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1988 Suburban Horrible MPG

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

  1. notaloafer

    notaloafer New Member

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    Now just to start off with, I know suburbans are not world-renowned for their gas mileage!

    I just picked up a 1988 suburban with the 350 engine and turbo 400 3-speed transmission. Besides sitting unused for the last 3 years, it's in good shape. I've run some Seafoam through it, as well as some injector cleaner. I also today just cleaned out the throttle body. It starts right up, and after cleaning the throttle body, the low-rev hesitation seems to have gone away for the most part. All in all mechanically it seems to be running well.

    After running my first tank of gas through it, I registered around 9.5 MPG with mostly freeway driving (55 - 70 MPH).

    Today I did a quick test run. I ran it on the freeway after filling it for about 50 miles and only pulled 8.7 MPG (averaging around 68 MPH), far below what I consider acceptable. My Dad's old suburban in the past pulled around 15 MPG average, so 8.7 is unacceptable.

    Other notes - The CEL only came on once after I first drove it away, and went out after about 10 seconds. I figured it was just running some old gas through the engine and that tripped a sensor. After cleaning the throttle body today, the CEL again came on for about 10 seconds and then went out again.

    I've noticed a slight engine ticking at around the 60 mph range. If I speed up to 70 it goes away, and if I slow back down below 60 again it also goes away. Sometimes it ticks (definitely affected by the rev's on the engine) after driving at 60 and then exiting the freeway, but the noise usually dissipates after a few minutes. Not sure if that is related or not, I just figured it was some valve noise.

    Any ideas as to why my mileage is so bad, and what else I can do about it?

    Thanks in advance.
    #1
  2. ChevyFan

    ChevyFan Administrator Staff Member

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    If it sat unused for 3 years then you're going to have to probably have it cycle a half-dozen times before you can figure out what's going on. A 1988 Suburban was made to cruise on the freeway at 55 MPH, so 70 MPH will give you about a 30-40% decrease in MPG over even 60 MPH.

    Need some more information. How many miles and what are the rear-end gears? What's the history of the vehicle also?
    #2
  3. oldmanstruck

    oldmanstruck New Member

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    Talking about 88....I just parted mine a week ago. I enjoyed it very much and your dad was right, usually get around 15. In my log, I got about 13 city and 17 hwy and my avg was around 15.7. But that's for a 2WD with 4 speed tranny that has over drive.

    Your 88 with 3 Speed, is it 4x4? May need a tune up since it hasn't been driven for long time? Under 10mpg is uncommon. How's the engine? does it idle smooth? sounds strong when rev high?
    #3
  4. notaloafer

    notaloafer New Member

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    The engine runs very strong from what I can tell. The only issue I have is when the engine is cold, it has a very small hesitation moving from a standstill. Other then that, it runs strong with no leaks or oil burning issues. One other minor thing is the drive belt slipping if I rev the engine hard off of a standstill. I will be changing that out very soon as well.

    I'll try slowing it down to 55 MPH on the freeway... it is just hard to do since it rolls so nicely @ 70MPH :)

    Right now the odometer is reading 87000 miles, and the previous owners who had it swear that it has not rolled over. From what I can tell based on engine and transmission strength, they may be telling the truth. The transmission shifts strong and smooth, and the engine runs great. Also the interior is in great shape.

    It is the 4x4 model. I am unsure what the gearing is in the rear-end... how would I go about telling that?

    As far as tune-ups go, what else could I do to help bring everything back up to speed?

    Thanks!
    #4
  5. notaloafer

    notaloafer New Member

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    New developments. I jumped the OBD-I computer ports and pulled two error codes: 33 and 43.

    33 "MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR (VOLTAGE HIGH-LOW VACUUM)"
    43 "ELECTRONIC SPARK CONTROL (ESC) CIRCUIT"

    Not sure what those mean exactly. I found the ESC module but I don't know where to look for the MAP sensor.

    I also researched and found out that both of these could be causing poor mileage and hesitation from a stop (my only two problems).

    What should be my next move? Anyone know how to get to the MAP sensor on this vehicle?
    #5
  6. notaloafer

    notaloafer New Member

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    More developments... hopefully I'm not breaking any rules by posting what is going on in hopes it can help someone else someday.

    Using the image below, I found the MAP sensor. However, the knock sensor is nowhere to be found. There are no wires going to that low passenger-side of the engine. On the driver's side though, I found what looks to be like some kind of sensor with one wire coming out of it. It's right in between the front two spark plugs, slightly higher. It looks like the knock sensor?

    Also besides finding the MAP sensor, I don't really know what else to do except stare at it, haha.

    Attached Files:

    #6
  7. oldmanstruck

    oldmanstruck New Member

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    sorry, I have no experience with any of the sensor you mentioned above. the 350 is a very common engine and there should be a lot of experience folks out there know about this. If no luck, you may want to just stop at the dealer and have them take care of this issue for you.
    #7
  8. notaloafer

    notaloafer New Member

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    It's pretty funny. The local Chevy dealer here didn't even know where the fuel filter was in this rig. He said he didn't have the service manual for anything before 1990 so he couldn't even work on it.

    Since this is my secondary vehicle, I'm just going to mess with it myself and see what I can do.
    #8
  9. Crawdaddy

    Crawdaddy Moderator

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    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.
    #9
  10. tbplus10

    tbplus10 Moderator

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    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.
    #10
  11. notaloafer

    notaloafer New Member

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    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.

    - - - Updated - - -

    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?
    #11
  12. Crawdaddy

    Crawdaddy Moderator

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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.
    #12
  13. notaloafer

    notaloafer New Member

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    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.
    #13
  14. wulk025

    wulk025 New Member

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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.
    #14
  15. notaloafer

    notaloafer New Member

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    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

    - - - 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....
    #15
  16. tbplus10

    tbplus10 Moderator

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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.
    #16
  17. oldmanstruck

    oldmanstruck New Member

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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.
    #17
  18. notaloafer

    notaloafer New Member

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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.
    #18

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