how to test fuel injectors
07-25-2014, 05:32 PM
#1
how to test fuel injectors
hey guys, ive been having a hard time figuring out why my new 350 tpi hasnt been making the appropriate power for the upgrades on it (it feels almost identical to my 178k 305, but that could atleast do a burnout) and its been getting a bit worse. lately when i come to a complete stop, or sometimes just slow down, the car dies. when the car is really hot it seems like its sputtering a bit while stopped, then when i hit the gas it seems almost like the injectors are loading up on fuel. also, when starting, it takes forever with cranking over to finally start up, and then i have to start getting on the gas to keep it running. i am wondering if maybe the injectors arent holding pressure, i dont know the history on the 24lb SVO injectors, just know i got them for free when i got my accel base. talked to a mechanic, and he was thinking they arent holding the pressure right either, if this is the case, i'm gonna order up some new ones from southbay
07-26-2014, 05:49 PM
#2
Supreme Member
Re: how to test fuel injectors
Since you're probably going to replace the injectors in sets you really don't have to know which individual injector is leaking. Just that there is leakage present.
Perform a fuel pressure drop test:
Connect a pressure gauge to the fuel rail and start the engine. Turn off the engine and note the pressure reading vs. time. The fuel system should hold this pressure for 30 minutes or so. It may drop a few PSI but that should be considered normal. If there's leakage, it will typically drop pretty quickly.
If the pressure drops quickly, that's an indication that there's leakage someplace. You need to confirm that the leakage is not at the pressure regulator or in the return line to the fuel tank (I think there's a check valve in the fuel pump).
Pressure Regulator:
You can pull the vacuum connection from the pressure regulator. No fuel here confirms the regulator has no leak.
Fuel Return:
To check the fuel return system, do the pressure drop test but pinch off the rubber return line to the tank after you shut down the engine and note pressure vs. time.
If you still get a pressure drop you've confirmed the leakage is at the injectors.
If you don't have access to a pressure gauge, you might be able to tell if there's injector leakage by running the engine and then shutting it down. After a few hours or so (or overnight) pull the plugs and see if they're wet with fuel. The only problem with this is you need to pull all the plugs because it might be just one injector that is leaking. Pull the easy to get to ones first!
Perform a fuel pressure drop test:
Connect a pressure gauge to the fuel rail and start the engine. Turn off the engine and note the pressure reading vs. time. The fuel system should hold this pressure for 30 minutes or so. It may drop a few PSI but that should be considered normal. If there's leakage, it will typically drop pretty quickly.
If the pressure drops quickly, that's an indication that there's leakage someplace. You need to confirm that the leakage is not at the pressure regulator or in the return line to the fuel tank (I think there's a check valve in the fuel pump).
Pressure Regulator:
You can pull the vacuum connection from the pressure regulator. No fuel here confirms the regulator has no leak.
Fuel Return:
To check the fuel return system, do the pressure drop test but pinch off the rubber return line to the tank after you shut down the engine and note pressure vs. time.
If you still get a pressure drop you've confirmed the leakage is at the injectors.
If you don't have access to a pressure gauge, you might be able to tell if there's injector leakage by running the engine and then shutting it down. After a few hours or so (or overnight) pull the plugs and see if they're wet with fuel. The only problem with this is you need to pull all the plugs because it might be just one injector that is leaking. Pull the easy to get to ones first!
07-27-2014, 11:44 AM
#4
Since you're probably going to replace the injectors in sets you really don't have to know which individual injector is leaking. Just that there is leakage present.
Perform a fuel pressure drop test:
Connect a pressure gauge to the fuel rail and start the engine. Turn off the engine and note the pressure reading vs. time. The fuel system should hold this pressure for 30 minutes or so. It may drop a few PSI but that should be considered normal. If there's leakage, it will typically drop pretty quickly.
If the pressure drops quickly, that's an indication that there's leakage someplace. You need to confirm that the leakage is not at the pressure regulator or in the return line to the fuel tank (I think there's a check valve in the fuel pump).
Pressure Regulator:
You can pull the vacuum connection from the pressure regulator. No fuel here confirms the regulator has no leak.
Fuel Return:
To check the fuel return system, do the pressure drop test but pinch off the rubber return line to the tank after you shut down the engine and note pressure vs. time.
If you still get a pressure drop you've confirmed the leakage is at the injectors.
If you don't have access to a pressure gauge, you might be able to tell if there's injector leakage by running the engine and then shutting it down. After a few hours or so (or overnight) pull the plugs and see if they're wet with fuel. The only problem with this is you need to pull all the plugs because it might be just one injector that is leaking. Pull the easy to get to ones first!
