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97 Chevy Truck Fuel and / or Spark Issue

Discussion in 'General Chevy & GM Tech Questions' started by dkeys23, Jul 18, 2011.

  1. dkeys23

    dkeys23 New Member

    I have a 97 chevy K1500 5.0L with approx 200K miles. Occasionally I would get a rough idle and stumbling at low speeds. I previously (bout 2 months ago) checked fuel pressure and it was a bit low at about 52psi. If I pinched the retrun line a bit, the pressure would spike up to 75 psi (I then shut it down so I didn't spike the pump with excessive psi). So I figured the fuel pressure regulator may be bad since it appeared the pump can supply the needed pressure. I replaced it and the pressure reading after was approx 58 psi and it would hold.

    About 2 months later, the stumbling started happening again and this time set an engine light of P1351 High ICM voltage. I was a little suspicious that this may be the result, and not the cause. I checked it out. It had good ground. I had approx 2.7vac between the ICM and the 12v while cranking (spec is 1-4 vac). I had continuity to the ecm connector, even while wiggling the harness. On a hunch, I checked the fuel pressure again since even though it felt like a spark/timing issue, it might be fuel related. With key on, it will run to 60psi and then as soon as the pump stops, it drops down to 0psi. The strange thing is that the truck will start.

    Has anyone seen a pump pressurize to 60psi, then drop to 0psi and still start? If the regulator was stuck open in bypass, would the injectors even fire? Once the engine starts and is running, I have approx 50 psi and it will jump towards 60 psi if I rev the engine a bit.
  2. stephan

    stephan Rockstar 4 Years 5000 Posts

    Has anyone seen a pump pressurize to 60psi, then drop to 0psi and still start?
    Yes it will still start because as soon as you cycle the key & crank it, it re-runs the fuel pump & you have pressure again. As for where your fuel pressure is going, it's either through the return line (bad FP regulator again) or is bleeding back through the supply line back through the pump. Do the same test that you did before by pinching off the return line to determine which line it's leaking back through. This is all assuming that the pressure isn't bleeding off through some bad injectors, but if it was it would flood it & probably wouldn't start at all without a whole lot of cranking...
  3. dkeys23

    dkeys23 New Member

    Hey Stephan, actually while you were posting I did venture out to see "where the fuel was going". I pinched the return line off and same thing. 57 - 60 psi, then bleed to 0psi within a couple of seconds. Then I had my wife cycle the key and while the pump was just stopping, I pinched the supply line hoping to see if I could trap the fuel in the line if it was bleeding back through the pump. Same thing. 57-60psi then bleed to 0 psi. Now I guess that leaves the injector, but as you mentioned, I would be surprised that it is starting so easy and running.

    Does this confirm injector(s), or does this info help ask another question?
  4. stephan

    stephan Rockstar 4 Years 5000 Posts

    I pinched the return line off and same thing. 57 - 60 psi, then bleed to 0psi within a couple of seconds.
    If I read that right, you pinched the return line off & the pressure held? If yes, then it's not bleeding back through the supply line & the injectors are good. It's a bad fuel pressure regulator.
    If you were to pinch off the return line & the pressure dropped to 0 psi, then it would be bleeding back through the supply line or the injectors.
  5. dkeys23

    dkeys23 New Member

    sorry if I created confusion.

    I pincehd the return line and the pressure built to 60 ish, then bled right down to zero (clamp still on the line) just as it does with no clamp. So that should rule the pressure regulator out. Then I had my wife key on to engage the pump and right as it was ending its cycle, I pinched the supply line (in hope to trap pressurized fuel in the rail to be read on the gauge). This had the same result, pressure built to 60 ish while the pump was running, but bled right down to zero (clamp still on the line). So I would think this rules out bleeding back through the pump.

