1988 Suburban Horrible MPG

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

  1. notaloafer

    notaloafer New Member

    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.
  2. ChevyFan

    ChevyFan The Sheriff Staff Member 5+ Years 1000 Posts

    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?
  3. oldmanstruck

    oldmanstruck Rockstar 100 Posts

    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?
  4. notaloafer

    notaloafer New Member

    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?

  5. notaloafer

    notaloafer New Member

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


    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?
  6. notaloafer

    notaloafer New Member

    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:

  7. oldmanstruck

    oldmanstruck Rockstar 100 Posts

    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.
  8. notaloafer

    notaloafer New Member

    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.
  9. Crawdaddy

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

    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.
  10. tbplus10

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

    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.

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