Perform a fuel pressure drop test:
Connect a pressure gauge to the fuel rail and start the engine. Turn off the engine and note the pressure reading vs. time. The fuel system should hold this pressure for 30 minutes or so. It may drop a few PSI but that should be considered normal. If there's leakage, it will typically drop pretty quickly.
If the pressure drops quickly, that's an indication that there's leakage someplace. You need to confirm that the leakage is not at the pressure regulator or in the return line to the fuel tank (I think there's a check valve in the fuel pump).
Pressure Regulator:
You can pull the vacuum connection from the pressure regulator. No fuel here confirms the regulator has no leak.
Fuel Return:
To check the fuel return system, do the pressure drop test but pinch off the rubber return line to the tank after you shut down the engine and note pressure vs. time.
If you still get a pressure drop you've confirmed the leakage is at the injectors.
If you don't have access to a pressure gauge, you might be able to tell if there's injector leakage by running the engine and then shutting it down. After a few hours or so (or overnight) pull the plugs and see if they're wet with fuel. The only problem with this is you need to pull all the plugs because it might be just one injector that is leaking. Pull the easy to get to ones first!
Alright thanks. I got a harbor freight tester so I'll give that a shot today. I plan on replacing all the injectors if this is the problem. Gonna get Bosch 24 lb Gen 3 from south bay along with an afpr maybe. Not sure how to set one of those though. But I will try all this first.
07-27-2014, 02:39 PM
#5
Re: how to test fuel injectors
alright, i got my testing done. i noticed as soon as i turn the key, it shoots up to about 35 psi, then drops to 0. when i run the motor, the psi fluctuates from about 30-40psi, but almost like its just jumping. i pulled the vacuum line off the FPR (just put a new diaphram in it) and no fuel came out of that, but it still dropped to 0. finally, i clamped off the fuel return line under the engine bay, and the psi remained at about 35, although very slowly dropped but never went to 0. so i guess my problem is the fuel pump then? not the injectors? not too sure where to go from here
07-28-2014, 06:16 PM
#6
Sponsor
Re: how to test fuel injectors
another test you can do for leaky injectors is to run the engine for a bit and then pull the plugs. If you have any wet cylinders then you have a leaky injector
07-28-2014, 06:39 PM
#7
Re: how to test fuel injectors
my father just bought a well running 1988 vette, gonna try this test on his, and see what the results are on his car.
one of the things which is peculiar to me, is the gauge never shows a steady pressure, its just bouncing about 10psi. i'm wondering if this may be part of my problem with why its not running right. although it seems like its getting too much fuel, and if the gas was all going back through the return too quickly, then i'd have a problem with not enough fuel. and the car doesnt have the punchiness to it like it would if it was low on fuel, instead it has the bogging down like its running too rich, and smells it too
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07-28-2014, 11:21 PM
#8
Supreme Member
Re: how to test fuel injectors
not sure i want to get my hands too close to the headers when they get hot lol. when i did the test, it never dropped pressure when i had the return line pinched with a pair of vice grips (atleast i think it was the return line, its the 5/16" rubber line right? lol) so thats why i think its the return line. i'm just not positive where to go from here, because i dont think i had this problem before the swap, dont know why it would just pop up like this.
my father just bought a well running 1988 vette, gonna try this test on his, and see what the results are on his car.
one of the things which is peculiar to me, is the gauge never shows a steady pressure, its just bouncing about 10psi. i'm wondering if this may be part of my problem with why its not running right. although it seems like its getting too much fuel, and if the gas was all going back through the return too quickly, then i'd have a problem with not enough fuel. and the car doesnt have the punchiness to it like it would if it was low on fuel, instead it has the bogging down like its running too rich, and smells it too
my father just bought a well running 1988 vette, gonna try this test on his, and see what the results are on his car.
one of the things which is peculiar to me, is the gauge never shows a steady pressure, its just bouncing about 10psi. i'm wondering if this may be part of my problem with why its not running right. although it seems like its getting too much fuel, and if the gas was all going back through the return too quickly, then i'd have a problem with not enough fuel. and the car doesnt have the punchiness to it like it would if it was low on fuel, instead it has the bogging down like its running too rich, and smells it too
.....
If you don't have access to a pressure gauge, you might be able to tell if there's injector leakage by running the engine and then shutting it down. After a few hours or so (or overnight) pull the plugs and see if they're wet with fuel. The only problem with this is you need to pull all the plugs because it might be just one injector that is leaking. Pull the easy to get to ones first!
If you don't have access to a pressure gauge, you might be able to tell if there's injector leakage by running the engine and then shutting it down. After a few hours or so (or overnight) pull the plugs and see if they're wet with fuel. The only problem with this is you need to pull all the plugs because it might be just one injector that is leaking. Pull the easy to get to ones first!
