Fuel Pressure Regulator

[GREGB1954]

What is the correct fuel pressure at idle, and when the ignition is turned to Run but not Start? I am getting 50psi at idle. At Run but not started, I am getting about 60psi until I hear the fuel pump stop, then it drops to 50.

My symptom is it really struggles to idle at cold start. Other than that it runs great including warm idle.

BTW, my 5.3l is in a custom car. But its crate engine is an early 2000 Gen III. The fuel pressure regulator is vacuum controlled and is located in the fuel rail.

 

[justirv]

What is the correct fuel pressure at idle, and when the ignition is turned to Run but not Start? I am getting 50psi at idle. At Run but not started, I am getting about 60psi until I hear the fuel pump stop, then it drops to 50.

My symptom is it really struggles to idle at cold start. Other than that it runs great including warm idle.

BTW, my 5.3l is in a custom car. But its crate engine is an early 2000 Gen III. The fuel pressure regulator is vacuum controlled and is located in the fuel rail.

What is the correct fuel pressure at idle, and when the ignition is turned to Run but not Start? I am getting 50psi at idle. At Run but not started, I am getting about 60psi until I hear the fuel pump stop, then it drops to 50.

My symptom is it really struggles to idle at cold start. Other than that it runs great including warm idle.

BTW, my 5.3l is in a custom car. But its crate engine is an early 2000 Gen III. The fuel pressure regulator is vacuum controlled and is located in the fuel rail.

Your fuel pressure sounds about right, and flex and gas have different ranges. Someone with more expertise will fill in the details here. My experience with rough cold idle is intake manifold gaskets. They shrink with age and the cold makes it worse. Once warmed up they expand and re-seal. Try to spray around there to see if your idle changes.

 

[Fless]

For fuel pressure, according to Haynes:
KOEO (key on, engine off)
Flex Fuel: 48-54 psi
Non-Flex: 55-62 psi

Running fuel pressure is usually in the mid-40s. If you have a decent scanner, see what the short- and long-term fuel trims look like. If it's constantly taking away fuel (negative trims) then maybe the pressure is too high.

The rough cold idle might be caused by a vacuum leak.

 

[nonickatall]

I don't think your problem is caused by wrong fuel pressure, sounds more like problems with intake air leak as justirv already said.

Why is thet cold a problem? Because you o2 sensors start working when engine reaches temp,. Before the ecm works with standard values. The O2 sensors can compensate leaking in air to a certain amount.

Make a smoke test to check for intake leaks..

 

[GREGB1954]

I discovered a fuel leak that is running down the back of the engine on the driver side. The fuel lines in that area seem dry. I am wondering if there is an intake manifold leak that is causing a vacuum issue. Is it possible that fuel could leak out of it? FYI: The engine has a Magnuson MP112 supercharger.

 

[Fless]

Since the fuel pressure drops right after the pump stops, there's likely a fuel leak somewhere on or near the top of the engine. The fuel pressure shouldn't drop more than a few lbs over a 5 minute time period after the pump stops.

 

[GREGB1954]

I found the leak from none other than my fuel pressure gage. So resolved it and now get this sequence:
1. Turn key to Run: 58psi as the fuel pump primes
2. Pressure drops to 40psi as the fule pump stops.
3. For some pecular reason the pressure actually rises very slowly to 46psi and holds it for some time.
4. When I turn the key to Start, the pressure holds at 50psi and the car stumbles a bit then finds a good cold start idle.

So, I think the fuel pump system is working correctlyi. I think I might have a small vacuum leak?

 

[nonickatall]

There can be a lot of problems which make this behavior.

Anything from problems with the o2 sensor to the air mass sensor, a defective temperature sensor, vacuum leak, defective spark plugs, defective ignition coils, anything can produce this behavior.

Yes, I know the o2 sensors only work when the engine warms up, but if incorrect adaptation values are stored, it can also lead to a poor cold start.

Did you check for error codes?

 

[GREGB1954]

There can be a lot of problems which make this behavior.

Anything from problems with the o2 sensor to the air mass sensor, a defective temperature sensor, vacuum leak, defective spark plugs, defective ignition coils, anything can produce this behavior.

Yes, I know the o2 sensors only work when the engine warms up, but if incorrect adaptation values are stored, it can also lead to a poor cold start.

Did you check for error codes?

We have chased down a few of those items which led us to evaluate the fuel pressure. The cold start idle problem is absent if you turn the key to Run, then off, then Start. It idles perfectly cold and warm. It also runs fine cold and warm if I do the Run/Off/Start routine. I have heard different thoughts on whether the pressure dropping to 40psi after the first prime is correct. Also, the pressure is 50psi when the engine is running. What is the correct pressure during these points?

 

[Fless]

The link below (for a 2000 2wd Tahoe with a 5.3L engine -- if yours isn't that, use the links at the top to navigate to the same section for yours) lists some troubleshooting items re fuel pressure and how much drop is allowed after the pump stops. It also has a test for how much the pressure drops with the engine running, using a test vacuum source (that can diagnose a faulty fuel pressure regulator).

Fuel Pressure: Testing and Inspection — 2000 Chevrolet C Tahoe 2WD V8-5.3L VIN T Service Manual | Operation CHARM

Detailed repair manual for the 2000 Chevrolet C Tahoe 2WD V8-5.3L VIN T.

charm.li

 

[GREGB1954]

The link below (for a 2000 2wd Tahoe with a 5.3L engine -- if yours isn't that, use the links at the top to navigate to the same section for yours) lists some troubleshooting items re fuel pressure and how much drop is allowed after the pump stops. It also has a test for how much the pressure drops with the engine running, using a test vacuum source (that can diagnose a faulty fuel pressure regulator).

Fuel Pressure: Testing and Inspection — 2000 Chevrolet C Tahoe 2WD V8-5.3L VIN T Service Manual | Operation CHARM

Detailed repair manual for the 2000 Chevrolet C Tahoe 2WD V8-5.3L VIN T.

charm.li

This is most helpful. I do not precisely know what equivalent Tahoe/Yukon year the engine is, but my best guess is an early 2000's Gen III with a vacuum pressure regulator. My configuration differs from the diagnosis link in that my fuel pump is not in the tank but is inline located just after the tank. I don't think that would make a difference with the diagnostics. Reading your diagnostic link it appears that the pressure should hold with no less than 5psi from a prime of 55-62. My prime is 60 and drops suddenly to 40 whe the pump stops. The next diagnosic step requires you put shut off valves on both the supply and return lines which I do not have. This allows you to isolate if the problem is with the pump or regulator. I also heard that a clogged filter could be the culprit too. Without those shut off valves I'm wondering how I can determine which part it is. The regulator holds vacuum so I wonder if that indicates it is good. I have read that this Walbro GSL 392 pump has been problematic and short-lived.

 

[GREGB1954]

Another thing to share with those following this thread: The car only has 2,000 mi on it. I had pulled all of the injectors about 4 years ago and had them tested. All were within spec. So I don't suspect they could be leaking. The car sits a lot and it could be the tank has developed some sludge which could have clogged the pre and post filters. So I think the next thing I will do is run the engine for a few minutes and see if the pump feels warm to the touch which is a symptom for this particular pump when its is failing (Walbro GSL 392). If it is hot i'll replace it along with the filters and see what I get. I will likely use a different pump than this model as it has a poor track record from the comments I have read on it.