge only goes to 15 lbs. I bet that if you put one that goes to 100 pounds, you wouldn't even see it. Also, I don't think that it can blow out the front seal, because the FSS is gutted, there is nothing stopping a little back pressure as you mentioned. I say party on and don't worry about it...Mark
#5
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Thread Starter
Found a new in the box gauge same make etc, it reads 5 lbs less. Pressure still spikes after shutdown but only goes to 12 psi.
Engine had set for 3 days, had to prime it before it would start.
Engine had set for 3 days, had to prime it before it would start.
#6
Administrator
Sounds like you have an air leak on the suction side...Mark
#7
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#8
Registered User
Thread Starter
Yea I've fought that monster before and now it's back.
edit: Really makes me want to replace all the flex lines with stainless steel braided hose........
edit: Really makes me want to replace all the flex lines with stainless steel braided hose........
#9
Administrator
#11
Administrator
The usual advice may not be good, because yours is in a scout. It may still be worth pressurizing the fuel tank with a few psi's to see if it shows up. Did you just plumb directly into the scout tank? or is it a transplant?...Mark
#12
Registered User
Thread Starter
The spiking lift pump pressure isn't.
The previous test was done on a cold engine, did the test with a hot engine, NO spike. False alarm.
Why the difference no idea.
The previous test was done on a cold engine, did the test with a hot engine, NO spike. False alarm.
Why the difference no idea.
#13
Administrator
Did you find the leak?
#14
Registered User
Thread Starter