Open the hood and leave it open. Headers don't stay hot for very long. Not like cast iron manifolds.
07-29-2014, 06:26 PM
#10
Supreme Member
Re: how to test fuel injectors
The tests call for pinching the line(s) after the fuel system is pressurized (key ON). Because you're losing pressure as soon as the pump prime cycle stops it would help to get someone to help you run the tests. Your helper will either need to operate the key or do the hose pinching.
The FSM also mentions that the best way to determine if you have leaking injectors is to remove and examine the plugs. If they're not wet or fouled, then follow the flow chart.
07-29-2014, 06:28 PM
#11
Re: how to test fuel injectors
alright, i tested the pressure on my dad's vette, his pressure goes from 45, and drops to about 42psi upon turning the key and holds pretty steady there without any problems. i think i can rule out the fuel injectors on my car and figure its the checkvalve.
also when his car was running, the pressure jumped around but only from about 40 to about 38 psi, my car is jumping from nearly 40 down to almost 27psi. which raises question on if it could possibly have anything to do with the FPR too, when rebuilding the motor, i replaced the diaphragm in the FPR, but would this cause such a jump in pressure or would that be the check valve still?
thirdly, how would this constant jumping from the fuel pressure effect the driveability? the car tries to die if i'm not keeping it at about 1k rpms constantly.
and finally, do you guys think there is a way to not have to pull the fuel tank all the way out in order to get the pump out? like just drop it and have it have it hang by the filler neck and still reach the pump? should i also replace the pulsator too?
also when his car was running, the pressure jumped around but only from about 40 to about 38 psi, my car is jumping from nearly 40 down to almost 27psi. which raises question on if it could possibly have anything to do with the FPR too, when rebuilding the motor, i replaced the diaphragm in the FPR, but would this cause such a jump in pressure or would that be the check valve still?
thirdly, how would this constant jumping from the fuel pressure effect the driveability? the car tries to die if i'm not keeping it at about 1k rpms constantly.
and finally, do you guys think there is a way to not have to pull the fuel tank all the way out in order to get the pump out? like just drop it and have it have it hang by the filler neck and still reach the pump? should i also replace the pulsator too?
07-29-2014, 06:38 PM
#12
Re: how to test fuel injectors
I dug out my FSM and found the flow chart you need to follow. It's attached. I was incorrect in describing the test procedure. I'm very sorry for that.
The tests call for pinching the line(s) after the fuel system is pressurized (key ON). Because you're losing pressure as soon as the pump prime cycle stops it would help to get someone to help you run the tests. Your helper will either need to operate the key or do the hose pinching.
The FSM also mentions that the best way to determine if you have leaking injectors is to remove and examine the plugs. If they're not wet or fouled, then follow the flow chart.
The tests call for pinching the line(s) after the fuel system is pressurized (key ON). Because you're losing pressure as soon as the pump prime cycle stops it would help to get someone to help you run the tests. Your helper will either need to operate the key or do the hose pinching.
The FSM also mentions that the best way to determine if you have leaking injectors is to remove and examine the plugs. If they're not wet or fouled, then follow the flow chart.
07-29-2014, 06:55 PM
#13
Supreme Member
Re: how to test fuel injectors
according to the flow chart, it would be fuel pressure regulator and not the pump. since it says if you pinch off the return line it would be the FPR. although i'm not sure which line it is talking about with "pinching flex hose" i pinch the rubber part of the return line, is that the same? or is that pinching the return line lol
The return line is the other line.
When the FSM instructs you to pinch the "flex hose" or "hose", it's referring to the hoses that are on the driver's side of the engine compartment.
07-30-2014, 09:09 AM
#15
Re: how to test fuel injectors
just redid the test, and its still coming up as the FPR. this fuel rail came with another motor i had bought which wound up having a blocked crack. couldnt remember if the fuel rail and injectors had gone underwater during hurricane sandy along with the rest of the motor, i know i had the plenum off but not sure of the rest, so i replaced the diaphragm just in case....i guess that FPR must have been totally garbage somehow. i'll try taking the old FPR that i still have from the old 305 and trying that out, since i know it worked, and see if that fixes this issue. thank you very much for the flow chart, be lost without you guys and this site, and my car would be in the junkyard by now lol
07-30-2014, 04:55 PM
#16
Update: tore it down, put on the regulator from the 305 motor and before rebuilding the whole runners and plenum again, hooked up the fuel rail and lines and hit the key to prime it. Pressure went up and held. Although I think due to the air in the lines right now, the pressure hasn't come up above like 30 psi yet, I think once I crank it over and get just fuel in there it should get a better pressure.
Thank you guys for all your help, it would be awesome if this solved my drieability problem too lol
Thank you guys for all your help, it would be awesome if this solved my drieability problem too lol
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