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GM Bob
GM Bob, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1103
Experience:  30 plus years. experience as GM technician, ASE Master Technician and GM Certified.
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When testing fuel pump pressure,

Customer Question

When testing fuel pump pressure, the shop manual says 57-63 psi. Is this at idle with the regulator vacuum hose disconnected and plugged? I am only getting 43 psi. with regulator connected at idle and 57 psi disconnected.
I am trying to solve a P300 multiple misfire error that happens at highway speeds.
Submitted: 2 years ago.
Category: Chevy
Expert:  George H. replied 2 years ago.
Hello I will help you with your question,
The specification is with the hose connected and the pressure gauge screwed into the test port.
It does sound like you have low fuel pressure even though you have replaced all the components in the system.
Have you checked your pressure gauge on another vehicle?
Customer: replied 2 years ago.
NO, but I can take one of my Freon gauges and do the same thing... the tester is brand new, I bought it for this...
Customer: replied 2 years ago.
Ok, using one of my refrigeration gauges I get the identical reading. 43 reg connected, and now 52 with vacuum hose off.
Expert:  George H. replied 2 years ago.
Ok, you have everything covered for the pressure test, you only have 43 psi and need 60. You need to go back and find where it is leaking past the check valve or see why you have such a wide spread with the regulator connected and disconnected. There should be about 5 psi not 14.
Sounds like it might be an autozone regulator?
Let me know how I can help
Thank you
Customer: replied 2 years ago.
Actually I had a NAPA regulator, and that was fluttering. Now I have a Delphi.
Customer: replied 2 years ago.
Leak down test holds the same pressure for over 10 minutes... does that eliminate the regulator?
Expert:  George H. replied 2 years ago.
I would not eliminate anything, the spread is too much while it is operating. I can't tell you what it is from here but you need more fuel pressure to get rid of the miss.
I will OPT OUT and maybe another tech will have a suggestion for you
Sorry we could not reach a solution
Customer: replied 2 years ago.
In ever got a solution, and I was billed for this. Please advise.
Expert:  GM Bob replied 2 years ago.
Hello my name is ***** ***** you still need help?
With your P0300 code
Customer: replied 2 years ago.
Yes Bob, I do. Have you seen the details of my issue?
Expert:  GM Bob replied 2 years ago.
Have you replaced any crank or cam sensors?
This only happens at highway speed?
Do you feel any misfire?
Can you scan the engine when driving show live data of the misfire?
If so does it show all cylinders misfire?
Sorry for so many questions
Thank you
Customer: replied 2 years ago.
Hello Bob,
Cam and crank sensors; I have not replaced any cam or crank sensors. I do not have a starting problem. This vehicle has been running like this for 20k miles.
Highway speed: The check engine flash is only at highway speed, and from the tach and scan tool I can see it raises the OD ratio so I am losing fuel economy by reving slightly higher.
Feel misfire? I can see a slight one at idle, and for a 6.0 it doesn't have the power it should. My 5.3 Tahoe will walk all over it at low end.
Scan tool; yes. I drove out west and back watching it. It is cylinder 6, a slight on 8 and 1.
Here is the work I have done to the vehicle to remedy to no avail:
1.Both fuel pumps, which were noisy and had bad sensors anyway.
2. Ping sensors and harness while replacing intake
3. sent fuels injectors out to be sonic cleaned, came back with 100% ok and I was told they weren't that bad anyway.
4. Plugs and wires, which are burning very light tan
5. Upper co2 sensors
6. Tried a Napa fp reg but it chattered, now I have a BWD on there.
Others have pointed to my fuel pressure. At idle, I have 43 psi with the regulator vacuum line connected, and 52 with vacuum hose off. This is a GM pump I bought on Amazon in April of 2014, and changed the filter twice since.
Your thoughts?
Expert:  GM Bob replied 2 years ago.
I have had several 6.0 engine do this.
The first thing I do with scan tool do a crankshaft position variation learn procedure. This fixed 80% of them.
The next if still showing misfire. I do a PCM reprogram amd crankshaft position variation learn.
Most the time it is a false misfire code setting.
If that don't work pull both front O2 sensors and drive it see if the power is back. May have catalytic converters plugging. They will cause misfire.
Let me know if this helps.
Customer: replied 2 years ago.
So it sounds like I need to take it to a dealer for this? A Snap On Solis scan tool will not to a relearn, correct?
Expert:  GM Bob replied 2 years ago.
I'm not sure about the scan tool.
If you have a special function test. May want to look in there or PCM setup.
I Hope this helps. Keep me posted
Thank you
Customer: replied 2 years ago.
I have a guy at a local garage checking to see if his new Snap On can do it on Saturday. Can I do this if I buy EFI live?
And you don't this fuel pressure is the issue, eh?
Expert:  GM Bob replied 2 years ago.
I have never used EFI live.
The fuel pump spec is 55-62 with engine of fuel pump running.
Yours look good
Have you drove it with fuel gauge on it see if it drops?
Customer: replied 2 years ago.
With Vac Hose off I get 52... on 44.
To test if while driving I need a mighty long hose. Its doable, should I try it?
Expert:  GM Bob replied 2 years ago.
I think your fuel pressure looks ok.
If you want to make sure its not dropping to low run with longer hose.
Most time when pump gets week the engine set lean codes. Also backfires on hard acceleration.
Customer: replied 2 years ago.
OK... looking to get my crank relearn done, will be in touch!
Expert:  GM Bob replied 2 years ago.
Ok sounds good.
Expert:  GM Bob replied 2 years ago.
Have any luck with the relearn?
Let me know if I was helpful.
Thank you

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