Pulsating idle, Code 83 thrown.

[JDegenhart]

My father asked me to come over and take a look at his 1995 GMC Safari VIN W.

Originally, he was having poor idle issues which prompted him to start to chase the problem. The van does have 113K on it, but was performing well until about a month or so ago. So far, he has pulled the EGR and found carbon build-up so he installed a new EGR valve with a screen gasket. This improved the situation, but did not solve it. He also said he heard an arcing noise from the dist so he replaced the cap, rotor, plugs and wires. He has run STP fuel injection cleaner through the last three tanks, all with no results.

The symptoms are poor idle. It has a pulsating vibration at idle increasing with the A/C compressor kicked on and even more pronounced when dropped into Drive. I ran the codes and received only code 83.

Code 83 is not listed specifically for VIN W although it is listed under general GM codes as a TCC solenoid circuit problem. The TCC solenoid has nothing to do with idle conditions, does it?

I did notice that at idle, when the A/C is kicked on and dropped into drive, the idle drops but does not return to spec. I took out the IAC valve, cleaned it and the port with carb cleaner and a toothbrush, check resistance (A-B 54.7 C-D 52.8, good to go) and re-installed. Idle still not returning after load is applied to the engine. I don’t know why the IAC did not kick a code 35 because the idle was definitely below 50 RPM off of spec for more than 30 seconds.

Replace the fuel filter and put in a new IAC valve, cleared the codes, buttoned her up and went for a test drive. Now when a load is applied to the engine, the idle drops and then comes back up. The IAC valve was bad and DID need to be replaced although it didn't kick a code. Hmmm... Also it sounds alot better, think I had the problem licked...read on...

Got down the block a stretch and as soon as it warmed up, say within a mile of driving off of cold, here it comes again! Pull up to a stop sign with the A/C on and this stinking, lousy pulsating idle returned. The whole van is shaking, erghhhhh.

Limp home, check the codes and still, only code 83 is thrown. Wait for it to cool down, restart and it sounds tolerable until it warms up. When it is warm and if you turn the A/C on, it is very bad, turn the A/C off and it is barely noticeable, but still there. Anything off of idle and the van runs good.

Ideas??? Seems to me if it was vacuum problem, it would be bad whether it was warm or cold, like if I deliberately pulled a main manifold line off, it would run bad warm or cold. Both myself and my father has checked for cracked, split or open lines and have found none. Also, the pulsing is rhythmic not wandering like a vacuum leak. I know that there is carbon buildup in the engine, could deposits in the combustion chamber be pre-igniting fuel when hot, and only after a mile of driving? Or maybe another sensor acting up without tripping a code? What’cha think?

Thanks, James