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  1. #11

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    Quote Originally Posted by tbplus10 View Post
    Unfortunately 1988 was a changeover year for trucks (mechanical and body) some of the changes found their way into Suburbans, early models for that manufacturing year could have a weird mix of new and old, not uncommon just a pain in the neck to sort out. If you can find a manual for it the manual will have a listing by vin of what's different.
    Th400, is this a 2500 series Suburban?

    Look in the glove box or door jam for an equipment sticker, glove box would be the best bet, this should tell you the gear ratio.

    Your noise issue is probably sticky lifters, not uncommon after getting build-up and sitting for years. Try an additive to loosen them up, I normaly like to add Marvel Mystery Oil to an older crank case just before oil change time and that'll loosen things up again. An oil change, if you havent done one yet, might do the trick too.
    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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    Quote Originally Posted by Crawdaddy View Post
    The knock sensor is that one you found in the side of the block on the lower drivers side. That's where it is on my 91. But, why are you looking for the knock sensor?

    The ESC module is either the module hanging off a bracket in the middle of space on the passenger side of the throttle body, or it's the module under the distributor cap. The manuals seem to be confusing about that and only mention those modules for certain applications (HEI for example) when they are present on all the ignition types I am aware of.
    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. #12

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    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.
    Christopher

    1991 Chevy Suburban 1/2 ton 2WD w/ chevy SBC 350-3/4 ton drivetrain upgrade w/4.10 gears 200K miles
    2005 Saturn ION-2 Stock 277K miles
    1982 Bronco, 1993 Bronco (sold), 1971 M35A2 Deuce and a Half



    There are 10 kinds of people, those who understand binary, and those who dont...

    Remember kids, the only difference between screwing around and science is writing it down- Adam Savage

  3. #13

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    Quote Originally Posted by Crawdaddy View Post
    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.
    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. #14

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    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. #15

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    Quote Originally Posted by Crawdaddy View Post
    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.

    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)

    Click image for larger version. 

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    - - - Updated - - -

    Code
    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. #16

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    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. #17

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    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. #18

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    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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