fuel pressure
Thread Starter
Heel & Toe
Joined: Jul 2017
Posts: 17
Likes: 0
fuel pressure
the car is 1987,the fuel pressure when key turned to power fuel pump was 10 psi,started car it was 36 psi,shut it down and it dropped down to 26 psi in 5 minutes,i went to recheck it again 3 hours later and had 2 psi with key on,then started it,no change,i shut it down and pushed in scrader and barely any gas came out,the reason for checking this is because after driving car for awhile it wont start unless I give it a little gas,but it starts great when I go start it to in morning,any ideas would be appreciated
Team Owner
Joined: Jul 2004
Posts: 28,461
Likes: 3,288
From: Hartford WI
I don't think you have any choice but to check things out. Crimp off the return line at the tank and fire up the pump by turning the key. That should prime for a couple seconds. See what the pressure is like and how long it holds and we can go from there.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
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From: near the thumb in the mitten
There's a fuel pressure issue since pressure is a low. I'm not sure it's the only one. I would also be sure throttle bore is clean of carbon build up. Are there any codes?
Le Mans Master
Joined: Oct 2004
Posts: 6,314
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From: South-central Missouri
I don't have the particulars (found in an '87 FSM) for testing the fuel pump, but this chart from Corvette Fuel Injection by Charles O. Probst might shed some light on the subject:
As I said, I'm not familiar with the '87s characteristics - someone please jump in here - but you'd want to know what the FSM says it should be (and I have a feeling it should be around the 37-42 psi range when cranking...no?).
Team Owner
Joined: Jul 2004
Posts: 28,461
Likes: 3,288
From: Hartford WI
I don't think you want to use cranking pressure. What does it tell you? You are losing pressure when the injectors open. I would turn it on and see what it goes to without cranking. If you can't even reach 37, you have either supply or regulator issues. In which case you crimp the return line. If you still cannot get it and it doesn't hold, you have supply issues.
Team Owner
Joined: Sep 2001
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From: Athens AL
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It should prime higher than that, it should basically be jumping up to run pressure at 42psi give or take a couple, then hold that pressure. As you fire it up, it should stay there.
Running pressure should stay around 42, until you goose the throttle, in which case it will likely drop a couple psi and recover within a second or two as the pump catches up.
Turn off car, pressure should remain high for quite some time, if it starts dropping immediately then an injector is sticking open. Sounds like you may have a slightly open injector, but that is not the main issue here.
Crimp the return line to check for regulator issues.
So from what you say I'd guess your pump is going out. Time to get the manual out and start testing that. It likely will never cause an SES light to come on.
Running pressure should stay around 42, until you goose the throttle, in which case it will likely drop a couple psi and recover within a second or two as the pump catches up.
Turn off car, pressure should remain high for quite some time, if it starts dropping immediately then an injector is sticking open. Sounds like you may have a slightly open injector, but that is not the main issue here.
Crimp the return line to check for regulator issues.
So from what you say I'd guess your pump is going out. Time to get the manual out and start testing that. It likely will never cause an SES light to come on.
Thread Starter
Heel & Toe
Joined: Jul 2017
Posts: 17
Likes: 0
so just turn key to energize pump,then turn back off keeping line pinched and let it sit
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Team Owner
Joined: Jul 2004
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From: Hartford WI
Pretty much. If the pressure holds for a while, the regulator is probably shot. If you are going there, might as well get reman injectors and do the regulator, disassemble the TB and clean all passages.
Drifting
Joined: Nov 2015
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Finalist 2020 C4 of the Year - Unmodified
Key on, engine not started, the fuel pressure on an '87 (like mine 
) should be 36+ psi. If not, look at the fuel pump or the mesh. Had similar myself, first started long, then started having issues had mid thottle and eventually, over a month or so, no start. Same problem at idle, the pressure looked okay then.
The fuel filter could be blocked too, that would limit fuel pump enough that the 2 seconds at key on wouldn't be enough to get pressure. Though, if that were the case, I'd expect problems at high to WOT also, it'd be running very lean.
Can also use ALDL to run the fuel pump a bit longer, see if pressure gets high enough.
) should be 36+ psi. If not, look at the fuel pump or the mesh. Had similar myself, first started long, then started having issues had mid thottle and eventually, over a month or so, no start. Same problem at idle, the pressure looked okay then.The fuel filter could be blocked too, that would limit fuel pump enough that the 2 seconds at key on wouldn't be enough to get pressure. Though, if that were the case, I'd expect problems at high to WOT also, it'd be running very lean.
Can also use ALDL to run the fuel pump a bit longer, see if pressure gets high enough.
Race Director
Joined: Apr 2002
Posts: 10,036
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From: Anaheim CA
When the key is turned on the pump will run for 2 seconds and the FPR (Fuel Pressure Regulator) will allow about 44 PSI (because there is no vacuum and the FPR thinks it's in a WOT condition. When the engine starts the vacuum will be maximum and the fuel pressure will be minimum (about 38 PSI).