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Losing Fuel Pressure

jtupper

By jtupper
in Ask A GM Technician

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jtupper Enthusiast

jtupper

Posted
Posted

I'm having issues with the fuel pressure dropping after it sits for a while. I had another thread going about it HERE. I ended up replacing the fuel pressure regulator and it was fixed...for about two days. Now it's doing it again, although not nearly as bad as it was. I don't see any fuel in the vacuum line on the FPR like I did in the old one.

 

Some background...

 

This is the 3rd FPR I've put in it since I bought it. Th two times before, I could turn the key over and let the fuel pump cycle and it would start right up. This time it wouldn't do that so I thought maybe it was the fuel pump going out. I ended up taking the chance and bought the FPR. Like I said, all was good for two days. Defective maybe???

 

I went downand picked up a fuel pressure gauge this afternoon and took some readings. When I turn the key on and let the fuel pump cycle, the pressure goes up to about 50 psi. When I start her up, the pressure drops to about 42-44 psi and holds steady. When I turn it off, the pressure builds back up to about 50 psi again over the course of about two minutes, then slowly begins to drop. The pressure dropped about 10 psi over the first half hour, then dropped all the way to zero within the next half hour.

 

It has a brand new fuel filter also. I can find no visible leaks nor do I smell fuel. This leads me to beleive that either the FPR is defective or maybe I have a leaking injector. The real question is how do I figure out exactly what the problem is? I could really use some help here.

 

Appreciate it guys... :fume:

txab Senior Enthusiast

txab

Posted
Posted

When the ignition switch is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gauge should read 379-427 kPa (55-62 psi). The spring pressure inside the fuel pressure regulator controls the fuel pressure.

 

A fuel system that drops more than 34 kPa (5 psi) in 10 minutes has a leak in one or more of the following areas:

 

• The fuel pump check valve

 

• The fuel pump flex pipe

 

• The valve or valve seat within the fuel pressure regulator

 

• The fuel injectors

 

A fuel system that drops more than 14 kPa (2 psi) in 10 minutes after being relieved to 69 kPa (10 psi) indicates a leaking fuel pump check valve.

 

Fuel pressure that drops-off during acceleration, cruise, or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. You can diagnose a lean condition using a scan tool. If an extremely lean condition occurs, the heated oxygen sensorss will stop toggling. The heated oxygen sensor output voltages will drop below 300 mV. The fuel injector pulse width will increase.

jtupper Enthusiast

jtupper

Posted
  • Author
Posted
The fuel injector pulse width will increase.

Wow! What a deep explanation. What does this mean exactly? Does that mean it will put out more fuel to compensate for the lean condition? I've noticed over the course of the last year that my fuel economy is progressively dropping.

 

I'm a relatively mechanically inclined person. I can replace an injector, if need be, but not quite sure about dropping the fuel tank. I can't exactly lift the bed off a Tahoe. More importantly, how can I determine with any certainty that it's the fuel pump and not an injector? I'm already about $80 in. I don't want to just throw money at parts without knowing if that's actually the culprit.

 

Thanks for the quick response.

txab Senior Enthusiast

txab

Posted
Posted

If lean, it will try to add fuel to correct the mix

 

Again, if you only get 50 psi, you are below spec.

 

 

If it had leaky injectors, you would be able to find out by pulling the plugs and looking for wet ones

jtupper Enthusiast

jtupper

Posted
  • Author
Posted
If lean, it will try to add fuel to correct the mix

 

Again, if you only get 50 psi, you are below spec.

 

 

If it had leaky injectors, you would be able to find out by pulling the plugs and looking for wet ones

Just for grins, I hooked up my gauge to my wife's 03 Tahoe and I got the exact same readings, minus the drop off. Kepping in mind that this is a cheap-o Harbor Freight tool, it probably is not what one would consider a precision instrument.

 

By your statement that a drop of more than 2 psi over 10 minutes when below 10 psi, doe that mean that it could ONLY be the fuel pump check valve? What really perplexes me is that it worked flawlessly for two days after replacing the FPR. Looks like I have a fuel pump replacement in my future.

 

So, I have two other questions...can you recommend a good aftermarket or do I go with OEM? Lastly, can anyone point me to a good how to on dropping the fuel tank? Everything I can find in the search talks about trucks and lifting the bed.

 

Again, thanks for the input txab!

jtupper Enthusiast

jtupper

Posted
  • Author
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can you recommend a good aftermarket or do I go with OEM? Lastly, can anyone point me to a good how to on dropping the fuel tank?

 

Anyone?

