99 Sub. 7.4 Missing

Discussion in 'GM Powertrain' started by Sahara Jungle, Mar 31, 2010.

  1. Sahara Jungle

    Sahara Jungle New Member

    Hello, I havent been on this forum in awhile and thought id come here for some thoughts on the problem with my truck. Its a 1999 Suburban 2500 with the 7.4L, and it has a whipple supercharger on it. The problem is that the truck is moody. Some days it will run fine, no problems at all. Others, it will miss, varying from a slight problem to heavy missing. Also on its good or bad days the truck cant really get over 60 mph and 2500 rpm because the truck is missing. But the odd thing is that it only misses while driving, i could have it in park and rev it up to 6k rpms if i wanted and it wouldnt skip a beat. Ive changed the spark plugs and fuel filter hoping they would be the problem. But of course they arent. So I had a local mechanic look at it and run a compression test on it and he said it could be a blown head gasket. I wouldnt have a problem with this diagnoses if the coolant level would actually drop some or if I would see water in the oil and such. Anyways too the point, Im asking for a second opinion for what the problem could be.

    P.S. I know this might be kind of stupid, but could it be the stall converter? Maybe its getting old and not working like it use too and slips when it shouldnt, idk just a thought.

    THanks for any help. :great:
  2. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    A miss will be noticeably different than a failing stall convertor, with a failing stall convertor the RPMs will actually jump up a little, a miss the RPMs will fall a little.
    It's not uncommon for a miss to happen under load.
    The easiest way to diagnosis this would be on a rolling Dyno.
    The reason could be many things but I would concentrate on electrical since it comes and go's, mechanical would be there all the time.
    Start making notes i.e. does it happen on humid or rainy days, are there any other similarities when it happens?
  3. Sahara Jungle

    Sahara Jungle New Member

    See thats what i was thinking too. Thats why i didnt understand that if was something like a blown head gasket i think it would be there all the time and not only somtimes. Maybe I do need to drive it around on rainy/ humid days to see if it misses then more than others. Also could it be in the ignition timing? I dont know how you change that in these newer cars but i know in older cars its not that hard when all u have to do is twist the distributor a little. Idk, just a thought. Thanks for the info and reply Tim
  4. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    As long as nobody has really messed with the timing the Computer will actually advance/retard your timing on its own... I dont know if you have a factory distributor. But i know that they went to a plastic model in the Vortec motors. You might want to take the cap off and see if there is any play in the distributor..
    I have had to replace 3 this year now in various years of Vortec motors... Just a thought.
  5. Sahara Jungle

    Sahara Jungle New Member

    Thanks for info murdog.
    So today, while i was with my gf my dad took the truck down to autozone and had them run a diagnostic on it to find its DTC's. which it had three, one with the crank/cam timing sensor and two with the oxygen sensors.
    So I just went under the truck to the crankshaft sensor and unpluged it and sprayed some wire cleaner on it and wire dryer on it. Pluged it back in and let it sit. Then i went to my gf's house and while i was out my dad tested the truck, when i got back home he said it ran good.:great:
    So I took it out for a test drive and it ran a whole lot better. I actually got up to seventy( didnt feel like going to much over speed limit) and there was no missing. Im hoping what i did fixed it, and tomorrow im gonna climb in the engine and look at the camshaft sensor and clean that out too.
    Im going to drive the truck over next few days to see what happens.
  6. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Glad you found something out. These things can be rather never racking...

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