'99 Suburban 7.4, low idle, hesitation, stalling

Discussion in 'GM Powertrain' started by TexasChris, Oct 7, 2010.

  1. TexasChris

    TexasChris Member

    I got a 99 burb with the 7.4 that I suspect was a shadetree job. **edit** and Tranny is 4L80e**

    She starts up, sounds great, but the idle continually slips from 1100 and down. Sometimes it settles @ 8 or 9 right after start up but continually slips either way.

    Sometimes she stalls and dies when you put it in reverse, if the engine doesn't die there's surging immediately afterwards. Dies at stopsigns, stoplights etc. There was also an issue with intermittent power under accelleration and some funny shifting. From a dead stop when you accelerate there's a pretty consistent *cough*hiccup* and then surge.

    There is/was also an issue shifting, wouldn't shift untill high rpm...wouldn't downshift under wot on the highway. I say is/was because it seems to have cleared up since I unhooked the battery fiddlin' with the EGR.

    Here is what I've done in the last three weeks in this order:

    --CES was on--

    New Fuel Pump
    New Fuel Filter
    New TPS (started up and idled @ 1100, went for a cruise, engine cutting out, idle would later drop lower and lower as same problem returned)

    ---still had problem with intermittent power/engine cutting out under acceleration at this point---

    New PCM from nelson Performance (has aftermarket GM cam*)

    -Started up, idled strong @ 1100 just like with the new TPS, but the problem returned also....went to autozone to pull the code...can't remember the exact code but I do have them written down somewhere.....

    Then while investigating EGR code and contemplating a new one discovered the wire loom had cooked on the exhaust return (it was just laying there!) and had melted through the plastic shield and one little gray wire was showing copper through its scorched insulation and making metal to metal contact with the return. 99% sure this wire was running to the EGR....

    Seperated and insulated the wire, suspended the loom...unhooked the battery for a minute.....hooked it back up, started up

    -CES was off (as a result of discon battery?)
    -Idle is still awful
    -No more cutting out (yet)
    -Still hesitates/stalls/dies when moving to Rev or Drive
    -Shifting was way better on the test drive this time, under 10 or 15% throttle just scooted right along through all 4 gears, also downshifted when gettin' after it on the highway.

    So here's where I'm at, I pulled the EGR, but it didn't look like to me like it was bad, so I didn't install the new one I'd bought (but I've still got it).

    How can I be sure that the EGR is good?

    Am I hunting a possible vacuum leak now?

    All input is appreciated.

    *it's a mystery cam, I was able after some sleuthing to track down the fella that put the cam in, he gave me a part number but never answered another phone call....so at the bottom of my list is I still have no idea what cam is in it and this could be a possible issue...



    I'm no expert and this is my first time to really go about sorting out an engine so If I ask a stupid question, please bear with me.
    Last edited: Oct 7, 2010
  2. stephan

    stephan Rockstar 4 Years 5000 Posts

    Hey Chris,
    First off there are no bad ques here so no worries. I'm not sure but from how I am understanding this everything has improved except:

    -Idle is still awful

    -Still hesitates/stalls/dies when moving to Rev or Drive

    I don't think these are EGR issues as it doesn't function at idle, (or at least it's NOT suppose to but it could be stuck open at idle) but rather an IAC problem.
    Enter idle problems or similar words in the search box above. ( IAC won't work for search as search word must be more than 3 letters) I have read some indepth posts recently about this and it should help you a lot. I would do the search for you but my GF is sceamin at me to get off here haha

    The cam could be some of the driveability problems but don't fault it yet. Continue trying to find out lift/duration/overlap though cuz it could be a contributing issue.
  3. TexasChris

    TexasChris Member

    ok, went ahead and replaced the IAC this afternoon. No change, idle is low and feels like it's about to die. 1000-1100 feels like a good idle, 500 and 800 doesn't feel so hot.

    I did look up the part number, and should have included that the new PCM was tuned for the cam....the ?? is wether or not this guy told me the correct information, or just told me something to get me off the phone. He wouldn't pick up phone calls from the previous owner (whom he sold it too) and only answered my initial phone call.

    Thoughts other than IAC?
  4. stephan

    stephan Rockstar 4 Years 5000 Posts

    It sounds like you're passing too much air at idle and it is just too lean. If the EGR is stuck open at idle from carbon build up this could cause it but you checked it didn't you? Ok, I just re-read your other post and you said the EGR looked good. Before I installed the new EGR if that's what you're thinking, I would block the old one off and see if it idles better. You can't get your money back if you bolt the new EGR on to test.
    This seems like a "lean" prob. cuz you never mentioned it smelling rich, AND the fact that higher RPM's cover up lean idle problems keeps telling me it is getting too much unmetered air at idle. Maybe try spraying a little carb cleaner around the intake gaskets and some of the vacuum hoses to see if the idle smoothes out. Go easy on the spray when you do find a leak cuz that stuff isn't good for oxy sensors. Acetelene also works if you have an oxy-acetelene torch BUT DON'T USE ANY OXYGEN! Use acetelene only. A small propane cannister also works.

    How is it that you happened to have a new EGR & IAC on hand? Did the previous owner suspect these, or did your mech tell you/him that it needed these?? Is this the same problem the previous owner was fighting and got frustrated with?
    If the previous owner did a lot of messin with this problem I would check everything and also make sure all the wires & hoses are hooked in the correct places.
    Did prev owner give you any history on the engine other than the new cam being installed?
  5. TexasChris

    TexasChris Member

    1st, could be air, although yesterday I did the brake clean test around the intake, throttle body etc and no discernable increase in RPM.

