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Fuel Pressure Problem DESPITE new Fuel Pressure Regulator
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Post Fuel Pressure Problem DESPITE new Fuel Pressure Regulator 
I'm getting confused.

Symptoms before fuel pressure regulator replacement:

(Other than occasional long cranking times to run, it ran great, snappy and has smooth power at all times and RPM)

Fuel pressure with engine off (after cycling the ignition a couple of times to make the pump prime): 60 psi.
Fuel pressure with engine off holds steady 60 psi for over 5 minutes with very little bread down.

Fuel pressure with engine running: 51 psi (engine sometimes barely starts)
Fuel pressure with engine running AT FLIP OF THROTTLE: 60 psi (jumps to 60 psi instantly with the flip of the throttle).

OK, so I thought with this throttle flip situation it has to be the fuel pressure regulator and I replaced it (I installed Standard Motor Products# PR 203).

The problem is that the symptoms have remained EXACTLY THE SAME despite the new fuel pressure regulator installed.

Is my newly installed fuel pressure regulator faulty as well? Or am I missing something?

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I think you should only have to turn the key once, if you are having to keep cycling the key it could be your pump is weak, what is the truck doing just hard to start? Is the problem the same cold or hot? Is this a l31 or lq4?

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It is a L31 all stock. Other than already described, it is simply a hard start condition. Once the vehicle is running you would not know the difference to new. It runs great. Also if you cycle the ignition (prime the lines), it starts well too. Starting is better with the engine warm e.g. after shutting it off 15-20 minutes and trying to restart.

Hopefully I did not install a new but dud fuel pressure regulator. I could live with having changed a good fuel pressure regulator for a new-good one and the problem being somewhere else.

I’m going to look into the fuel pump as well. It is a Bosch 67368 and it is not too old. I installed it about 75k miles ago.

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What's your fuel pressure when you cycle the key once?

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I have checked again.

After manually reliefing pressure and starting with 0 psi, while cranking, the pressure quickly went up to 60 psi and the engine started. It needed longer than usual, about 1.5 second, but it did start and ran. I guess in those past times when it did not start well, the pressure did not rise quite fast enough.

Then when running the pressure settled at a steady 52 psi. Flipping to throttle it would go to 60 psi and beyond for a blink of an eye and would settle back at 52 psi. No hesitation all smooth. While slowly raising RPM, the pressure would hold 52 psi almost steady. Throttle response would be good at any RPM.

It is almost as the fuel pressure regulator was set to 52 psi operating pressure (but then again, so was the one that I removed/replaced as well).

After turning the engine off, the pressure would climb to 65 psi (for the few seconds that the pump is running at engine shutoff), then with the pump stopped fuel pressure would instantly go to 60 psi.

I can repeat these test as many times as I wanted and it would always be the same.

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Your pump is fine if you turn the key on and get 60psi with out having to keep turning on and off

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In my desperation, today I changed the fuel filter, but no improvement of course. I knew it would not help, because I recently changed it just a few thousand miles ago, but I changed it again anyway.

So this is where I am. I'm getting beautifully regulated 52 psi while running. Once started the engine runs smooth and strong and without hesitation, instantly responds to throttle at any rpm and any load.

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hello good sir
u should do this
the GM MPFI conversion

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Mr.Pro you might be on to something.

Today I installed a new fuel pump. But no changes.

However probing pump voltage, I have noticed about 1 volt lower voltage at the pump (measured grey pump power wire against a good ground) than battery voltage. I measured voltage on the pump while running and then went ahead and measured voltage at the battery.

Pump ground seem to have been OK.

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This morning I found my battery negative contact side terminal bolt completely without threads. So I installed new side terminal bolt.

I slightly dropped the tank in order to this time more accurately probe grey feedwire voltage at the pump against battery voltage. Measured 0.63 Volt difference with engine running. According to Delphi the difference shall not exceed 0.2 Volt, according to Airtex 0.5 Volt, for the feedwire respectively. I checked ground connection as well against battery and it is solid.

