[vikasintl]

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?

 

[wis bang]

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.

 

[gridlock]

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

 

[stephan]

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

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.

 

[dipstick]

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.

 

[gridlock]

I think what you're describing is what to do to read the trouble codes from the computer.

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.

 

[stephan]

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.

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.

 

[d-monikgmcs15]

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

 

[tudorhtop]

exact same problem!

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

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

 

[91s10clay]

does anyone know where the wires on the back of the data link'diagnostic port go for 1991 s10,2.5

 

[Crispyt]

I've always unplugged the little wire to unplug the computer. I think crossing the A and B pins puts the computer into diagnostic mode which from what I was told (atleast on full size trucks) is a bad idea when its running. However maybe on the S10's its differnent. I would just unplug the little wire that way you know for sure its unpluged. The 8 degrees needs to be set without the computer controlling the timing. If your unable to find a timing mark on the pulley you can take the number 1 cylinder spark plug out and use a flashlight to watch and see when the pistion is all the way at the top on the compression stroke. (Thats TDC). Then take some whiteout and mark a line on your pulley to match up to the 0 degrees TDC mark on the indicator bracket. The other way is you can take a coat hanger and set it on top of the piston through the spark plug hole. Then slowly bar the engine over till the coat hanger just stops moving up then its at TDC. But ususally theres a mark on the balancer.

And I dont know for sure where the wires go but I belive pin A is just a ground and pin B is the data link wire.