 

I'm looking to pick up a new pump this week and try to get it installed this weekend.

jtupper Enthusiast

jtupper

Posted
  • Author
Posted

Okay, this is getting really frustrating. I picked up a new Delphi fuel pump this week and installed it. What a royal PITA it was to drop that tank. Anyway, I got it swapped out and put it all back together only to discover that I'm still losing fuel pressure.

 

The numbers did come up a little bit. The prssure reads about 53-54 psi when the pump cycles and about 48 psi at idle. it's lost about 10-12 psi over 30 minutes. I still have the gauge on it and will see how much more it drops. I'm about $325 ionto this and still don't have the issue resolved.

 

Where should I look next?

jtupper Enthusiast

jtupper

Posted
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I'm still looking for some help here guys. So far, the fuel pressure has not dropped below 20 psi, but I haven't been able to leave the gauge on it for more than 4 hours. I'm going to hook it up tonight and leave it until morning and see where it sits.

 

Since I have already replaced the pump, regulator and filter, I'm thinking the injectors are all that's left. Seems to me I should be able to pull the fuel rails and pressurize the system to see if any of the injectors are leaking. Apparently, these injectors a known for lraking issues.

caddytech10 Enthusiast

caddytech10

Posted
Posted

I would double check the new fuel pressure regulator you just installed. There is a small o-ring on the end of the regulator that fits into a small boss in the fuel rail. If that o-ring gets damaged during installation it will allow fuel to bypass the regulator. When installing the regulator I like to coat the o-ring and the boss with transmission assembley lube (vasaline will work fine also) to prevent tearing the o-ring. It's just a guess, but I have seen a few of them installed incorrectly before. Hope that helps.

nluchau Enthusiast

nluchau

Posted
Posted

My mechanic checked my fuel pressure on Monday and with the key in the on position (pump running) it was at 59. Engine running it was at 55. Removed the vaccum line from the fuel pressure regulator and it went back up to 59. He said my fuel pump and (newly installed FPR) was working fine. Long story short the FPR was leaking fuel out the vaccum line, this is why it was replaced.

 

This is a simple test to determine if your FPR is working as it should.

 

Oh almost forgot, I have an 02 Silverado 5.3

jtupper Enthusiast

jtupper

Posted
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I'm fairly certain it is NOT the FPR. I thought about this myself already and had O'Reilly swap out the FPR under warranty thinking it might be defective. The O rings and screen were fine. I will pull it again tonight and check the O ring though.

 

I left the fuel pressure gauge on it all night and it bled down to zero when I checked it this morning. I'm really thinking it has to be the injectors. I just don't know how to find out for sure. I pulled all of the plugs, but I wasn't real sure what I should be looking for. None of the tips were wet although the threads were on a few, but did not smell like gas.

jtupper Enthusiast

jtupper

Posted
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I'm really at my wits end with this thing. I pulled the FPR and all o-rings were fine and no fuel in the vacuum line. After that I decided to pull the fuel rails to get a look at the injectors. I pressurized the system and I can't see any of the injectors leaking. I checked all of the fuel lines real well before I bolted the tank back up and There is no leak at the fuel filter.

 

I'm ready to pull my hair out and I ain't got much as it is. :rolleyes:

caddytech10 Enthusiast

caddytech10

Posted
Posted

I have also seen bad check valves in brand new fuel pumps before too. Really the only way to isolate your fuel pressure drop is blocking off the feed line and the return line with a fuel line block off kit. I don't know if you can get one or not, but it's basically a set of hoses with fittings on each end that connect between your fuel rail and your feed and return lines. They have valves on them so you can open and close them. What you do to check for an injector leak is pressurize your fuel system (key on) with a fuel pressure guage attached to the service port. Then close both valves on the return and feed block off lines and monitor your guage. If the system keeps pressure you know that injectors are ok and the problem may be down stream (fuel pump or fpr). On the flip side, if it didn't hold pressure then it could be an injector or an fpr leaking into the vacuum line. To check the fuel pump, presurize the fuel system and only block off only the return line. If it holds pressure then you know the the fuel pump is ok and the problem may be with the fpr. If you loose pressure, the pump check valve is bad and you need a fuel pump. To check the fpr, pressurize the system and then block off only the feed line. If the system holds pressure then the fpr is ok. If you loose pressure then the fpr is allowing fuel to pass into the return line. In that case you have a bad fpr.

Stevenduchnych Enthusiast

Stevenduchnych

Posted
Posted

I did the exact same thing> what i would recommend is going to a shop ask them to do a injector calibration to find out which injector is bad. i am guessing you are having a hard starting issue after it sits for awhile.

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