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Pete
Pete, GMC Technician
Category: GM
Satisfied Customers: 15518
Experience:  14 years GM dealership technician and training.
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94 GMC: jump..Fuel pump, fuel injector assembly..fuel filter..No leaks

Customer Question

94 GMC JIMMY- Fuel Pressure

Description: After two seconds of turning key fuel builds up to 54 lbs then when relay kicks out pressure drops to 10 lbs. Will not crank during cold weather sitting overnight, it will crank and run fine after using a jump booster for battery and cycling fuel pump relay several times.

History: Fuel pump, fuel injector assembly, fuel pump relay, fuel pump and sock, and fuel filter are new and have been installed within last two weeks. No leaks detected on any fuel line.
Submitted: 3 years ago.
Category: GM
Expert:  Pete replied 3 years ago.
Hello,I will be glad to try and assist you with this,Welcome to Just Answer.

Hi.

I just need a little more info to assist better.

When you replace the fuel injector unit was this the entire unit with the fuel pressure regulator attached?

Also when it does start does it run rough or rich at all at first?

Have you had the battery,alternator or draw tested for the battery concern?

Let me know so we can continue,Thanks Pete!
Customer: replied 3 years ago.
When you replace the fuel injector unit was this the entire unit with the fuel pressure regulator attached? Yes

Also when it does start does it run rough or rich at all at first? Runs smooth

Have you had the battery,alternator or draw tested for the battery concern? New Battery full charge and amps, 14.5 Volts from alternator.
Expert:  Pete replied 3 years ago.
O.k.

First to test to see why and where the fuel pressure is dropping of at there should be two rubber hoses in between the metal fuel feed and return lines under the vehicle near the frame rail on the drivers side up front,pressurize the fuel system then before the pump stops running clamp both of these fuel feed and return lines,if your fuel pressure drops then the drop is up front in the engine area.The most common cause is the fuel injector unit but another very common cause in the engine for the fuel pressure to drop is the nylon/plastic fuel lines that go into the fuel injector unit,these crack and can leak fuel into the engine especially when removing them to install a new injector unit,this is when they usually crack the most.

These become very brittle after years of use and I have made it a practice to replace these short lines under the upper intake when replacing the fuel injector unit.One way you can tell if this is the leak down for sure is by removing the upper intake and turning the key on to prime the pump,then look to see if these lines are leaking down the fuel pressure,most likely they are and need replaced.You can get these lines for around $50.These are very common to leak out the pressure.

if you clamp the hoses and the pressure does not drop then the leak down of the fuel pressure is in the tank and is the fuel pump check valve,even if the pump has been replaced you may have gotten a faulty fuel pump and it may need replaced again,the parts warranty will cover giving you a new part in exchange for this one.


As for the battery we know the battery tests good and is new and the alternator is charging so this only leaves a draw on the battery system.You will have to disconnect the negative cable from the battery,hook e multimeter in between the negative cable and the negative post on the battery,then turn the meter on to the amp scale and read how many amps the vehicle is drawing with all doors closed and the hood light off.If it is above 30 or 40 milliamps then you have a draw that is draining the battery down over night.Then you can pull each fuse one at a time to locate which circuit the draw is on,once the milliamp reading drops on one particular fuse this is the circuit at fault,then it can be determined what is on this circuit and each component on this circuit unplugged until the draw is found,then that component repaired or replaced.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 3 years ago.

What is the best way to check pressure aft of the clamped hoses? If the pressure does not drop for the fuel injector assbly then release clamp and look for a pressure drop or connect pressure gage to fuel line near fuel filter? Fuel pump is an Airtex (auto zone special), original equipment was a Carter.

Expert:  Pete replied 3 years ago.
Air tex is a good design for the fuel pump but can be faulty in some cases.

You should have a fuel pressure test port on the engine at the back of the upper intake,test the pressure here.

Clamp the lines when testing fuel pressure here,then if it drops still then the leak is under the upper intake and remove the intake for inspection,prime the pump and see where it is leaking from.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Pete, GMC Technician
Category: GM
Satisfied Customers: 15518
Experience: 14 years GM dealership technician and training.
Pete and 3 other GM Specialists are ready to help you
Customer: replied 3 years ago.

Thanks Pete,

We will use your procedure, if we cannot locate the problem we will get back to you.

 

Happy holidays,

 

Leon

 

Expert:  Pete replied 3 years ago.
Sounds good,let me know what you come up with,happy holidays to you as well.

Thanks Pete!

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