I probed the fuel pump relay contacts 87 and 30 with running engine against battery as well (pin 87 is switched to pin 30 by the relay, pin 30 becomes grey fuel pump feed wire). Pin 87 had 0.16 Volt difference and pin 30 had 0.17 Volt difference. Difference from pin 30 to pin 87 was 0.01 Volt. I think the relay is good.

I checked connector C103. It connects the grey wire that is coming from pin 30 of the relay to the grey wire in the vehicle harness. It is located under the brake booster and is a flat 4 pin Metripack 280 connector with only 2 pins used (this is possibly for 1999 only). I opened the connector to looks for anything unusual, but the contacts looked like new. I measured resistance from pin 30 to the connector pin and got 0.4 Ohm. So I believe that should be good as well. I was trying to measure resistance from the other end of this connector to the fuel pump connector, but I shamefully gave up due to lack of a long proper gauge wire. I was thinking to run the fuel pump via jumper wire straight from battery voltage as well, but just as shamefully gave up for the same lack of long proper gauge wire. At this point I started to doubting pump voltage problem anyway. So I put everything back together and called it a day.

I will keep testing and report back here.

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Post Re: Fuel Pressure Problem DESPITE new Fuel Pressure Regulato 
I've been out of the game for a while but was reading this and was wondering what your the pressure problem was. Is the problem you are seeing that the pressure drops to 51 psi while the engine is running? If it is, I think that is normal. At ambient atmospheric pressure (no vacuum or boost) the regulated pressure should be at 60 psi on our trucks. When the engine is running and there is a vacuum inside the manifold the regulated pressure will drop to somewhere around 48psi to 52psi (from memory). Once you blip the throttle (no vacuum), the pressure goes back to 60 psi for that second. From what you are describing, its doing exactly that. But maybe I am misunderstanding the problem.

-Archie

1999 Tahoe 4x4 wrote:
I'm getting confused.

Symptoms before fuel pressure regulator replacement:

(Other than occasional long cranking times to run, it ran great, snappy and has smooth power at all times and RPM)

Fuel pressure with engine off (after cycling the ignition a couple of times to make the pump prime): 60 psi.
Fuel pressure with engine off holds steady 60 psi for over 5 minutes with very little bread down.

Fuel pressure with engine running: 51 psi (engine sometimes barely starts)
Fuel pressure with engine running AT FLIP OF THROTTLE: 60 psi (jumps to 60 psi instantly with the flip of the throttle).

OK, so I thought with this throttle flip situation it has to be the fuel pressure regulator and I replaced it (I installed Standard Motor Products# PR 203).

The problem is that the symptoms have remained EXACTLY THE SAME despite the new fuel pressure regulator installed.

Is my newly installed fuel pressure regulator faulty as well? Or am I missing something?


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Hi Archie, thanks for chiming in.

Yes, you are reading correctly. Regulated 52 psi pressure with the engine running. Throttle blip brings it up to 60 psi, with quick acceleration pressure briefly rises as well and settles in at 52 psi at any rpm.

Problem is, that according to GM Service Information, idle fuel pressure can't be less than 55 psi (but that is really the one and only reference I can find about idle fuel pressure in the GM Service Information Book - all other printed testing reference is with the engine off or throttle flip). Could this 55 psi even be a misprint? Although according to some unofficial non-GM internet sources idle fuel pressure supposed to be 55-57 psi.

BUT, if you are saying that you (and perhaps others) are running 52 psi as well, then I might be chasing a ghost here.


So far my hard start issue seem to have been gone with the new fuel pump installed. At least it has not done it in the short time since the install. So I'm wondering if the fuel pump was on it's way out and had some intermittent issue and I just stumbled upon the 52 psi reading and started chasing that instead of the pump. The next few days will show if the hard start condition is 100% cured.

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