Odd readings for a stumbling, hesitating vortec 5.7L

Discussion in 'Chevy C/K Truck Forum' started by jcaldwell109, Jul 24, 2013.

  1. jcaldwell109

    jcaldwell109 New Member

    New to the forum with (maybe) an old problem.

    After letting my 1996 Chevrolet 1500 work truck with 5.7L vortec v-8 and 4L60e trans (203K on the engine, recent rebuild on trans) sit in the driveway for a couple weeks, I hopped in to head to the parts store. And...

    At very low load, rpm between 1000-1500, it accelerates normally for a couple seconds, then nearly dies for about 2-3 seconds, then "catches up with itself" and pulls hard. Did the same thing for nearly every upshift. When I tried, from a dead stop, to accelerate enough to get the truck moving then floored it, same thing. It "died" enough to throw me forward, then caught up and pulled hard to 5000+ rpm.

    So, I headed out today to try to get some more info--and buy a can of MAF cleaner, just in case.

    Here's what I found, and it puzzles me some.

    There are no codes or Check Engine Light. The hesitation is initially there as before, but after I drove for about 15-20 min, it seemed to be gone. While I was driving, I was watching my scanner which was set to the monitor mode. Every thing looked pretty normal, except the timing. At idle, the timing was -15 or -16 degrees. When I accelerated, it went up(?) to -33 degrees. When the hesitation hit, it dropped to -20, -12, even as low as -4 degrees, only to instantly go back to -33 when the hesitation stopped. Remember that this hesitation lasts only about 1.5-2 seconds. Much later, when I got back home, the idle timing was -24 degrees, but the driving timing was still usually -33. There were some times that it was mid-20s. When I step on the gas, from a setting of -33, timing changes, very rapidly and of short duration, to around -19 degrees. Then it goes back "up". I checked the TPS and it usually showed 3-8% open. Load was 4-18%, depending on going uphill or downhill.

    Now, it's been a while since I really played with things like timing and carburation (I told you it was a while ago), but I remember setting my chevy v-8 at about 10 degrees at idle and having about 32-34 degrees at max advance. I don't even remember getting an engine to run with negative timing.

    The readings from all O2 sensors look correct. The front ones rapidly flucuate from about .12 to .87. The rear fluctuate from about .06 to .18. The TPS shows good throttle position throughout the range. One thing that I'm unsure about is the Long Term Fuel settings. The scan tool showed them at -11%. That seems a little high to me, but I don't really know what they should be. Inlet air temp looked correct at about 102. Air flow seemed reasonable for the load factor.

    The fuel filter is about 5k miles old, so I'm not convinced it is at fault. I can't, thus far, find my fuel pressure gauge to check for low pressure. However, the way the truck pulls at full throttle, after it gets over its hesitation, makes me think the pump is working properly.

    I thought the crankshaft position sensor might be a culprit, until I did a web search for how it works. Seems to be an either/or. Either it works, and the engine runs, or it doesn't, and the engine won't start. I couldn't find anything that talked about variable timing being generated by the CPS.

    I'll clean the MAF, since I got the cleaner at the parts store, but don't expect that to be the problem.

    I guess next up is the distributor. I've heard that the drive gear can wear badly and cause problems. However, a bad gear would seem to be a candidate for problems throughout the operating range, and not just at low rpm. Am I wrong on that? Wouldn't the distributor also cause misses throughout the operating range?

    The fact that this hesitation/stumble seems to occur only at fairly low rpm--and now seems to go away after 15-20 minutes--really has me puzzled.

    Any ideas????


  2. Conlan Rose

    Conlan Rose Epic Member 5+ Years 1000 Posts

    Your Vortec has variable timing. This means that the ECM choses how it should adjust the ignition depending on engine load and other variables. this allows the truck to get the most power possible out of each power stroke and to ignite the fuel/air mix at just the perfect time.

    I own a 96 Tahoe which has the same engine and the timing varies all the time to help increase power and use less fuel. It really sounds like the ECM is forgetting your hitting the throttle and not giving the engine the right amount of fuel.

    - - - Updated - - -

    I just went out to the truck and attached my OBDII bluetooth adapter to see the timing change at idle and at 1500 rpm. When at idle when the my truck is running open loop for fuel is at about -22. When I hit the gas a bit it changes to about -11. On the Highway at higher rpm its at around -34. So I think the timing has 0 to do with the hesitation its just dependent on the engine's rpm since the numbers I'm seeing on my healthy truck are the same as yours.
  3. jcaldwell109

    jcaldwell109 New Member

    Thanks for checking the timing with your Tahoe. I feel a lot better knowing that my timing readings are in the ball park.

    When I saw the timing numbers drop with the hesitation, I didn't know if timing caused the hesitation or was a result of the hesitation. I tend to agree with you now that the problem is fuel supply. I cleaned the MAF with the spray, though it didn't really look dirty. I'll probably wait until tonight or tomorrow to check whether it had an effect. I want to make sure the engine is cold when I try my test. Murphy says it has to be something more convoluted than just cleaning the MAF.

  4. jcaldwell109

    jcaldwell109 New Member

    Cleaned the MAF. Let the truck sit overnight and drove it cold. No improvement, maybe worse than before.

    At about 1500-1600 rpm, truck stumbled/hesitated/died back to about 1000 rpm for about 1.5 seconds, then recovered. Did the same thing again, a couple times. First time seemed to coincide with 1-2 shift. Others did not coincide with shift.

    I already know from monitoring the scan tool that the timing changes a lot when the truck stumbles/hesitates/dies, then recovers when the rpm recovers.

    Still haven't found my pressure gage to check the fuel pressure. I'm wondering about the possibility of fuel pressure problems, though, because of how the problem lessens as the truck warms up. Seems that fuel pressure would get worse with higher temps.

    Any ideas?


  5. BackwodsChevy

    BackwodsChevy New Member

    Ok the pump should put out 13 to 9 lbs if you have a throttle body .I would check the TPS and see if it is puting out the correct voltage at full throttle.I think it is 5 volts.It should increase and decrease as you open and close the butterfly
  6. jcaldwell109

    jcaldwell109 New Member

    Checked the throttle position sensor using the scan tool. It checks good, from idle to full throttle, in that the scan tool shows throttle position. It derives position from the sensor, so the sensor must be working correctly.

    The truck is a 96, with Vortec FI instead of a throttle body. I've read that even fuel pressure of 50psi is not enough for the engine to run properly, but can't really verify that information.

  7. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    I'll verify the 50 psi number. About a year ago, my '98 would stumble and hesitate on acceleration and was intermittently hard to start. When it was having trouble, the fuel pressure was between 50 and 60 psi.
  8. jcaldwell109

    jcaldwell109 New Member

    So the solution was to replace the fuel pump? Did you do that and solve the problem? What pressure does the new fuel pump put out?


  9. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    Yes, I replaced the pump. Fuel pressures are now consistently 60+ psi (depending on engine vacuum). Of course, there was more to diagnosis than just reading the fuel pressure. There were a few other tests to make sure it wasn't a problem with the regulator or the injectors. If you have low pressure, then clamp off the return line and the pressure jumps up, that indicates that the regulator is opening prematurely, for example.
  10. Rebel_Bowtie

    Rebel_Bowtie Rockstar

    jcaldwell it really sounds like your distributor. That's the same thing my 93 did 3 days before it gave up on a hill. New distributor and timing adjustment and it was good as new.

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