88 Suburban 350 running badly, poor mileage. TBI rebuild time.

Discussion in 'Engine Performance + Maintenance' started by geocrasher, Jul 6, 2019.

Car Parts
  1. geocrasher

    geocrasher Newbie

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    Howdy everyone,

    Sorry for the long first post. TL;DR: My Suburban idles badly, gets bad mileage, and so I'm rebuilding the TBI and need suggestions for what else I may need to do.

    My 1988 Suburban 1/2 ton 4x4 has been running poorly. The PO said the rig had been in his wifes family since new, had been through several engines and transmissions over the years. He said the engine in it is a Goodwrench 350 and I believe him- it's got more guts than the previous two 350 Suburbans I've owned. I haven't check the block numbers though.

    It's got a 700r4 with about 80K mile, and 3.73's. And to complete the specs, its rolling on 31" mudders that I got for way too little money last year.

    The last 3 years or so I've hardly driven it, and last summer it started running really badly. At idle it'll be fine, then it'll miss for just a second, followed by what sounds like a 1500-2000rpm surge (no tach) that lasts several seconds before it settles down. Sometimes when cold, it'll die when I put it into gear. The exhaust also smells gassy when its cold.

    I replaced the fuel filter, because I thought maybe I ran the tank too low and clogged something. I was broke at the time and had run it down to probably 2-3 gallons in its 40 gallon tank, against my own better judgement.

    The filter didn't help. Neither did a new TPS, O2 sensor, and IAC. I recently filled it, and drove around for about 140 miles, and got 11.5mpg... I could have understood 13 or 14 considering that some of that was around town driving and that I'm on mud tires, but my previous Suburbans got 15-17 on the highway and they were heavier 3/4 ton rigs.

    I mentioned these problems to a friend who's a mechanic, he suggested giving the TBI a good cleaning. After doing some youtubing around, I decided to just pull the TBI unit and rebuild it. Then I saw the Ultimate TBI mods here and decided that might be fun to look into.

    Tonight I tore into it and in about 15 or 20 minutes had the TBI in hand. That was far easier than I expected! One thing I noticed though was that the wiring harness to the passenger side injector was showing copper between the harness and the connector. I'm thinking that can't be all that good. I'm going to test it for continuity of course.

    Disassembling the TBI unit showed that the bolts holding the injector pedestal on were pretty loose, and the area around the fuel pressure regular looked a bit varnished. The rest of the TBI looks in really good shape. I'm guessing it has been rebuilt in the last decade or so.

    My intention is to rebuild it while cutting off the big "gates" at the top of the throttle body to clean up airflow, and smooth them after cutting. I'm not going for the super pretty jobs that some folks do, and I'm not looking to remove a lot of material- just get rid of the obvious obstruction. The vehicle is bone stock other than the replacement engine, so I don't see the point in getting too crazy with the TBI mods beyond that.

    After the TBI rebuild and getting the truck running again I'm going to check timing. Something tells me it's off. It's also due for an oil change. The goal right now is to just get it up to the 15-17mpg I'm used to. The truck has some other issues (soft brakes, inoperable front windows etc and more) but I'd like to get this solved first.

    I'd love any input on this and suggestions for getting it running well, including any extra work that I hadn't discussed. Thanks!

    [​IMG][​IMG][​IMG] IMG_20190705_203846.jpg
     
  2. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    After rebuilding your TB you should calibrate it. If you don't you run the risk of burning up the IAC stepper motor.

    http://www.gmcmidwestclassics.org/Web pages/Tuning the TBI.pdf

    Before you change more parts, check your fuel pressure and, like you planned, base timing. Fuel pressure specs are 9 - 13psi, preferably 11 - 13. Then put it on a scanner to see if any other issues exist.
     
    bigfutz likes this.
  3. Schurkey

    Schurkey I'm Awesome

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    Waste of effort and time. Given the engine and the typical RPM range, airflow is not a problem.
     
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  4. geocrasher

    geocrasher Newbie

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    Thank you- I'll just do the rebuild normally then. I really do appreciate the honest response. It looks like I'm going to have to build a scanner. WinALDL and an Arduino should do the trick, and then I need to chase down a fuel pressure gauge. Thanks for the tips!
     
  5. kauila

    kauila Newbie

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    If the TBI fix does not solve the problem, maybe it could be an old coil starting to go bad.
     
  6. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    WinALDL is pretty much obsolete, try TunerPro RT v5+ it even works with Windows 10
     
  7. geocrasher

    geocrasher Newbie

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    Interesting! I will consider that. Thank you.

    Thank you, this I did not know. From what I've read, my '88 ECM is 8192 baud, TTL serial, and I can use an Arduino with the chip removed, TX/RX pins shorted to each other in A and ground going to M. Does that sound about right? I'm still trying to determine what is accurate and what is old (like WinALDL). I'd love to be able to use this for diagnostics, seems like a great tool. Thanks again.
     
  8. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    No, it's 160 baud unless someone updated your 88 (odds are it's a 1227747) ECM to a 92+ year model like the 16197427. Here's a cheap way to data log and troubleshoot with a laptop.

    12 pin for 88 - 93, 16 pin for some 94 and all 95 ($60)
    http://aldlcable.com/aldl.asp

    Free software (or a contribution)
    http://www.tunerpro.net/

    And some reading
    http://www.gearhead-efi.com/Fuel-Injection/forumdisplay.php?27-TunerPro-Tuning-Talk
     
  9. geocrasher

    geocrasher Newbie

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    Thank you @PlayingWithTBI I appreciate the direction. Can you direct me to a way to make the ALDL cable? Electronics is a hobby, and so I'd rather build than buy if at all possible. Or will the aforementioned TTL serial work fine?
     
  10. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    Can't help you there but, you can google it. For me it's so much easier to invest a paltry $60 and know it's not the reason if I can't get online with the ECM.
     

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