    No, I didnt' happen to have the spare parts, I had it scanned.

    Previous owner said the hesitation was because the PCM wasn't tuned to the cam, although right now I'm thinking I'm the second person to order a pcm for this truck in the last 6 months.....

    I'm thinking this is absolutely the reason he wanted rid of it.

    Should I just get a chiltons and start from the fuel tank? It's funny that you mention double checking everything because I'm so irritated with the way the wiring was done I called our local dealership and inquired about a new wiring set up for the whole kitty. 1100 dollars.......so I can't go the all new route...but I'm tracking stuff down as I learn what's what. This is my first go around with all the electronic fanciness.

    How do you block the EGR off?

    I found my codes list, these were before after fuel pump replacement and fuel filter replacement.

    po 121
    po 122 (low intermittent voltage to TPS)

    po 141 (o2 bank 1 sensor 2)

    po 405 (egr sensor)
    po 463 (fuel level sensor)

    p 1121
    p 1122 intermittent voltage (lo and hi I believe)

    p 1153 o2 bank 2 sensor 1 (insufficient switching)

    ----Then TPS replacement------

    po 122
    p 1122 (tp or APP sensor 1 circuit low/tp circuit intermittent low voltage)

    ---- then discon battery---

    which cleared everything and I've been driving it around a bit with no codes, but the hesitation/stalling is still occuring.

    ---- and Thanks for the help here, it's appreciated man.----
  6. stephan

    stephan Rockstar 4 Years 5000 Posts

    OK good that you found your list of codes. Murdog or someone with a key to the codes should be able to decipher all those. I wouldn't even consider buying the wiring loom yet. You can repair individual wireing as you find burnt/bad ones & a lot cheaper than a new loom.
    I think you have already fixed the midrange & high rpm running, shifting, down shifting etc with your repair of the burnt wireing you found. If someone doesn't decipher the codes here then start a new thread and title it trouble codes or decipher codes, and that should get some responses. As far as blocking the EGR to test, you can disconnect the plumbing and plug inlets/outlets, or R&I EGR and block off the mani. but before you do that try spraying all around the EGR & plumbing and see if idle improves. If the idle improves then EGR is stuck open/leaking at idle & you need to find out why. Look for carbon sticking the valve open, loose etc.
    Do you have a vacuum guage? If you do hook it up & see what kind of vac this eng. pulls. It's probably going to be random because of the cam overlap but we still might be able to figure out smth. Don't buy a guage just borrow one from a friend or borrow/rent from one of the auto parts stores that loan tools to customers.

    Hey Chris, just put this for another poster, & thought it might be helpful to you too so you don't have to spend a zillion dollars for a Chiltons manual.

    click on the chilton library.com logo
    type "nwls" for the password
    click proceed

    Or click on this one if the other one doesn't work. There was some question as to how long these links are good for as they change them from time to time. http://infotrac.galegroup.com/itweb/...40539003520151

    Chris here's another link I found that addresses idle probs. Not the same engine but same probs and all engines are just different versions of the same fancy smancy air pumps. lol
    Last edited: Oct 8, 2010
  7. TexasChris

    TexasChris Member

    OK, to plug the EGR stuff.....I see one hose coming in, the exhaust return, so I unhook that and plug it with _____?

    Then R&I EGR = remove and inspect?

    Once removed what do I plug the two holes in the mani with?

    I'm not sure I completely understand how the EGR works, I'll remove and put up pics later this evening, gotta run at present.

    OMT though......can the PCM be programmed to ignore all these sensors? what would that do?

    thanks again,
  8. stephan

    stephan Rockstar 4 Years 5000 Posts

    Pictures would be good Chris. Photo from both sides.

    Ok EGR simply stands for "exhaust gas recirculation" and is only suppose to work during mid range rpms not at idle. Plug the holes with anything that you have that is suitable that will completely seal them off. When I tested mine I wanted to try it while driving it for a few days so I made a block off plate and bolted it on and put a gasket with it to make sure it was completely sealed. If you don't have time to make a block off plate, just make sure you don't use smth that might fall in or get sucked in and it must seal good, otherwise you have just created your own vacuum leak.

    For inspecting the EGR valve; Take it off & look and see if the valve is sealing. It should be CLOSED. It should be sealed and not allow ANY AIR TO PASS. Blow in it or spray it and see if it leaks to the other side. If it does, it's not sealing. IF is passes this test, it could still be opening at idle when installed on the engine because of false/incorrect inputs/vacuum/electrical/sensors ie: something telling the computer that the engine is running mid range when it is actually at idle. This is why I suggest making a permanent block off plate so you can run and test drive your rig a couple days and see if this has cured your probs.

    I don't know if all the sensors can be bypassed unless you go old school, & pull everything off it, and put a real carbuerator and a real distributor on it:lol:
  9. TexasChris

    TexasChris Member


    found short in o2 bank 1 and 2 sensor 2. No cats on exhaust, o2 sims on their way.
  10. k.daddy1

    k.daddy1 Rockstar 100 Posts

    As far as your idling and shifting problems, your first code show the TPS has low voltage. The throttle position sensor controls your idling and also controls the shift points through the computer to the tranny. The TPS are relatively cheap 35-40 bucks. I've had several vehicles come in for different shifting issues, scan the system and the TPS is usually low. Not saying that is 100% yours but you are describing issues the TPS will fix.

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