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Loping idle on my new engine (and old one).

Discussion in 'Chevy Truck Talk & GM News' started by Ape, May 10, 2012.

  1. Ape

    Ape Member 100 Posts

    Those of you who read my last thread ( ) will know that I just replaced my old worn out (about to blow up) engine (350/5.7L) in my 89 Burb with a GM crate motor.
    It took me 2 weekends of hard work, one pulling and one dropping it in, along with some time inbetween doing easy stuff like switching the intake manifold and TBI to the new engine. That and I cleaned up alot of the old stuff and engine bay.
    I never thought about it at the time, but the old engine had this "loping idle" thing that it did from the time I got it about a year ago. I just ignored it since I knew the engine was tired and it only did it in neutral or park. But if I would've thought about it while switching engines I might have done some reading to see what the issue might have been related to.
    So you can see where this is leading to.....Now with the new engine in and running nicely (13 miles on the break in so far, along with a bunch of non miles running time in the garage), the "loping idle" thing is still there. :-(

    So I've done some reading and figured the issue was related to my EGR system. Consequently I started out by replacing my EGR valve. The old one was sticky so I figured that must be it.... Not! So next I replaced the Idle Air Control Valve and it's still loping.....Today I got my EGR Solenoid Control Valve in and installed it. At this point I thought it had to be the culprit since it idles just fine when I disconnect the vacuum hose connected to it from the TBI. Yet it still lopes as if it's laughing at me! lol
    Now I'm thinking that maybe the power going to the EGR Solenoid Control Valve is either weak or nonexistant? And I'd check to see if my hunch was correct if I had an Ohm meter. That and I knew how to use and read one! lol
    So I called my local mechanic that I use ocassionally and made an appointment to have them check and see if my hunch is correct. I have to drop it off on Tuesday morning first thing. I told them that I just want it checked and not fixed if it's gonna cost more than a standard one hour labor charge. If it's not getting any power then I'll trace the wiring myself and see if I can mend the issue. After all, I decided to pull and install the engine myself to save thousands of dollars, so I'm not gonna spend hundreds of dollars having something fixed or adjusted that wont hurt the truck.

    SOOOOOOO.....After that long drawn out diatribe, my question is this.......Has anyone else ever run into this problem? And if so, am I barking up the wrong tree or am I headed in the right direction?
    Oh, and foir what it's worth, the "loping idle" thing is a non issue when it's in gear (700R4). And my TV cable is set up according to all the instructions I've read and it drives just fine. The only thing I haven't done is tune the engine with a timing light. I went old school and did it by ear. SO I suppose that could be the issue. But as far as I'm aware the old engine was timed professionally?

    ---------- Post added at 10:53 PM ---------- Previous post was at 10:47 PM ----------

    And another note.....The up and down idle is at worst a 400 or so rpm change ever second or two. And at best it's about 200 rpms in change.
  2. Ape

    Ape Member 100 Posts

    No ideas or experiences from anyone huh? :-/
  3. Coach24

    Coach24 Rockstar 3 Years 5000 Posts

    Try using the timing light. It may be that simple.
    It may be less expensive to just purchase a meter that will have simple instructions, than pay a mechanic.
    In gear it goes away? Idle sensor? vacuum leak ? check all hoses for age related breakdown
  4. giorge

    giorge Rockstar 100 Posts

    It almost sounds like a vacum leak and at what rpm do you have it idling at? I have also seen fuel problems cause that, fuel pressure or dirty fuel filter. your timing could be part of problem.
  5. Ape

    Ape Member 100 Posts

    I was going to time it with the light I have, but to be honest I wasn't sure what wire it is that I'm supposed to disconnect to time it? I read about it in my Hanes manual, but it doesn't tell me where the heck the wire is supposed to be.

    As for vacuum leaks, I've replaced all of the vacuum lines with new ones and don't hear anything obvious.
    And like I said above, I replaced the IAC valve hoping that was the issue to no avail. :-/

    ---------- Post added at 07:41 PM ---------- Previous post was at 07:38 PM ----------

    Rpm idle when warm is about 800 or so. Maybe even a bit less, but it's hard to tell since it jumps 2 to 400 rpms while idiling.

    The fuel filter is only a few months old, but lately I've been wondering if my issue would be my fuel pump. From the little bit of reading I've been doing online it seems to be a common faulty part with this year Suburban.
    I don't have any way of testing the pressure on my own aside from cutting the line and installing a gague. But since I'm going to have my local mechanic look at it Tuesday morning, I may just have them test the pressure while they're there.
  6. Coach24

    Coach24 Rockstar 3 Years 5000 Posts

    Timing light should just clip on to number one ignition wire and then the other two wires go to power and ground
  7. Ape

    Ape Member 100 Posts

    LOL.....I should've explained more. I've used a timing light plenty of times in the past for my older non computer, non emissions controlled vehicles such as my 66 Chevelle. But the manual on this Burb says you're supposed to disconnect some sort of timing wire to set the timing with a light.
    I'd have to look it up again to see exactly what it calls the wire........But suffice it to say that it's supposed to have "that" wire disconnected from the engine electronics before using a timing light.
  8. giorge

    giorge Rockstar 100 Posts

    It is probably talking about your timing advance wire.
  9. Ape

    Ape Member 100 Posts

    That sounds right......Any ideas on where it is???
  10. giorge

    giorge Rockstar 100 Posts

    Last edited: May 13, 2012

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