Low initial fuel pressure
03-21-2015, 07:28 PM
#1
Intermediate
Thread Starter
Low initial fuel pressure
1996 lt-1. I've done dozens of searches and followed their advice. I have an FSM and followed the trouble shooting guide. No lights and no codes. Fuel filter replaced last fall (about 2000 miles ago). A couple of times within the last month or so I had a hard start condition after the car sat for a week or two without being started. This was the first time I've ever experienced this even when sitting for longer periods. This presented as multiple long cranks before starting, a moment of stumbling, then running smoothly and strongly. Restarts no problem and no problems with hot starts or when started daily or even every few days.
I purchased a pressure gauge and discovered that my best initial "key on"- "car off" pressure reading after a couple of cycles of this with 10 seconds in between was 20 psi. After starting the car, the pressure increased to 38 psi and to 42 psi with vacuum hose off the FPR.
After turning car off, the pressure slowly dropped off to 25 psi after an hour, 15 psi after 1.5 hours, and down to 0 psi after 2 hours.
No gas or smell in regulator vacuum hose, however there appeared to be some leaking around the body of it and possibly down on the engine. Because of this and since I think it was original to the car, I replaced it with an AC/Delco unit. After installation, my initial pressure readings remained unchanged although after starting car and building up to 38 psi then turning it off, the pressure dropped to 26 psi after 10 minutes, then to 16 psi after 3 hours and held there for 14 hours until I checked it the next morning.
The car starts quickly and runs fine with no definative symptoms, even started right up after sitting for 5 days. As mentioned, I followed the trouble shooting chart in the FSM to the best of my ability and ruled out electrical, no obstructions to flow, and was left with the fuel pump as the possible culprit. I thought maybe it was just getting weak since it too was an original unit. I changed that today along with the strainer (sock) and I am still getting the exact same readings as before.
20 psi at "key on", 38 psi at "ignition/start up" 42 psi at WOT (vacuum hose off FPR. No visible leaks.
There's not much else i can think of other than the injectors and I have not done any testing on them. I thought I might of had one or more leaking which caused some flooding of a cylinder/s and the hard start after the prolonged sitting. Plugs are such a pain to pull, I haven't bothered to to check them for fouling.
Could a leaking injector/s cause this low initial pressure, especially in the absence of poor performance and fuel smell/rich smelling exhaust?
Sorry for the long narrative. I just wanted to be thorough. I could sure use some ideas!
I purchased a pressure gauge and discovered that my best initial "key on"- "car off" pressure reading after a couple of cycles of this with 10 seconds in between was 20 psi. After starting the car, the pressure increased to 38 psi and to 42 psi with vacuum hose off the FPR.
After turning car off, the pressure slowly dropped off to 25 psi after an hour, 15 psi after 1.5 hours, and down to 0 psi after 2 hours.
No gas or smell in regulator vacuum hose, however there appeared to be some leaking around the body of it and possibly down on the engine. Because of this and since I think it was original to the car, I replaced it with an AC/Delco unit. After installation, my initial pressure readings remained unchanged although after starting car and building up to 38 psi then turning it off, the pressure dropped to 26 psi after 10 minutes, then to 16 psi after 3 hours and held there for 14 hours until I checked it the next morning.
The car starts quickly and runs fine with no definative symptoms, even started right up after sitting for 5 days. As mentioned, I followed the trouble shooting chart in the FSM to the best of my ability and ruled out electrical, no obstructions to flow, and was left with the fuel pump as the possible culprit. I thought maybe it was just getting weak since it too was an original unit. I changed that today along with the strainer (sock) and I am still getting the exact same readings as before.
20 psi at "key on", 38 psi at "ignition/start up" 42 psi at WOT (vacuum hose off FPR. No visible leaks.
There's not much else i can think of other than the injectors and I have not done any testing on them. I thought I might of had one or more leaking which caused some flooding of a cylinder/s and the hard start after the prolonged sitting. Plugs are such a pain to pull, I haven't bothered to to check them for fouling.
Could a leaking injector/s cause this low initial pressure, especially in the absence of poor performance and fuel smell/rich smelling exhaust?
Sorry for the long narrative. I just wanted to be thorough. I could sure use some ideas!
03-21-2015, 09:40 PM
#3
Intermediate
Thread Starter
I didn't really think it was injector related either, John. I'm just out of ideas. I even considered the new fuel pump may be bad, but since I got the exact same readings as the original pump, that seemed unlikely. Old pump was likely still good, but maybe on borrowed time with 90,000 miles on it.
I suppose the fuel pressure gauge could be suspect?
I suppose the fuel pressure gauge could be suspect?
03-21-2015, 10:31 PM
#4
I didn't really think it was injector related either, John. I'm just out of ideas. I even considered the new fuel pump may be bad, but since I got the exact same readings as the original pump, that seemed unlikely. Old pump was likely still good, but maybe on borrowed time with 90,000 miles on it.
I suppose the fuel pressure gauge could be suspect?
I suppose the fuel pressure gauge could be suspect?
03-21-2015, 10:52 PM
#5
Intermediate
Thread Starter
Jay-I replaced the FPR with a new AC/Delco unit because the old one seemed a little stained on the outside, possibly by leaking fuel. Initial pressure reading after installation of new FPR did not increase, however pressure held much longer. Leak down tests on the new one confirm it is good.
03-22-2015, 11:48 AM
#6
Intermediate
Thread Starter
Thinking about it all night.
When I replaced the pump, I did not replace the pulsator with the section of rubber hose. I left the original pulsator in place.
Can a worn/leaking/defective pulsator cause low static fuel pressure?
Would you recommend going back in and doing this since I think I've covered all the other bases? Or, just wishful thinking?
When I replaced the pump, I did not replace the pulsator with the section of rubber hose. I left the original pulsator in place.
Can a worn/leaking/defective pulsator cause low static fuel pressure?
Would you recommend going back in and doing this since I think I've covered all the other bases? Or, just wishful thinking?
03-27-2015, 04:29 PM
#7
Intermediate
Thread Starter
I don't like threads with no closure since I don't learn anything definitive from them. So, in order to not leave any hanging/loose threads, (pun intended) I thought I'd follow up on my own post. Prior to doing anything else, I decided to drive the car some to see how it felt. The next day I put the pressure gauge back on the car and, voila' 42 psi at key on!
I have attributed this to my not having purged the fuel lines completely of air. I did use the pressure relief valve on the gauge until I had a steady stream of gas, but apparently this wasn't enough.
So, moral to the story, I guess, is to make sure and run the engine a while prior to rechecking your fuel pressure after opening up the lines.
Thanks to both of you who offered input and I hope this helps some one else down the line.
I have attributed this to my not having purged the fuel lines completely of air. I did use the pressure relief valve on the gauge until I had a steady stream of gas, but apparently this wasn't enough.
So, moral to the story, I guess, is to make sure and run the engine a while prior to rechecking your fuel pressure after opening up the lines.
Thanks to both of you who offered input and I hope this helps some one else down the line.