    Does this sound like injectors? Can they be replaced individually (assuming I pull the upper intake and see one bleeding fuel) or do they all have to be swapped? I see from the reciepts that I got with the truck that the previous owner swapped entire spider about 5 years ago to the day with a unit from Standard Motor Products. If I end up swapping these out I will most likley go for Delco or Delphi parts.
  6. stephan

    stephan Rockstar 4 Years 5000 Posts

    I really doubt that it's leaking through the injectors cuz it would run like crap if it ran at all. As fast as it's dropping to zero it would flood the snot out of it, it would be difficult to start, & it would be smoking black out the exhaust. I think what's happening is that you aren't able to completely pinch off the line. Before I faulted the injectors I would fit a plug to the return line & re-run your test. Just plug it completely off & have your wife turn on the key for a second to energize the pump while you watch the guage & see if it holds after she turns it off.
  7. dkeys23

    dkeys23 New Member

    I am fairly certain I got the hose pinched off (same manner with hose pinch off pliers that I used in the past to diagnose the regulator), but I will run another test tomorrow as you have a fair point and it is worth checking. For S&Gs I wanted to check and make sure my gauge was good and that the bleed wasn't stuck open. I rigged the gauge up to a set of A/C manifold gauges and hit it with 70 psi of compressed air and closed the manifold valve. The fuel gauge held the pressure.

    As far as what the vehicle is doing, it hesitates and has horrible throttle response with light throttle inputs. If I stand on it, it "catches up" and runs fairly well. Almost like a carbureted vehicle that has too big of an accelerator pump or boosters. If it were just one injector, it would only impact 1 cylinder, correct? The engine could run on 7 cyl. If that is the case I would think I should be able to see this in the plugs if I check them after an extended idle low load drive. Anything with heavy throttle may clear it up. Just typing as I am thinking "out loud". Any comments, questions, confirmations?
  8. stephan

    stephan Rockstar 4 Years 5000 Posts

    It's good that you double checked your guage.
    If it were just one injector, it would only impact 1 cylinder, correct? Yes, but I'm still thinking 60 psi of fuel leaking through 1 or 2 or even all 8 injectors into the cylinders everytime you shut it down would flood the crap out of it & this would be obvious on your next restart cuz that's a LOT of fuel, & yes this should show up on your plugs. The injectors may not be the greatest, but I think the fuel is returning to the tank.
    Have one further que. After re-reading your posts above, when you say: With key on, it will run to 60psi and then as soon as the pump stops, it drops down to 0psi. You mean within a few seconds or minutes right? You're not talking about doing the one hour leak down test, correct?
  9. dkeys23

    dkeys23 New Member

    I am tending to agree with you after logic had a chance to sink in over predetermined emotion. I blame the heat index of 110F that I was working it :)

    Yes, the pressure would bleed off 0 psi in a matter of 2-3 seconds. As I said, I was pretty confident that I had the return hose pinched off, but can't say the same on the supply line due to the trying to pinch it off at the end of the pump cycle. The return line I have all the time in the world to pinch it off before the pump engages.

    Before focusing on the return line, I want to isolate the pump. My intention is to remove the fuel filter and come up with an adapter to get my gauge right on the line at the filter. This will take the regulator and return line out of the loop. I will have to make sure I don't pressure spike the pump, but if your intial assumption (and lately mine too) is correct, then if the pressure bleeds off immedieately, then it is definately the pump. If it passes that test then I will plug the return line and test.
  10. dkeys23

    dkeys23 New Member

    Thanks again for the help. Previously I isolated the pump by making an adapter to hook my fuel pressure guage right to the filter. Again, it would not hold pressure. After I hooked everything back up, I plugged the return line at the union and it again would not hold pressure. I ordered a Carter fuel pump module from Rock Auto for about $150 (had to wait a week, but it was at least a $100 cheaper than the stores) since I read to stay away from the other brands esp Airtex. Swap was relatively straight forward. I ended up replacing the straps since I had to sacrafice one of the bolts. New pump is very quiet. Key on pressure goes to 58psi, then drops to about 54psi after the pump stops. This is a bit under the spec in the manual, but it is running really well right now. Maybe tomorrow I will measure the voltage that is going to the pump. If I don't quite have 12v (or more like 13+v), that may explain the slightly low pressure. Pump module came with new wire harness. I also ground down the rust on the frame where the ground attaches. Again, thanks for the help!

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