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'99 7.4 Vortec Rough Idle

Discussion in 'Chevy Trucks' started by MikeRi24, Oct 10, 2011.

  1. MikeRi24

    MikeRi24 Senior Member
    Messages: 562

    Truck recently had an exhaust manifold replaced. When its cold (as in first thing in the morning), it fires right up and high-idles for a few seconds and runs fine. After the first few seconds, its idles rough. The engine struggles to keep it above 7-800 RPM. When its in gear at idle (like a stop light), it struggles to stay above 500 RPM, and will occasionally stall. On any start after the initial cold start in the morning, it is hard starting. You have to feather the gas a little and it shudders but it will start then you have to give it a little gas or it'll stall. Driving, its fine as long as you are moving. It runs like a champ. Upon acceleration from a stop, there is a slight hesitation but once it's over that it runs flawlessly until it needs to idle at a stop again. I've been reading through the factory service manual, and theres more information in there on how to diagnose a squeaking serpentine belt than an issue like this. There is no CEL on and I scanned the computer for codes and nothing in the memory. I'm stumped.

    What I've tried/done so far:
    -New Plugs/Wires/Cap/Rotor
    -Remove and clean Idle Air Control, test resistance and it falls within factory spec
    -Replace PVC valve and vacuum line for it
    -Clean Mass Air sensor
    -Replace Intake Air Temp Sensor
    -Run a fresh tank of gas with fuel injector cleaner
    -Remove and clean throttle body
  2. the new boss 92

    the new boss 92 2000 Club Member
    Messages: 2,989

    o2 sensor, sounds dumb my 92 did this and i replaced that and it worked. not saying its a cure cause your truck should have 3 and mine had 1. bust out your ohm meter and test them to see what happens. although mine tested good it was still shyt!
  3. Milwaukee

    Milwaukee 2000 Club Member
    Messages: 2,180

    Can you test oxygen sensors though scanner with real live data?

    That sound like egr issues but that on my 94 explorer it would run poor no good acceleration after unplug vac line to EGR it run great. Mileage went up from 15 to 19-20 mpg.
  4. MikeRi24

    MikeRi24 Senior Member
    Messages: 562

    I will check that and look through the shop manual to see what the ohm spec is.
  5. MikeRi24

    MikeRi24 Senior Member
    Messages: 562

    did you just plug the vac line up? and yes you can check the output of the o2 sensors with the scan tools, but it cant hurt to double check. Although if the scan tool was picking up a fault with them, then theoretically the CEL would be on, especially in an OBDII vehicle I'd think.

    THEGOLDPRO PlowSite Veteran
    Messages: 3,136

    My 97 454 has a rough idle as well, I also tried a bunch of things that haven't fixed it either, Ny idle isnt as rough as yours tho it never stalls out or really stumbles.
  7. MikeRi24

    MikeRi24 Senior Member
    Messages: 562

    I can understand it having a little rumble to it, from what I understand they put a good size cam in these things to get power, but nothing like what I have haha
  8. NWanner

    NWanner Member
    Messages: 64

    I'd look at the upstream O2 sensor as well, especially if the manifold was just replaced. The downstream sensor is just a nanny for the cat and doesn't effect fuel/air at all. The computer only throws a light when it starts throwing bad voltage, but just because the voltage is within specs doesn't mean it isn't fubar'd. EGR is only supposed to operate at highway speeds, but could be something to look at as well.
  9. MikeRi24

    MikeRi24 Senior Member
    Messages: 562

    I looked at the EGR alright....unbolted it and when I pulled it off to inspect it the mounting flange cracked in my hands. $90 at the parts store later, it has a new EGR and it still idles rough so thats not the problem hahaha
  10. the new boss 92

    the new boss 92 2000 Club Member
    Messages: 2,989

    you can only test an os if it throws a code, unless you have a really high tech scaner( witch i dont) then you have to ohm it and even then they test good sometime till they get hot from driving and they go ham and send weird shyt through the computer.
    in order to test the egr properly you have to pull it and see if it hold vaccume, cause most of the time when an egr goes the diaphram splits inside. it still kinda functions but will do what this guys trucks doing!
  11. Holland

    Holland Senior Member
    Messages: 605

    Fuel pressure regulator. And it sucks to get to. Time to pull the upper plenum off. Pretty common on the 7.4. They start to leak fuel into the intake.
    Last edited: Oct 11, 2011
  12. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Ditto. Get a scanner on it and check the LTFT's. If they're pegged at +25 then the FP regulator is leaking. Very very common issue on the 7.4's. If you don't have scanner access connect a FP gauge, cycle the fuel pump a couple times and then watch the gauge for drop off. If it drops more than about 10 PSI in 20 minutes or so the reg is leaking into the intake.

    They'll also lose injectors sometimes also. They'll stick open thus any time there's fuel pressure it's injecting unmetered fuel into the intake creating a rough idle/rich running condition. Those can easily be diagnosed with a temp gun on each exhaust port.
  13. crete5245

    crete5245 Member
    Messages: 59

    I agree with the o2 sensor check, but there is a problem using a scan tool. It is not fast enough to see the voltage change above and below the thresh hold voltage of .450.
    You need a lab scope that will take reading as fast as several thousand a second.
    If you can get a shop to scope it, make sure it switches from rich to lean. And make sure it has a good sweep.
  14. 70monte

    70monte Senior Member
    Messages: 468

    I have a 98 K3500 with the 7.4L and it idles rough. I have replaced the distributor, cap, rotor, coil, plugs, wires, fuel filter, and both pre-cat O2 sensors. I would also guess the fuel pressure regulator or injectors because of your hard starting symptom. As others have stated, these are problem areas with this engine.

    A lot of people replace the factory multec injectors with Bosch 19 lbs injectors which from what I understand are a drop in replacement. They supposedly are a better design than the factory ones. Some people on another forum I go to that use the Bosch ones really like them.

  15. MikeRi24

    MikeRi24 Senior Member
    Messages: 562

    Actually ended up being an EGR issue. After I replaced the EGR valve, it finally threw an EGR code, I cleared the code and it has been fine since!
  16. Milwaukee

    Milwaukee 2000 Club Member
    Messages: 2,180

    Glad to know it was EGR valve issue Thumbs Up When it get bad then your mileage GO SUNK.
  17. dieseld

    dieseld Senior Member
    from 90210
    Messages: 627

    Great reading here. Just bought a 1998 K2500 7.4L myself.