1988 Suburban Horrible MPG

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by notaloafer, Oct 14, 2012.

  1. notaloafer

    notaloafer New Member

    I just changed my oil with 5W-30. The beast took 9 quarts!! I'm assuming that it has some kind of large oil pan mod as well. At first I thought I had forgotten to put the plug in the bottom, but when I refilled the quarts with the used oil, it seemed to match up fairly closely. (7.5 quarts, but I spilled a ton as well). Would it be safe to just add the Marvel Mystery Oil at this point and change my oil again after 3000 miles? I'll try to drive it for a bit now and see if just the change helped.

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    I found this from this site: http://www.theimportkiller.com/forums/index.php?topic=420.0

    CODE 43
    Trouble Code 43 indicates that there may be a malfunction in the Electronic Spark Control (ESC) circuit. ESC is used to sense spark knock (pinging) and retard the timing to eliminate it. The knock sensor is located at the rear of the engine block. The ECM will retard the timing by as much as 20 degrees in 1 degree increments. A loss of knock sensor signal or loss of ground at the ESC module will cause the signal at the ECM to remain high. The ECM will act as if no knock is present, and
    may possibly result in engine damage, due to detonation.
    Code 43 is set when:
    ? Voltage at Knock Sensor is above 4.8 volts or below .64 volts.
    ? Either condition is met for about 10 seconds.

    Possible causes:
    1. Open or shorted knock sensor
    2. Loose knock sensor
    3. Excessive mechanical noise within engine
    4. Improper or incorrectly installed MEM-CAL in the ECM or defective ECM
    5. Intermittent open in the EST line to the ignition module

    I figured it might just be a faulty knock sensor causing my problem. Am I on the right track with that?
  2. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    I'm not going to say it's NOT the knock sensor, but I'd say the ignition modules fail more than knock sensors. Check for bare, chafed, or broken wires. When you worked on the engine, did you mess with a 1-wire connector over by the master brake cylinder?

    Also, with a Chevy 350, that's a 5 quart motor stock. Unless it has a 4 quart additional pan, which would be quite obvious under the truck, you're way overfilled on oil, possibly to the point of engine damage.

    My 91 Suburban with the same motor and body style will run 15mpg all day with no problem.
  3. notaloafer

    notaloafer New Member

    I thought the same thing - I have always changed the oil in all of my vehicles, and I checked the dipstick after adding every quart past 5 to ensure that I wasn't overfilling it. Finally at 9 quarts I had a reading at the bottom end of operating range on the dipstick. I haven't done anything yet (no driving) except starting it to park it after changing it, so I'll double-check today and make sure it's not way overfilled. I did fill back up the empty quarts with the used oil and came up with around 7.5 quarts (but I also spilled at least a quart's worth as well). The pan under the truck looked gigantic compared to other vehicles I've worked on, but then again who knows.

    I haven't checked the wiring yet, but I'll look for that today when I get back home. To the best of my knowledge, I haven't noticed/messed with the 1 wire connector.
  4. wulk025

    wulk025 New Member

    I would pull your distributer and check to see how warn thesplines on the shaft. If they are good i would then check to see how yourtiming is set you might have it retarded which would cause bad gas mileage. Inthat case advance the timing till you hear a slight pinging a W.O.T. and set the timing there you should getbetter mileage from that point on.
  5. notaloafer

    notaloafer New Member

    To put the oil issue to bed, I just double-checked and the dipstick is showing right in the middle of the operating range. The thing must have some kind of extra pan on it. When the truck is cold the oil pressure max's out the gauge @ 60 PSI, and after it warms up it settles around 45 (moves with throttle), which I think is normal?

    Just to help with troubleshooting, I went ahead and took some pictures. I figured if you guys were going to help me, I might as well try to make it easier. One of the pictures is the code list inside the glove box, and some of the others show the engine bay, and what I believe to be the engine knock sensor (which looks intact)

    2012-10-17_18-18-38_393.jpg 2012-10-17_18-17-25_485.jpg 2012-10-17_18-15-20_569.jpg 2012-10-17_18-15-15_603.jpg 2012-10-17_18-15-06_842.jpg 2012-10-17_18-14-59_969.jpg 2012-10-17_18-14-56_700.jpg 2012-10-17_18-14-46_616.jpg 2012-10-17_18-14-08_534.jpg 2012-10-17_18-14-00_889.jpg 2012-10-17_18-13-48_839.jpg

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    GT5 4.10:1 Rear Axle Ratio
    GT5 Axle Rear 4.10 Ratio (DUP With GT8)

    I think that may be a contributor to bad MPG....
  6. tbplus10

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

    Yea that gear ratio may be part of the problem.
    Its okay to add Marval Mystery Oil now and leave it in until the next change, just make sure you dont overfill the oil.
    Considering the year it doesnt look bad at all under the hood.
    I'd probably let her warm up a bit before reving it since your oil pressure on cold start does seem a touch high.
  7. oldmanstruck

    oldmanstruck Rockstar 100 Posts

    Try list a gig in craigslist about the problem. I had some good outcome going that route and found a father/son family they are chevy guys. When I stopped by their house seeing those older chevy trucks, I know I got some help.
  8. notaloafer

    notaloafer New Member

    Based on the feedback from this thread and the position of the MAP sensor and no indication of leakage on the line leading into the TBI, I am going to take the throttle body off and replace the gasket? underneath in a leap of faith attempt at hoping to fix this issue.

    Does anyone have any recommendations on how to go about this? I'm trying to avoid spending more $ on a manual to just do this one job.

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