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1989 gmc s15 pickup low vacuum and how to check timing?

Discussion in 'Chevy S10 Forum (GMC Sonoma)' started by vikasintl, Aug 18, 2011.

  1. vikasintl

    vikasintl New Member

    1989 Gmc s15 pickup 2wd , odometer - 112000 4 cylinder 2.5 liter automatic

    I checked vacuum is approx. 15 and goes little lower...

    and when I start its high idle like 1050 rpm and than after a while it calms down

    I tried to check timing which should 8 btdc...but I can't see timing mark

    can you guys suggest me how can I find timing mark? I can see 12,8,4,0 marks but noting on pulley..

    also what could be main reason for low vacuum like 15? and how to trouble shoot?
  2. wis bang

    wis bang Rockstar 100 Posts

    That era had an OBD I computer and Throttle Body Fuel injection. The timing is controlled buy the computer. You set the 'base timing' by un-pluging the computer. The wire is under the glove box near the upper [firewall] edge of the carpet a single wire w/ a purple stripe and this tiny little plug about 3/16ths" by a 1/2" long. Set the timing and then plug it back in to let the computer work it.
  3. gridlock

    gridlock New Member

    Can one just short the computer by crossing the A and B ports on the OBD I plug in under the drivers side dash just below steering wheel?
    That's the way I was told to do it and that's the way I did, didn't know you could just unplug the computer...... lol
  4. stephan

    stephan Rockstar 3 Years 5000 Posts

    I think what you're describing is what to do to read the trouble codes from the computer. What Wis describes above is the ticket for setting timing, although the connector locations & the wire colors change over the years. I have an '88 & the wire is outside the firewall & it's brown with a black tracer.

    OP Vika,
    Is your vacuum reading steady at 15 or does it bounce around? Does it ever go up above 15 when you are backing off the throttle going downhill on compression? Google "vacuum guage readings" & you'll find some places that show 14-16 various vac guage readings that will help you diagnose yours. I used to have the link to one but I can't find it in my files:neutral:

    **From what I remember "a little low but steady needle" indicates somewhat low compression, but not necessairly too low to make it run bad. A wildly swinging needle indicates worse problems like sticking valves, burnt valves, blown head gaskets etc. There's also one for ignition timing being too advanced or retarded (which could be yours) but I don't remember what the needle indication is for that one.
    Last edited: Dec 10, 2011
  5. dipstick

    dipstick Member 2 Years 100 Posts

    15 not to bad should be 18 to 21.. long as said not erratic.
    You will only have a notch/slot on balance mark it with white chalk ect. to see better.
    You should have a tan wire with black stripe to disconnect when timing. Should have a black box on passenger side fierwall / cowl.
    Have a cover labeled fuel pump fuse.. relay.. juction block . Dis connect will be here.
  6. gridlock

    gridlock New Member

    Nah, Im just describing how I read it here http://www.s10forum.com/forum/f104/help-need-timing-advice-now-91-a-code-2-5-a-183703/
    and that's the way I set my timing, was this incorrect? can the timing not be set per this method? Im not sure but that was the way I done it and it seemed to work, as soon as I plugged in jumper wire I was able to read timing marks but if that was not correct I'll have to go back and do it again but from what they say in link I posted you set timing that way and also can pull the codes.
  7. stephan

    stephan Rockstar 3 Years 5000 Posts

    Interesting Grid. I've never heard of that or ever seen it in a manual either. I've always set them with the same procedure as WIS is describing above. Just to see if it works & you get the same timing result, just disconnect the comp wire, hook up your timing light & see if your timing marks are in the same place as they were when you set it by bridging the A&B connectors.
  8. d-monikgmcs15

    d-monikgmcs15 New Member

    hey I've got a 89 gmc s15 2.5 would the computer wire be white on that year I need to reset it having timing issue it will start but die when i try to bring up the rpms stauls at stops and will fire back up but run like crap, crap meaming sluggis no power I've replaced the injector, plugs, plug wires, fuel pump, fuel filter, rotor, and cap, EGR valve
  9. tudorhtop

    tudorhtop New Member

    exact same problem!


    HELP!92 Sonoma 2.5....what did you do?

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