timing light does not shoot at steady 8

[vikasintl]

we have 1989 Gmc s15 truck 112k miles

when I check timing instead showing at 8 degree
it flickers between 6 to 10..sometimes 6 sometimes 8 or 10 etc.

same thing with vacuum ..flickers between 14 to 17 etc.

also when we start the truck it vibrates heavily and when stomp on gas pedal vibration stops...when we drive it does not vibrates..so am not sure...if this is due to engine mounts. I also think when engine is warm it does nor vibrates...


changed spark plugs, checked wires for resistance, cleaned cap and rotor...


what should be reason? and what's solution?

[68C15]

check for vacuum leak. IAC is attempting to bring things back to normal as well as ECM changing timing which accounts for fluctuating timing.

[BMERDOC]

I would agree with 68C15, if it fluctuates its internal like a miss. Remember, there is such thing as a lean miss which would be where he is pointing you. Simply enough, it is an internal problem and not a problem with mounts or something similar.

[vikasintl]

Originally Posted by 68C15

check for vacuum leak. IAC is attempting to bring things back to normal as well as ECM changing timing which accounts for fluctuating timing.

Someone suggested to check for pressure regulator spring and its not broken...how do I check for vacuum leak?

also how do I check for IAC? if its working properly or something wrong there?

[68C15]

Originally Posted by vikasintl

how do I check for vacuum leak?

also how do I check for IAC? if its working properly or something wrong there?

spray carb clean, just a shot here and there. when the idle changes you have found it. you can also use a mechanic's stethoscope with just the hose to focus your hearing while you listen for a hiss.

the IAC is like the sheriff in town. if he is arresting a bunch of drunk drivers he is not the problem, he's just doing his job.

[vikasintl]

Originally Posted by 68C15

spray carb clean, just a shot here and there. when the idle changes you have found it. you can also use a mechanic's stethoscope with just the hose to focus your hearing while you listen for a hiss.

the IAC is like the sheriff in town. if he is arresting a bunch of drunk drivers he is not the problem, he's just doing his job.

Thanks I found a vacuum leak and fixed the hose and it runs smooth now..truck or engine is not shacking any more...thanks for advice..

I did not understand what you meant for IAC though...

[BMERDOC]

IAC= Idle Air Control

[95 S_Trucker]

when you check timing, are you unplugging the timing connector under the pass. side carpet?

[vikasintl]

Originally Posted by 95 S_Trucker

when you check timing, are you unplugging the timing connector under the pass. side carpet?

We have 89 gmc s15 2.5 Liter ..does it have timing connector?

[Tink]

Yup, all computer controlled vehicles do.

[vikasintl]

Originally Posted by Tink

Yup, all computer controlled vehicles do.

Can you point me to an image or video so I can see where is this timing connector ? I can't find under passenger side carpet...

[95 S_Trucker]

its up close to the fire wall and kick panel. a single wire connector, i cant remember what color the wire is

[68C15]

tan with a black stripe