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2004 gmc envoy_slt
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Discussion Starter · #21 ·
Is it common for fuel pumps in these vehicles to go bad, or suddenly get very weak, without tripping codes?

I swear it has the symptoms of a bad fuel pump, but no code. At a constant speed, say on the interstate, going up a hill you can feel it stutter, and it will "fall on its face" acceleration-wise and then you've got to stab the throttle just to maintain speed.

The hard start issue remains, and sometimes holding the key turned will take 15-ish seconds to get it started. I have seen the digital mpg readout lose 0.3mpg just starting it. once. It also has, what feels like a random stutter at idle that you can feel in the floor/pedal, just noticed that today...almost like it has idle problems, but its not "searching for idle" or surging/idle fluctuation. I swear the pump is bad, but its weird theres no code, maybe its "on its way out" but just wont die and throw the code.

I've had the FPR & Filters, and gas cap, replaced as far as the fuel system goes.

As for the transmission, we did the filter/fluid replacement and that helped with the vibration somewhat. It did drive better, and got better mpg, but there is more in there.

As for the mpg, well its getting about 280-300 miles to the tank now, up from 210, 211, and 220 when i first discovered this problem.
 

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Put a fuel pressure gage on the fuel rail supply and stick the gage under the wiper arm with some duct tape - drive full throttle to 6300 upshift and watch the pressure - does it drop ?
 

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Discussion Starter · #23 ·
where did i see a diagram of that schrader valve?
 

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Discussion Starter · #24 ·
ok. My buddy called to tell me the fuel pressure goes to 0 almost instantly after shutting it off. He used the valve at the fuel filter. They drove around a bit and got on it a little, no pressure drop, he said it stayed at ~58psi

They have 2 snap on scanners they used to look at things and reset them each, and for some reason some things are giving off really wacky readings. He told me it saw 13,000rpms on the scanner, the intake temp was 240 degrees, and that it read my coolant temp was (-40) degrees, etc etc

wtf?
 

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Discussion Starter · #25 ·
we are convinced the fuel pressure relief/check valve is bad, is it serviceable or is that one of those buy the whole new pump assembly things?

gonna hook it up to his HP Tuners to watch a few variables to see if it just doesn't like Snap On stuff tomorrow. if they are still throwin out wacky variables to those sensors he says i may have some "5 volt issue" gremlin

anyone?
 

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Discussion Starter · #26 ·
didnt like snap on stuff. the hptuner variables looked within range although it was seeing 8 degrees of retard on acceleration....

up to about 15.5mpg and around 300 miles per tank.
 

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Discussion Starter · #27 ·
just thought about an update....a month or so ago, i finally got around to throwing some money at it again. Replaced both motor mounts with OEM mounts, and before i even drove it again, i held the removed mounts in my hands to inspect. No tears, no leaks, no visible signs of damage....Held it by the bolt and shook them a little and there's where it was. One of them, presumably the passenger side mount, was loose/rattled there in my hands, so it was surely amplified under the motor.

No more vibration/shake.
 

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Pretty sure I've just got 2 issues that combined threw me off. The hard start issue, might be a bad relief valve. The vibration it seems is coming from my transmission, but being amplified in the front end of the car. Checked the tranny fluid yesterday and it didn't smell good, wasn't the cherry red you'd like it to be, and had some grainy material in it, possibly "clutch" material. Gonna change the filter at least once and maybe twice to clean it out. The "shudder" it seems comes from the tq converter, as it feels a little loose and that's making it drive a little less efficiently.

hopefully check valve can be verified this week and filter replacement on Sunday.
Do you remember how it ran afterwards? I just got my tranny rebuilt and my TB is acting the exact same, new cats are clogged, fuel in my intake not getting burned right and my brake booster hose Po off. Has a hard time starting, large Lea 455 code and 420 catalytic converter codes. What helped? Veteran, wife and toddler need help. Fan clutch pulley does have a bit of play upwards, was louder after p/s line replacement.
 
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